The digestive system

The principal structure of the digestive system is an irregular tube, open at both ends, called the alimentary (al-i-MEN-tar-ee) canal or the gastrointestinal (GI) tract.
In the adult, this hollow tube is about 9 meters (29 feet) long
Think of the tube as a passageway that extends through the body like a hallway through a building. Food must be broken down or digested and then absorbed through the walls of the digestive tube before it can actually enter the body and be used by cells
The teeth are used to physically break down food material before it is swallowed. The churning of food in the stomach then continues the mechanical breakdown process.
Chemical breakdown results from the action of digestive enzymes and other chemicals acting on food as it passes through the GI trac
In chemical digestion, large food molecules are reduced to smaller molecules that can be absorbed through the lining of the intestinal wall and then distributed to body cells for use.
This process of altering the chemical and physical composition of food so that it can be absorbed and used by body cells is known as digestion, and it is the function of the digestive system
Part of the digestive system, the large intestine, serves also as an organ of elimination, ridding the body of the waste material, or feces, resulting from the digestive process.
Table 15-1 names both main and accessory digestive organs. Note that the accessory organs include the teeth, tongue, gallbladder, and appendix, as well as a number of glands that secrete their products into the digestive tube
Foods undergo three kinds of processing in the body: digestion, absorption, and metabolism
Digestion and absorption are performed by the organs of the digestive system
Metabolism, on the other hand, is performed by all body cells.
The digestive tract has been described as a tube that extends from the mouth to the anus
The wall of this digestive tube is fashioned of four layers of tissue
The inside, or hollow space within the tube, is called the lumen.
The four layers, named from the inside coat to the outside of the tube are as follows: 1. Mucosa or mucous membrane
2. Submucosa
3. Muscularis
4. Serosa
Although the same four tissue coats form every organ of the alimentary tract, their structure varies from organ to organ
The mucosa of the esophagus, for example, is composed of tough and stratified abrasion-resistant epithelium
The mucosa of the remainder of the tract is a delicate layer of simple columnar epithelium designed for absorption and secretion
The mucus produced by either type of epithelium coats the lining of the alimentary canal.
The submucosa, as the name implies, is a connective tissue layer that lies just below the mucosa.
It contains many blood vessels and nerves
The two layers of muscle tissue called the muscularis have an important function in the digestive process
By way of wavelike, rhythmic contractions of the muscular coat, a motion called peristalsis, food material is moved through the digestive tube the contraction of the muscularis also assists in the mixing of food with digestive juice and in the further mechanical break- down of larger food particles.
The serosa is the outermost covering or coat of the digestive tube
In the abdominal cavity it is composed of the visceral peritoneum
The loops of the digestive tract are anchored to the posterior wall of the abdominal cavity by a large double fold of peritoneal tissue called the mesentery
The mouth, or oral cavity, is a hollow chamber with a roof, a fl oor, and walls. Food enters, or rather is ingested into, the digestive tract through the mouth, and the process of digestion begins immediately.
As with the remainder of the digestive tract, the mouth is lined with mucous membrane.
Typically, mucous membranes line hollow organs, such as the digestive tube, that open to the exterior of the body
Mucus produced by the lining of the GI tract protects the epithelium from digestive juices and lubricates food passing through the lumen.
The roof of the mouth is formed by the hard and soft palates
The hard palate is a bony structure in the anterior or front portion of the mouth formed by parts of the palatine and maxillary bones
The soft palate is located above the posterior or rear portion of the mouth. It is soft because it consists chiefly of muscle.
The uvula and the soft palate prevent any food and liquid from entering the nasal cavities above the mouth.
The tongue is made of skeletal muscle covered with mucous membrane
It is anchored to bones in the skull and to the hyoid bone in the neck
A thin membrane called the frenulum (FREN-yoo- lum) attaches the tongue to the floor of the mouth.
Occasionally a person is born with a frenulum that is too short to allow free movements of the tongue
Individuals with this condition cannot enunciate words normally and are said to be “tongue-tied.” that the tongue can be divided into a blunt rear portion called the root, a pointed tip, and a central body.
