3.11: Alternate Text Description

Figure 3.1.1: Components of the Digestive System

This image shows the human digestive system arranged vertically from the head to the lower torso, with labels identifying each structure and its role in the pathway food follows through the body. At the top, the mouth is shown as the entry point where food enters the digestive system. Inside the mouth, the tongue is labeled, indicating its role in positioning food during chewing and swallowing. Three sets of salivary glands are labeled near the mouth:

• The parotid gland, located near the cheek area
• The sublingual gland, located beneath the tongue
• The submandibular gland, located below the jaw

These glands produce saliva that begins chemical digestion and lubricates food. From the mouth, the pathway continues to the pharynx, which serves as a passage guiding swallowed food toward the esophagus. The esophagus is shown as a long vertical tube descending through the chest region, transporting food from the pharynx to the stomach through muscular contractions. In the upper abdominal region, the stomach is labeled. The stomach acts as a holding chamber where food is mixed with digestive secretions for further breakdown. Surrounding the stomach are accessory digestive organs:

• The liver, located above and to one side of the stomach, produces substances that aid digestion
• The gallbladder, positioned beneath the liver, stores and releases bile needed for fat digestion
• The pancreas, located behind the stomach, releases enzymes that support chemical digestion

Below the stomach begins the small intestine, which is divided into three segments:

• The duodenum, the first section where digestion continues
• The jejunum, the middle section where nutrient absorption occurs
• The ileum, the final section that completes absorption and connects to the large intestine

The large intestine surrounds the small intestine and is labeled in several parts:

• The ascending colon, which carries material upward
• The transverse colon, which runs horizontally
• The descending colon, which moves downward
• The sigmoid colon, which curves toward the rectum
• The cecum, located at the beginning of the large intestine
• The appendix, attached to the cecum

At the end of the digestive tract, the rectum stores waste before elimination, and the anal canal and anus serve as the exit point for waste removal.

Figure 3.1.2: Layers of the Alimentary Canal

Figure 3.1.3: The Peritoneum

This image shows a horizontal cross-section of the abdomen, as if the body has been sliced across the midsection. The organs are arranged within a circular body outline, with labels identifying both digestive organs and surrounding structures. At the center top of the image is the vertebra, with the spinal cord located just in front of it. These structures provide support and protection for the central nervous system.

On both sides of the spinal cord are the kidneys, positioned toward the back of the abdominal cavity. Near one kidney, the spleen is labeled, and on the opposite side, the liver is shown as a large organ occupying significant space in the upper abdomen. Beneath the liver is the gallbladder, which stores bile.

Near the middle of the abdomen is the pancreas, positioned behind the stomach and extending across the midline. The stomach is located toward the front portion of the abdominal cavity. From the stomach, the digestive tract continues into the small intestine, shown as coiled loops occupying the central region of the abdomen.

Surrounding the small intestine is the large intestine, which forms a thicker tube that frames the smaller loops of the small intestine. The diagram highlights the peritoneum, a continuous membrane that lines and supports the abdominal organs. It is divided into:

• The parietal peritoneum, which lines the inner surface of the body wall
• The visceral peritoneum, which covers the surfaces of the organs

Between these two layers is the peritoneal cavity, a space that allows movement of the organs while reducing friction. The diagram uses a highlighted boundary to show how the peritoneum wraps around and encloses the organs, forming a supportive and protective structure that helps maintain the organization of the abdominal contents.

Figure 3.2.1: Digestive Processes

This image consists of three side-by-side diagrams showing a sequence of events in peristalsis, the process that moves food through the digestive tract. Each diagram shows a vertical tube representing a section of the digestive tract. Inside the tube is an oval-shaped mass representing a bolus of food. Arrows indicate both the direction of food movement and the actions of the surrounding muscles.

1. Left Diagram: Start of Movement - The bolus is located near the upper part of the tube. The text “direction of food” and a downward arrow indicate that movement occurs from top to bottom. The tube is narrowed just above the bolus, showing contraction of circular muscles behind the bolus. Below the bolus, the tube is wider, indicating relaxation of muscles ahead of the bolus.
2. Middle Diagram: Progression - The bolus has moved downward. The region of contraction has also shifted downward to remain just behind the bolus. Arrows on the sides of the tube indicate inward pressure behind the bolus and reduced pressure ahead of it. This demonstrates that muscle contraction follows the bolus, continuing to push it forward.
3. Right Diagram: Continued Movement - The bolus is now positioned further down the tube. The same pattern continues: contraction occurs behind the bolus, and relaxation occurs ahead of the bolus, maintaining a one-directional push. Arrows indicate the continued downward movement.

Figure 3.2.2: The Human Digestive Tract

The image is the human digestive tract from the head to the lower body, with arrows tracing the movement of food and labels identifying the key processes that occur at each stage.

At the top, the mouth is labeled with ingestion of food, indicating the entry point where food is taken into the digestive system. From the mouth, a continuous pathway leads through the pharynx and into the esophagus. A labeled section titled Propulsion explains how food moves through the digestive tract. Two mechanisms are listed:

• Swallowing, which moves food from the mouth through the oropharynx and into the esophagus
• Peristalsis, which occurs in the esophagus, stomach, small intestine, and large intestine, pushing food forward through coordinated muscle contractions

The pathway continues into the stomach, where arrows inside the organ indicate mixing movements. A labeled section titled Mechanical digestion explains that physical breakdown occurs through:

• chewing in the mouth
• churning in the stomach
• segmentation in the small intestine, which mixes contents

Another labeled section titled Chemical digestion shows that digestive substances act throughout the tract to break down food into smaller components suitable for absorption. From the stomach, the pathway continues into the small intestine, shown as a long, coiled tube. Arrows along this region indicate both movement and mixing. A labeled section titled Absorption explains that:

• nutrients and water move into blood vessels and lymphatic vessels in the small intestine
• water is absorbed into blood vessels in the large intestine

The pathway then continues into the large intestine, shown as a broader tube surrounding the end of the small intestine. Near the bottom, the remaining material is labeled feces, indicating the waste left after digestion and absorption. The final section shows the anus, labeled with defecation, where waste is eliminated from the body. Some chemical digestion occurs in the mouth. Some absorption can occur in the mouth and stomach, for example, alcohol and aspirin.

Figure 3.3.1: Anatomy of the Oral Cavity

This image shows an anterior view of the open mouth, with the lips parted and the internal structures of the oral cavity clearly labeled.

At the top are the superior lip and the superior labial frenulum, a thin fold of tissue connecting the upper lip to the gum. The gingivae (gums) surround the upper teeth and provide support. The roof of the mouth is divided into two regions:

• The hard palate, located toward the front, provides a firm surface that helps press food during chewing.
• The soft palate, located toward the back, is more flexible and helps direct food toward the throat during swallowing.

Hanging from the soft palate is the uvula, a small projection that helps close off the nasal passage during swallowing. At the back of the oral cavity are the palatoglossal arch and palatopharyngeal arch, which form the boundaries of the fauces, the opening between the mouth and the pharynx. Between these arches is the palatine tonsil, which helps with immune defense. On the sides of the mouth are the cheeks, which form the lateral walls of the oral cavity.

The lower portion of the diagram shows the tongue (underside), which is lifted to reveal structures beneath it. The lingual frenulum is the fold of tissue connecting the tongue to the floor of the mouth. Near this area is the opening of the duct of the submandibular gland, where saliva enters the oral cavity. The diagram also labels different types of teeth, arranged from front to back:

• Incisors, used for cutting food
• Cuspids (canines), used for tearing
• Premolars, used for crushing
• Molars, used for grinding

The oral vestibule, the space between the lips or cheeks and the teeth, is labeled along the lower outer region. At the bottom are the inferior lip and the inferior labial frenulum, which connects the lower lip to the gum.

A fleshy bead of tissue called the uvula drops down from the center of the posterior edge of the soft palate. Although some have suggested that the uvula is a vestigial organ, it serves an important purpose. When you swallow, the soft palate and uvula move upward, helping to keep foods and liquid from entering the nasal cavity. Unfortunately, it can also contribute to the sound produced by snoring. Two muscular folds extend downward from the soft palate, on either side of the uvula. Toward the front, the palatoglossal arch lies next to the base of the tongue; behind it, the palatopharyngeal arch forms the superior and lateral margins of the fauces. Between these two arches are the palatine tonsils, clusters of lymphoid tissue that protect the pharynx. The lingual tonsils are located at the base of the tongue.

Figure 3.3.2: Surface Anatomy of the Tongue and Associated Oral Structures

This image shows a superior view of the tongue and adjacent throat structures, with the tongue extended forward and the back of the oral cavity visible. At the top of the image is the epiglottis, positioned behind the tongue. It helps close the airway during swallowing.

On both sides of the upper region are the palatopharyngeal arches and palatoglossal arches, which form the boundaries of the passage between the oral cavity and the pharynx. Between these arches are the palatine tonsils, which help support immune defense by detecting pathogens. At the base of the tongue is the lingual tonsil, another area of immune tissue.

Across the surface of the tongue is a V-shaped groove called the terminal sulcus, which separates the anterior portion of the tongue from the posterior portion. The surface of the tongue, labeled as the dorsum of the tongue, contains several types of papillae, which contribute to both mechanical processing and taste:

• Circumvallate papillae are large, rounded structures arranged in a row along the terminal sulcus. They contain taste buds and play a major role in detecting taste.
• Foliate papillae are located along the sides of the tongue near the posterior region and also contain taste receptors.
• Fungiform papillae are scattered across the surface of the tongue, especially toward the front, and contain taste buds.
• Filiform papillae are the most numerous and cover much of the tongue surface. Unlike the other papillae, they do not contain taste buds. Instead, they provide friction and help move food.

Figure 3.3.3: Salivary glands

This image shows a side view of the lower face and oral cavity, focusing on the three major salivary glands and the ducts that transport saliva into the mouth.

Near the upper cheek region is the parotid salivary gland, the largest of the salivary glands. Extending forward from this gland is the parotid duct, which travels across the cheek and opens into the oral cavity near the upper teeth. This duct delivers saliva into the mouth to begin chemical digestion and lubrication.