A typical tooth can be divided into three main parts: crown, neck, and root.
The crown is the portion that is exposed and visible in the mouth
It is covered by enamel—the hardest tissue in the body.
Enamel is ideally suited to withstand the grinding that occurs during the chewing of hard and brittle foods
In addition to enamel, the outer shell of each tooth is covered by two other dental tissues—dentin and cementum
Dentin makes up the greatest proportion of the tooth shell. It is covered by enamel in the crown and by cementum in the neck and root areas.
The center of the tooth contains a pulp cavity consisting of connective tissue, blood and lymphatic vessels, and sensory nerves.
The neck of a tooth is the narrow portion, shown in Figure 15-5 that joins the crown of the tooth to the root
It is surrounded by the pink gingival or gum tissue.
A general term for mild, local- ized, and often transitory inflammation of the gums or gingiva is called gingivitis
The root of the tooth fi ts into the bony socket which surrounds it in either the upper or lower jaw bone.
A fibrous periodontal (pair-ee-oh-DON-tull) membrane lines each tooth socket and anchors the tooth to the bone.
Periodontitis (pair-ee-oh- don-TYE-tis) is a generalized and serious type of inflammation and infection. It is often a complication of untreated gingivitis unlike gingivitis, which is generally limited to superficial gingival or gum tissues, periodontitis involves the deeper periodontal membrane and other support tissues, including bone, that surround the teeth.
The bacteria responsible for periodontal disease cause the formation of “pockets” under the gingiva that deepen and enlarge over time
As the infection worsens it will cause loss of both periodontal membrane and bone, resulting in loosening and eventually complete loss of teeth
The bacteria may even travel through the bloodstream to affect the heart and other areas of the body. Periodontitis is the leading cause of tooth loss among adults
Tooth decay, or dental caries, is a disease of the enamel, dentin, and cementum of teeth that results in the formation of a permanent defect called a cavity.
Decay occurs on tooth surfaces where food debris, acid-secreting bacteria, and plaque accumulate
The shape and placement of the teeth (Figure 15-4) assist in their functions. The four major types of teeth are as follows:
1. Incisors
2. Canines 3. Premolars
4. Molars incisors have a sharp cutting edge. They have a cutting function during mastication (mas-ti-KAY-shun), or chewing of food.
The canine teeth are sometimes called “eyeteeth” or cuspids (KUSS-pedz). These pointed teeth are prominent in dogs and other meat eating-animals
After food has been chewed, it is formed into a small rounded mass called a bolus (BOW-lus) so that it can be swallowed
When an infant begins “teething” at about 6 months of age, the fi rst deciduous (deh-SID-yoo- us) teeth, or baby teeth, to erupt through the gums are the eight incisors.
Three pairs of salivary glands—the parotids, submandibulars, and sublinguals—secrete most (about 1 L) of the saliva produced each day in the adult
The salivary glands (Figure 15-7) are typical of the accessory glands associated with the digestive system. They are located outside of the digestive tube itself and must convey their secretions by way of ducts into the tract.
The parotid (pah-ROT-id) glands, largest of the salivary glands, lie just below and in front of each ear at the angle of the jaw an interesting anatomical position because it explains why people who have mumps (an infection of the parotid gland) often complain that it hurts when they open their mouths or chew; these movements squeeze the tender, inflamed gland
To see the openings of the parotid ducts, look in a mirror at the insides of your cheeks opposite the second molar tooth on either side of the upper jaw
The ducts of the submandibular (sub-man- DIB-yoo-lar) glands open into the mouth on either side of the lingual frenulum
The ducts of the sublingual (sub-LING-gwal) glands open into the floor of the mouth.
Saliva contains mucus and a digestive enzyme that is called salivary amylase
Mucus moistens the food and allows it to pass with less friction through the esophagus and into the stomach.
Salivary amylase begins the chemical digestion of carbohydrates.
The pharynx (FAIR-inks) is a tube-like structure made of muscle and lined with mucous membrane. its location in Figure 15-1. Because of its location behind the nasal cavities and mouth, it functions as part of the respiratory and digestive systems.