Beneath the jaw is the submandibular salivary gland, positioned along the lower part of the face. The submandibular duct extends from this gland and opens into the floor of the mouth. This gland produces a large portion of saliva and contributes both fluid and enzymes.

Located in the floor of the mouth is the sublingual salivary gland, which is smaller and positioned beneath the tongue. Multiple sublingual ducts extend from this gland and open directly into the oral cavity. These ducts release saliva continuously to help maintain moisture and assist in digestion.

All three glands are connected to the oral cavity through ducts, which are shown as narrow channels directing secretions into the mouth. The diagram emphasizes that:

• The parotid gland secretes saliva through a single long duct into the cheek area.
• The submandibular gland releases saliva through a duct into the floor of the mouth.
• The sublingual gland releases saliva through multiple small ducts under the tongue.

Figure 3.3.4: Permanent and Deciduous Teeth

This image shows three views of human dentition, focusing on tooth types and their eruption timelines. The views include an upper jaw for permanent teeth, a smaller upper and lower jaw for primary teeth, and a lower jaw for permanent teeth. Each tooth is labeled by type and typical age of eruption.

1. Upper Permanent Teeth - The upper dental arch is shown from a top view and labeled upper teeth. Teeth are arranged from front to back as follows:
   • Central incisors: 7 to 8 years
   • Lateral incisors: 8 to 9 years
   • Cuspids or canines: 11 to 12 years
   • First premolars: 9 to 10 years
   • Second premolars: 10 to 12 years
   • First molars: 6 to 7 years
   • Second molars: 12 to 13 years
   • Third molars or wisdom teeth: shown without a specific age
2. Primary Teeth - Smaller upper and lower arches represent the primary dentition, with eruption ages listed in months.
   1. Upper Primary Teeth
      • Central incisors: 8 to 12 months
      • Lateral incisors: 12 to 24 months
      • Cuspids or canines: 16 to 24 months
      • First molars: 12 to 16 months
      • Second molars: 24 to 32 months
   2. Lower Primary Teeth
      • Central incisors: 6 to 8 months
      • Lateral incisors: 12 to 15 months
      • Cuspids or canines: 16 to 24 months
      • First molars: 12 to 16 months
      • Second molars: 24 to 32 months
3. Lower Permanent Teeth - A larger lower dental arch is shown with eruption ages in years:
   • Central incisors: 7 to 8 years
   • Lateral incisors: 7 to 8 years
   • Cuspids or canines: 9 to 10 years
   • First premolars: 9 to 10 years
   • Second premolars: 11 to 12 years
   • First molars: 6 to 7 years
   • Second molars: 11 to 13 years
   • Third molars or wisdom teeth: shown without a specific age

Figure 3.3.5: The Structure of the Tooth

This image shows a cross-sectional view of a single tooth embedded in the jaw, with labels identifying its major structural regions and internal components.

The tooth is divided into three main regions:

• The crown, which is the visible portion above the gum line and is involved in chewing
• The neck, which is the narrow region at the gum line where the crown meets the root
• The root, which extends below the gum and anchors the tooth in the jawbone

The outermost layer of the crown is enamel, which forms a hard protective covering that resists wear from chewing. Beneath the enamel is dentin, a dense tissue that forms most of the tooth structure and supports the outer layer while transmitting forces from chewing.

At the center of the tooth is the pulp cavity, which contains blood vessels and nerves. These structures provide nourishment to the tooth and allow it to respond to stimuli. The pulp cavity extends downward into the root as the root canal, carrying blood vessels and nerves toward the base of the tooth.

Surrounding the root is cementum, a layer that covers the outer surface of the root and helps anchor the tooth.

Between the cementum and the surrounding bone is the periodontal ligament, a connective tissue that secures the tooth within the socket while allowing slight movement during chewing.

The gingiva (gum) surrounds the tooth at the level of the neck, providing protection and support where the tooth meets the oral cavity. The tooth is embedded in bone, which forms the socket that holds the root in place.

Figure 3.3.6: Pharynx

This image shows a side (sagittal) view of the head and neck, showing the pathways for both air and food and how these pathways intersect and separate.

At the upper front of the diagram is the nasal cavity, located above the roof of the mouth. It connects to the nasopharynx, the upper part of the throat behind the nasal cavity, which serves as a passageway for air. Below the nasal cavity is the oral cavity, which extends from the lips to the back of the mouth. The roof of the oral cavity is divided into two regions:

• The hard palate at the front, which provides a rigid surface
• The soft palate at the back, which is flexible and helps direct substances during swallowing

Extending downward from the soft palate is the uvula, which helps close off the nasal passage during swallowing so food does not enter the nasal cavity. Behind the oral cavity is the oropharynx, which serves as a shared pathway for both air and food. Below this region is the laryngopharynx, which directs air toward the trachea and food toward the esophagus.

At the lower front of the throat is the larynx, which is part of the airway and leads into the trachea. The trachea carries air to and from the lungs. Located above the larynx is the epiglottis, which acts as a movable flap. During swallowing, the epiglottis folds downward to cover the airway opening and helps prevent food from entering the trachea.

The glottis is also labeled as the opening within the larynx through which air passes. Behind the trachea is the esophagus, the tube that carries food from the pharynx to the stomach. It lies posterior to the airway and functions only in food movement. A legend identifies the major regions:

• Nasal cavity, the upper air passage
• Oral cavity, an entry point for both food and air
• Pharynx, the shared passage with three regions: nasopharynx, oropharynx, and laryngopharynx
• Larynx, the airway structure leading to the trachea

Figure 3.3.7: Anatomy of the Esophagus and Esophageal Sphincters

This image shows a side (sagittal) view of the head, neck, and upper torso, focusing on the esophagus and the structures that control the movement of food between the throat and the stomach.

At the upper portion of the image, the pathway from the mouth leads to the pharynx and then into the esophagus, a long muscular tube that descends vertically through the chest.

At the upper end of the esophagus is the upper esophageal sphincter, located near the transition between the throat and the esophagus. This sphincter acts as a controlled opening that relaxes to allow swallowed material to enter the esophagus and then contracts to help prevent air from entering and material from moving back upward.

The esophagus is shown as a continuous tube extending downward behind the airway. Adjacent to it and positioned in front is the trachea, which carries air to the lungs. The separation between these two tubes helps ensure that food and air follow different pathways. At the lower end of the esophagus is the lower esophageal sphincter, located just before the stomach. This sphincter helps:

• allow food to enter the stomach
• prevent backflow of stomach contents into the esophagus

The stomach is shown at the bottom as the receiving organ where food is delivered after passing through the esophagus.

Figure 3.3.8: Pharyngeal Contractions During Swallowing

This image consists of three side-by-side sagittal views of the head and neck, showing the pharyngeal phase of swallowing and how muscles contract in sequence to move a bolus downward. Each panel shows a vertical pathway from the mouth to the esophagus. A rounded mass labeled bolus represents food. Arrows and highlighted muscle regions show the direction of movement and the sequence of muscle activity.

1. Left Panel: Initiation of Swallowing - The bolus is located in the back of the mouth near the entrance to the pharynx. The superior pharyngeal constrictor muscle is labeled and shown contracting around the upper pharynx. The medial pharyngeal constrictor muscle is also labeled just below it. This contraction narrows the passage behind the bolus and begins to push it downward from the oral cavity into the pharynx.
2. Middle Panel: Progression Through the Pharynx - The bolus has moved farther down into the pharynx. The labeled region shows the medial and inferior pharyngeal constrictor muscles contracting in sequence. The contraction now occurs lower than in the first panel, showing that muscle activity follows the bolus downward and continues to push it through the pharyngeal passage.
3. Right Panel: Entry Into the Esophagus - The bolus is positioned at the lower end of the pharynx near the esophagus. The diagram labels the inferior pharyngeal constrictor muscles and esophageal constrictor muscles. These muscles contract to push the bolus into the esophagus, completing the pharyngeal phase of swallowing. The contraction is now at the lowest level in the sequence.

Figure 3.4.1: The Stomach

This image shows a cutaway view of the stomach, connected to the esophagus above and the duodenum below, with labels identifying its regions, internal folds, and muscle layers. At the top, the esophagus enters the stomach at a region labeled the cardia, which serves as the entry point for food into the stomach.

Above and to the left of this entry is the fundus, a dome-shaped region that functions as a storage area for ingested material. The largest portion of the stomach is the body, located centrally, where mixing and digestion primarily occur.

The inner space of the stomach is labeled the lumen, which contains partially digested material. The internal surface of the stomach lining forms folds called rugae of the mucosa, which allow the stomach to expand as it fills and also help mix contents. Along the outer shape of the stomach are two borders:

• The lesser curvature, a shorter inner curve
• The greater curvature, a longer outer curve

Arrows along these curves indicate the general direction of movement and mixing within the stomach. Toward the lower end of the stomach is the pyloric region, which includes:

• The pyloric antrum, where food is further processed
• The pyloric canal, a narrowing passage
• The pyloric sphincter, which regulates the movement of stomach contents into the next structure

Beyond the sphincter is the duodenum, the first part of the small intestine, which receives partially digested material from the stomach. The wall of the stomach is labeled muscularis externa, consisting of three layers of muscle:

• An outer longitudinal layer, which shortens the stomach
• A middle circular layer, which constricts the lumen
• An inner oblique layer, unique to the stomach, which enhances mixing

Together, these layers generate strong mixing movements that mechanically break down food. The outermost covering of the stomach is the serosa, which provides protection and reduces friction with surrounding structures.

The convex lateral surface of the stomach is called the greater curvature; the concave medial border is the lesser curvature. The stomach is held in place by the lesser omentum, which extends from the liver to the lesser curvature, and the greater omentum, which runs from the greater curvature to the posterior abdominal wall.

Figure 3.4.2: Layers of the Stomach Wall and Gastric Glands

This image combines a cross-sectional view of the stomach wall with progressively magnified views of the gastric mucosa and a gastric gland. The figure illustrates how the layers of the stomach wall relate to the structures responsible for gastric secretion.