Air must pass through the pharynx on its way to the lungs, and food must pass through it on its way to the stomach.
The pharynx as a whole is subdivided into three anatomical components
The esophagus (eh-SOF-ah-gus) is the muscular, mucus-lined tube that connects the pharynx with the stomach
It is about 25 centimeters (10 inches) long. The esophagus serves as a dynamic passage- way for food, pushing the food toward the stomach.
The production of mucus by glands in the mucosal lining lubricates the tube to permit easier passage of food moving toward the stomach
The stomach (Figures 15-8 and 15-13, A) lies in the upper part of the abdominal cavity just under the diaphragm
After food has entered the stomach by passing through the muscular gastroesophageal or cardiac sphincter (SFINK-ter) at the lower end of the esophagus, the digestive process continues.
Sphincters are rings of muscle tissue. The cardiac sphincter keeps food from reentering the esophagus when the stomach contracts the opening in the diaphragm that permits the passage of the esophagus into the abdomen is enlarged. This may permit a bulging of the end of the esophagus and part or even all of the stomach upward through the diaphragm and into the chest. The condition, called a hiatal hernia may result in backward movement or reflux of stomach contents into the lower portion of the esophagus.
The resulting symptoms are referred to as gastroesophageal reflux disease, or GERD
Contraction of the stomach’s muscular walls mixes the food thoroughly with the gastric juice and breaks it down into a semisolid mixture called chyme
Gastric juice contains hydrochloric acid and enzymes that function in the digestive process
Chyme formation is a continuation of the mechanical digestive process that begins in the mouth there are three layers of smooth muscle in the stomach wall.
The muscle fibers that run lengthwise, around, and obliquely make the stomach one of the strongest internal organs—well able to break up food into tiny particles and to mix them thoroughly with gastric juice to form chime
Stomach muscle contractions result in peristalsis (pair-i-STAL-sis), which propels food down the digestive tract
Mucous membrane lines the stomach; it contains thousands of microscopic gastric glands that secrete gastric juice and hydrochloric acid into the stomach.
When the stomach is empty, its lining lies in folds called rugae
The three divisions of the stomach shown in Figure 15-8 are the fundus, body, and pylorus
The fundus is the enlarged portion to the left of and above the opening of the esophagus into the stomach.
The body is the central part of the stomach the pylorus is its lower narrow section, which joins the first part of the small intestine
Partial digestion occurs after food is held in the stomach by the pyloric (pye- LOR-ik) sphincter muscle
The smooth muscle fibers of the sphincter stay contracted most of the time and thereby close off the opening of the pylorus into the small intestine. the upper right border of the stomach is known as the lesser curvature, and the lower left border is called the greater curvature
After food has been mixed in the stomach, chyme begins its passage through the pyloric sphincter into the first part of the small intestine.
Note that the folds of mucosa are covered with villi and that each villus is covered with epithelium, which increases the surface area for absorption of food
The small intestine seems to be misnamed if you look at its length—it is roughly 7 meters (20 feet) long. However, it is noticeably smaller in diameter than the large intestine, so in this respect its name is appropriate
Different names identify different sections of the small intestine. In the order in which food passes through them, they are the duodenum (doo-oh-DEE-num), jejunum (jeh-JOO-num), and ileum (IL-ee-um).
The mucous lining of the small intestine, as with that of the stomach, contains thousands of microscopic glands.
These intestinal glands secrete the intestinal digestive juice. Another structural feature of the lining of the small intestine makes it especially well suited to absorption of food and water; it is not perfectly smooth, as it appears to the naked eye
Instead, the intestinal lining is arranged into multiple circular folds called plicae
These folds are themselves covered with thou- sands of tiny “fi ngers” called villi
Under the microscope, the villi can be seen projecting into the hollow interior of the intestine
Inside each villus lies a rich network of blood capillaries that absorb the products of carbohydrate and protein digestion (sugars and amino acids). each villus in the intestine contains a lymphatic vessel, or lacteal (LAK-tee-al), that absorbs lipid or fat materials from the chyme passing through the small intestine
In addition to the thousands of villi that in- crease surface area in the small intestine, each villus is itself covered by epithelial cells, which have a brush-like border composed of microvilli
Most of the chemical digestion occurs in the fi rst subdivision of the small intestine or duodenum.