On the left, a small stomach diagram identifies the region being enlarged. The enlarged cross-sectional view shows the four major layers of the stomach wall arranged from the lumen outward.

Figure 3.4.3: Neural and Hormonal Regulation of Gastric Secretion

This image is a flowchart divided into three major sections: the cephalic phase, gastric phase, and intestinal phase. Each section shows pathways that either stimulate or inhibit stomach secretory activity. Arrows connect the steps and show the direction of signals and responses.

Figure 3.5.1: Anatomy of the Small Intestine and Large Intestine

This image shows a front view of the abdomen with the digestive organs simplified and labeled, focusing on the small intestine and large intestine. The small intestine appears as tightly folded loops in the center of the abdomen. It is divided into three labeled sections:

• The duodenum, located at the top, receives material from the stomach and begins further digestion.
• The jejunum, located in the middle, continues digestion and is a major site of nutrient absorption.
• The ileum, located in the lower portion, completes absorption and connects to the large intestine.

Surrounding the small intestine is the large intestine, shown as a thicker tube forming an outer frame around the smaller loops. At the bottom, the large intestine narrows into the rectum, which stores waste before elimination.

Figure 3.5.2: Structural Adaptations of the Small Intestine for Absorption

This image shows three levels of structural detail of the small intestine, along with matching microscopic images, to explain how the intestinal lining is specialized for absorption.

In the image, at the broadest level, the small intestine is shown as a tube with a central space labeled the lumen. The inner lining forms large ridges called circular folds that project into the lumen. Covering these folds are many small, finger-like projections called villi. Together, the folds and villi greatly increase the surface area available for absorption. The outer wall also includes muscle layers that move intestinal contents, and a labeled blood vessel indicates circulation involved in nutrient transport.

The middle level enlarges the villi to show their internal structure. Each villus is covered by absorptive cells that form the outer surface. Between these cells are goblet cells, which secrete mucus to lubricate and protect the lining. Inside each villus is a capillary network connected to an artery and vein, which carries absorbed nutrients into the bloodstream. Each villus also contains a central lacteal, a lymphatic vessel that absorbs certain nutrients. Between the villi are intestinal crypts, which are downward extensions of the lining that contain cells involved in secretion and renewal. At the base of the mucosa is the muscularis mucosae, a thin muscle layer that helps move the villi slightly. Below this are larger lymphatic vessels and a labeled duodenal gland, showing additional support structures.

The most magnified view in Figure \(\PageIndex{2}\) shows a single absorptive cell. Its apical surface is covered with tiny projections called microvilli, together known as the brush border. These structures increase surface area even further at the cellular level and contain enzymes that support the final stages of digestion and absorption.

A row of three microscope images corresponds to these same levels of structure. The first shows large folds with villi projecting outward. The second shows a closer view of densely packed villi lining the intestinal surface. The third shows the brush border at very high magnification, revealing the closely spaced microvilli.

Figure 3.5.3: Segmentation

This image shows three vertical panels arranged from left to right, illustrating segmentation, a mixing movement that occurs in the small intestine. Each panel shows a long, narrow intestinal tube divided into repeated segments. Inside the tube are rounded portions of chyme arranged along its length.

In the left panel, the intestinal tube is relaxed. The chyme is arranged in alternating segments, with different portions separated along the tube. There are no arrows yet, indicating that active mixing has not started.

In the middle panel, localized contractions appear at several points along the intestinal wall. These contractions divide the chyme into smaller portions and push it in opposite directions in neighboring segments. Arrows inside the tube point both upward and downward, showing that the contents are moving back and forth rather than in a single forward direction.

In the right panel, the chyme has become more evenly mixed. The contents that were previously separated are now more uniformly distributed within the segments. Arrows continue to show localized movement, but there is still no overall forward progression along the tube.

Overall, the figure demonstrates that segmentation involves alternating contraction and relaxation of intestinal segments. These contractions occur in non-adjacent regions, dividing and recombining chyme. The movement is bidirectional within short sections rather than one-way. The main function of segmentation is to mix chyme with digestive secretions and increase contact with the absorptive surface of the small intestine, which supports digestion and absorption.

Figure 3.5.4: The Large Intestine

This image shows a front view of the large intestine, with labeled sections showing its structure and its connection to the small intestine and the anal canal.

The large intestine forms a broad outer frame around the central abdominal area. At the lower right side of the figure, the ileum, which is the last part of the small intestine, connects to the cecum, a pouch-like structure that marks the beginning of the large intestine. Attached to the cecum is the vermiform appendix, shown as a narrow, finger-like extension.

From the cecum, the pathway continues through the colon in sequence. The ascending colon extends upward along one side of the abdomen. At the top, it bends at the right colic (hepatic) flexure and continues across the abdomen as the transverse colon. On the opposite side, it turns at the left colic (splenic) flexure and then travels downward as the descending colon.

At the lower end, the large intestine narrows into the sigmoid colon, which follows a curved, S-shaped path. The sigmoid colon connects to the rectum, which stores waste material before elimination. The rectum continues into the anal canal, the final segment through which waste leaves the body.

Overall, the figure shows the continuous pathway of the large intestine from the ileum to the anal canal. It also shows the major functions of the large intestine: receiving undigested material from the small intestine, absorbing water and electrolytes, compacting waste into feces, storing feces in the rectum, and eliminating waste through the anal canal.

Figure 3.5.5: Histology and Functional Organization of the Large Intestine Mucosa

This image combines a labeled diagram and a matching microscopic image to show the structure and function of the large intestine mucosa.

In the top portion, a simplified image of the large intestine is shown with an arrow pointing to a magnified view of its inner lining. The lining appears relatively flat and contains many straight, vertical intestinal glands, also called crypts, which open at the surface and extend downward into the tissue.

Two main cell types are shown in the lining. Absorptive cells line the surface and are responsible for absorbing water from intestinal contents. A magnified view shows that these cells have microvilli, small projections that increase surface area for absorption. Goblet cells are scattered among the absorptive cells and secrete mucus, which lubricates the intestinal contents and helps move waste through the colon.

Beneath the glands is the lamina propria, a connective tissue layer that contains support structures and a labeled lymphatic nodule, indicating immune activity. Below this is the muscularis mucosae, a thin layer of smooth muscle that allows minor movement of the mucosa. Beneath that lies the submucosa, a deeper supportive connective tissue layer. A label for smooth muscle fiber indicates contractile tissue involved in movement through the colon.

The lower portion shows a microscopic image of the large intestine lining. It shows closely packed, straight intestinal glands extending downward and many pale spaces representing goblet cells filled with mucus. The surface is relatively even and lacks large finger-like projections, confirming that the large intestine does not have villi.

Overall, the figure demonstrates that the large intestine mucosa is specialized for:

• absorption of water and electrolytes, mainly by absorptive cells
• secretion of mucus, by goblet cells to lubricate and protect the lining
• immune support, through lymphatic tissue
• compaction and movement of waste, supported by smooth muscle activity

Figure 3.5.6: External Features of the Large Intestine

This image shows a close-up view of the outer surface of the large intestine, highlighting three external features that distinguish it from the small intestine: haustra, teniae coli, and epiploic appendages.

The large intestine appears as a thick, segmented tube with an uneven outer surface. Instead of being smooth, it is divided into a series of rounded pouch-like sections.

These bulging sections are labeled haustra. Haustra are sac-like compartments formed by uneven contraction and tension in the intestinal wall. They divide the colon into segments, slow the movement of contents, and allow more time for water and electrolyte absorption.

Running lengthwise along the surface are narrow bands labeled teniae coli. These are thickened bands of longitudinal smooth muscle. They help maintain tension along the colon, contribute to the formation of the haustra, and assist with the movement and mixing of contents through coordinated contractions.

Attached along the outer surface are small, rounded structures labeled epiploic appendages. These are fat-filled pouches that project from the colon wall. They serve as characteristic external features of the large intestine and provide fat storage, cushioning, and support.

Overall, the figure shows how these three structures are related:

• teniae coli create longitudinal tension
• that tension helps form the haustra
• epiploic appendages provide external support

Together, these features help regulate the mixing, movement, and compaction of waste material as it travels through the large intestine.

Figure 3.6.1: Liver, Gallbladder, Pancreas, and Biliary Duct System

This image shows a front view of the upper abdominal region, highlighting the liver, gallbladder, pancreas, spleen, and the duct system that connects the liver, gallbladder, and pancreas to the small intestine.

The liver is the largest structure in the image and occupies the upper right portion of the abdomen. It is divided into four labeled regions: the right lobe, left lobe, quadrate lobe, and caudate lobe. The liver produces bile, which helps digest fats.

Beneath the liver is the gallbladder, a small sac that stores bile. A branching network of ducts connects the liver and gallbladder. The right hepatic duct and left hepatic duct carry bile from the liver and join to form the common hepatic duct. The cystic duct connects the gallbladder to this pathway. These ducts then combine to form the common bile duct, which carries bile toward the small intestine.

Below the liver is the pancreas, shown as an elongated organ extending horizontally. Running through it is the pancreatic duct, which carries digestive enzymes produced by the pancreas. These enzymes help digest carbohydrates, proteins, and fats in the small intestine.

The spleen is shown near the pancreas to indicate its anatomical position, although it is not directly involved in digestion.

Figure 3.6.2: Microscopic Structure of a Liver Lobule

This image shows both a macro-to-micro view of the liver and a three-dimensional view of a single liver lobule, illustrating how liver tissue is organized to process blood. A simplified outline of the liver appears at the upper left, with one region highlighted to show where the enlarged tissue view comes from.

At the upper right, the liver tissue is shown as a repeating pattern of lobules separated by connective tissue. Each lobule contains a central vein at its center. The main image shows one liver lobule in three dimensions. At the center is the central vein, which collects blood after it has passed through the lobule. Surrounding it are plates of hepatocytes arranged like spokes radiating outward. These cells carry out key liver functions such as metabolism, detoxification, and bile production.

Between the hepatocyte plates are irregular blood channels called sinusoids. Blood flows through these channels from the outer edge of the lobule toward the central vein. This arrangement allows close contact between the blood and the hepatocytes so that nutrients, toxins, and other substances can be processed efficiently. At the outer corners of the lobule are groups of structures called the portal triads. Each portal triad includes:

• a portal venule
• a portal arteriole
• a bile duct

Blood enters the lobule from the portal venule and portal arteriole, then passes through the sinusoids toward the central vein. An interlobular vein is also shown, indicating how blood leaving the lobule joins larger venous pathways in the liver.

This organization allows the liver to continuously filter blood, process nutrients, detoxify substances, and regulate blood composition before the blood returns to the general circulation

Figure 3.6.3: Anatomy and Histology of the Pancreas

This image combines a gross anatomical view of the pancreas with a magnified view of pancreatic tissue to illustrate how the pancreas functions as both an exocrine digestive organ and an endocrine gland.

Figure 3.6.4: The Gallbladder

This image shows a close-up view of the liver and gallbladder, highlighting the structure of the gallbladder and the duct system that carries bile from the liver toward the small intestine.

The liver occupies the upper part of the figure. Two ducts emerge from it: the left hepatic duct and the right hepatic duct. These ducts drain bile produced in the liver and join to form the common hepatic duct, which carries bile downward.

Attached to the underside of the liver is the gallbladder, shown as a pear-shaped organ. It is divided into three labeled regions:

• the fundus, the rounded wider end
• the body, the central portion
• the neck, the narrowed end that connects to the duct system

These regions show how the gallbladder is shaped to store and concentrate bile before release. The fundus and body serve mainly as the storage area, while the neck forms the connection to the outflow pathway.

The cystic duct extends from the neck of the gallbladder and joins the common hepatic duct. Together they form the common bile duct, which carries bile toward the small intestine.

Overall, the figure shows the pathway of bile movement:

• bile is produced in the liver
• it travels through the right and left hepatic ducts
• enters the common hepatic duct
• may be diverted into the gallbladder for storage and concentration through the cystic duct
• and then passes into the common bile duct for delivery to the small intestine

The diagram emphasizes that the gallbladder functions as a storage and concentration organ for bile, while the duct system provides a continuous route for bile to move from the liver to the digestive tract, where it helps with the digestion of fats.

Figure 3.7.1: The Digestive Tract

This image shows a vertical view of the digestive tract from the mouth to the anus, with labeled organs on the left and text boxes on the right summarizing the main digestive functions that occur in each region.

At the top is the mouth and oral cavity, which leads into the esophagus. The text for this region explains that mechanical digestion begins with chewing and swallowing, while chemical digestion begins with the breakdown of carbohydrates and fats.

Below this, the esophagus is shown as a vertical tube leading to the stomach. The liver and gallbladder are also shown nearby as accessory digestive organs. The text associated with this region explains that mechanical digestion includes peristaltic mixing and propulsion, while chemical digestion includes the digestion of proteins and fats. It also notes that the stomach carries out limited absorption of lipid-soluble substances, including alcohol and aspirin.

Below the stomach is the small intestine, shown as a coiled tube connected near the pancreas. The pylorus marks the exit from the stomach into the small intestine. The text for this region explains that mechanical digestion includes mixing and propulsion, mainly through segmentation. It also states that chemical digestion of carbohydrates, fats, polypeptides, and nucleic acids occurs here. In addition, this region is identified as the main site for absorption, including peptides, amino acids, glucose, fructose, fats, water, minerals, and vitamins. The large intestine appears below as a wider tube leading downward to the rectum and anal sphincter. The text for this region explains that mechanical digestion includes segmental mixing and propulsion. It states that there is no chemical digestion except by bacteria. It also identifies this region as a site of absorption of ions, water, minerals, vitamins, and organic molecules.

Figure 3.7.2: Digestion of Carbohydrates

This image is a flowchart showing the step-by-step digestion of carbohydrates, beginning with large polysaccharides and ending with absorbable monosaccharides. Arrows indicate the direction of chemical breakdown, and each step is labeled with the enzyme responsible for the change.

At the top, the process begins with starch and glycogen, which are large carbohydrate molecules found in food. An arrow labeled salivary amylase shows that digestion starts in the mouth, where this enzyme begins breaking these polysaccharides into smaller carbohydrate fragments.

From this first step, the pathway continues into two main groups of intermediate products. One branch leads to short branched polysaccharides, including limit dextrins. These partially digested carbohydrate fragments are further broken down by the enzyme alpha-dextrinase, producing glucose. A second branch leads to disaccharides, which are pairs of sugar units. Three disaccharides are shown:

• maltose
• sucrose
• lactose

Each disaccharide is paired with a specific enzyme:

• maltase breaks down maltose
• sucrase breaks down sucrose
• lactase breaks down lactose

The final products at the bottom of the flowchart are monosaccharides, which are the simple sugars that can be absorbed in the small intestine:

• limit dextrins, broken down by alpha-dextrinase, produce glucose
• maltose, broken down by maltase, produces two glucose molecules
• sucrose, broken down by sucrase, produces one glucose and one fructose
• lactose, broken down by lactase, produces one glucose and one galactose

Overall, the figure shows that carbohydrate digestion occurs in a stepwise sequence. Large polysaccharides are broken into smaller fragments and disaccharides, and these are then reduced to monosaccharides such as glucose, fructose, and galactose. These final products are the forms that can be absorbed into the bloodstream and used by body cells.

Figure 3.7.3: Protein Digestion, Absorption, and Distribution of Amino Acids

This image shows a front view of the digestive system with labeled organs and explanatory text describing how proteins are digested, absorbed, and distributed through the body. At the top, the oral cavity and esophagus are shown as the pathway that food follows into the digestive tract. No protein digestion is identified in this region.

The stomach is labeled as the site where protein digestion begins. A text note explains that digestion starts here through the action of hydrochloric acid and the enzyme pepsin. The acid creates the environment needed for pepsin to begin breaking proteins into smaller fragments.

Next to the stomach, the pancreas is identified as a source of protein-digesting enzymes. These enzymes are released into the small intestine, where they continue the breakdown of protein. The small intestine is shown as the major site of protein digestion and absorption. The labels explain that:

• the small intestine is the primary location where protein digestion is completed
• protein-digesting enzymes are secreted from the brush border
• the final products of digestion are amino acids
• these amino acids are absorbed into the blood and then carried to the liver

The liver is labeled as the organ that regulates the distribution of absorbed amino acids to the rest of the body. This indicates that amino acids do not simply circulate unchanged, but are processed and directed as needed after absorption.

In the lower part of the figure, the large intestine is shown with a note indicating that a small amount of dietary protein is lost in the feces. This shows that most protein is digested and absorbed before the remaining waste reaches the end of the digestive tract.

Figure 3.7.4: Stages of Protein Digestion

This image is a vertical flowchart showing the step-by-step digestion of proteins from large food molecules to absorbable amino acids. Rectangular boxes are connected by downward arrows, indicating the direction of chemical breakdown.

At the top is a box labeled protein, representing large, complex protein molecules from food. An arrow labeled pepsin indicates that this enzyme begins the digestion process by acting on proteins at the first stage.

The next box is labeled large polypeptides. This shows that proteins are first broken into smaller chains of amino acids called polypeptides. In this diagram, pepsin is the enzyme responsible for that initial breakdown.

Below that is a box labeled short peptides and amino acids. This indicates that the large polypeptides are digested further into smaller peptide fragments and some individual amino acids by additional digestive enzymes.

At the bottom is a box labeled amino acids, which are the final end products of protein digestion. These are the small molecules that can be absorbed through the lining of the small intestine and carried into the bloodstream.

Figure 3.7.5: The Digestive Tract

This image shows a front view of the digestive tract from the mouth to the rectum, with arrows and labeled volumes showing how fluid moves, is added, and is reabsorbed throughout the digestive system. At the top, fluid enters the digestive tract through the mouth. Two sources are shown:

• dietary intake: 2000 mL
• saliva: 1500 mL

These combine as the initial fluid entering the digestive tract. As contents move downward into the stomach, additional fluid is added through gastric secretions: 1500 mL. The total fluid volume at this stage is labeled 5000 mL. When the contents enter the small intestine, three more sources of digestive fluid are added:

• liver bile: 1000 mL
• pancreatic juice: 1000 mL
• intestinal secretions: 2000 mL

These additions bring the total fluid volume in the small intestine to 9000 mL. A labeled arrow shows that the small intestine reabsorbs 7800 mL of this fluid. After that absorption, the remaining volume entering the large intestine is labeled 1200 mL. In the colon, another fluid source is added:

• colonic mucus secretions: 200 mL

This increases the volume in the large intestine to 1400 mL. Another labeled arrow shows that the colon reabsorbs 1250 mL. After this reabsorption, the remaining 150 mL is labeled as lost in feces.

Figure 3.7.6: Absorption and Transport of Lipids in the Small Intestine

This image shows a cross-sectional view of the intestinal lining, illustrating how the products of lipid digestion are absorbed by epithelial cells and transported away through the lymphatic system.

At the top is the lumen of the intestine, which contains digested lipid materials. Below the lumen is a layer of absorptive epithelial cells that line the intestinal surface. Beneath these cells are two transport structures: a blood capillary and a lacteal, which is a lymphatic vessel. Arrows show the movement of substances from the lumen, through the epithelial cells, and into the transport vessels.

In the lumen, lipid digestion produces fatty acids and monoglycerides. These molecules are shown combining into emulsified droplets and then into micelles. The micelles carry lipid components to the surface of the epithelial cells.

At the epithelial surface, the diagram shows that fatty acids and monoglycerides leave the micelles and enter the epithelial cell. The micelles themselves do not enter the cell. Instead, they deliver the lipid components to the membrane, where those components pass inward.

Inside the epithelial cell, the fatty acids and monoglycerides are reassembled into triglycerides. Near a labeled Golgi apparatus, these triglycerides combine with proteins to form chylomicrons, which are lipid transport particles.

The diagram then shows chylomicrons leaving the epithelial cell and entering the lacteal. A final label indicates that lymph in the lacteal transports chylomicrons away from the intestine.

Although both a capillary and a lacteal are shown, the figure emphasizes that lipids follow the lymphatic pathway rather than entering the blood capillary directly.