The duodenum is C-shaped (Figure 15-10) and curves around the head of the pancreas. The acid chyme enters the duodenum from the stomach
This area is the site of frequent ulceration
The middle third of the duodenum contains the openings of ducts that empty pancreatic digestive juice and bile from the liver into the small intestine. the two openings are called the minor and major duodenal papillae
Occasionally a gallstone blocks the major duodenal papilla, causing symptoms such as severe pain, jaundice, and digestive problems.
Smooth muscle in the wall of the small intestine contracts to produce peristalsis, the wavelike contraction that moves food through the tract.
The liver is so large that it fi lls the entire upper right section of the abdominal cavity and even ex- tends part way into the left side. Because its cells secrete a substance called bile into ducts, the liver is classified as an exocrine gland; in fact, it is the largest gland in the body the hepatic ducts. They drain bile out of the liver, a fact suggested by the name “hepatic,” which comes from the Greek word for liver
Obstruction of the hepatic, or common, bile duct by stone or spasm blocks the exit of bile from the liver, where it is formed, and prevents bile from being ejected into the duodenum. the duct that drains bile into the small intestine (duodenum), the common bile duct. It is formed by the union of the common hepatic duct with the cystic duct.
Chemically, bile contains significant quantities of cholesterol and substances (bile salts) that act as detergents to mechanically break up, or emulsify (eh-MUL-seh-fye), fats.
Because fats form large globules, they must be broken down, or emulsified, into smaller particles to increase the surface area to aid digestion.
In addition to emulsification of fats, bile that is eliminated from the body in the feces serves as a mechanism for excreting cholesterol from the body
Both emulsification of fats and elimination of cholesterol from the body are primary functions of bile.
When chyme containing lipid or fat enters the duodenum, it initiates a mechanism that contracts the gallbladder and forces bile into the small intestine.
Fats in chyme stimulate or “trigger” the secretion of the hormone cholecystokinin (koh-lee- sis-toh-KYE-nin), or CCK, from the intestinal mucosa of the duodenum.
This hormone then stimulates the contraction of the gallbladder, and consequently bile flows into the duodenum.
Between meals, a lot of bile moves up the cystic duct into the gallbladder on the undersurface of the liver. The gallbladder thus concentrates and stores bile produced in the liver.
Visualize a gallstone blocking the common bile duct shown in Figure 15-10. Bile could not then drain into the duodenum
Feces would then appear gray-white because the pigments from bile give feces its characteristic color.
Furthermore, excessive amounts of bile would be absorbed into the blood. A yellowish skin discoloration called jaundice (JAWN-dis) would result
Obstruction of the common hepatic duct also leads to jaundice.
Because bile cannot then drain out of the liver, excessive amounts of it are absorbed. Because bile is not resorbed from the gallbladder, no jaundice occurs if the cystic duct is blocked.
The pancreas lies behind the stomach in the concavity produced by the C shape of the duodenum
. It is an exocrine gland that secretes pancreatic juice into ducts and an endocrine gland that secretes hormones into the blood.
Pancreatic juice is the most important digestive juice. It contains enzymes that digest all three major kinds of foods
It also contains sodium bicarbonate, an alkaline substance that neutralizes the hydrochloric acid in the gastric juice that enters the intestines
Pancreatic juice enters the duodenum of the small intestine at the same place that bile enters the common bile and pancreatic ducts open into the duodenum at the major duodenal papilla.
Between the cells that secrete pancreatic juice into ducts lie clusters of cells that have no contact with any ducts
These are the pancreatic islets (of Langerhans), which secrete the hormones of the pancreas
The large intestine is only about 1.5 meters (5 feet) in length
It forms the lower or terminal portion of the digestive tract
Undigested and unabsorbed food material enters the large intestine after passing through a sphincter-like structure (Figure 15-12) called the ileocecal (il-ee-oh-SEE-kal) valve
The word chyme is no longer appropriate in describing the contents of the large intestine
Chyme, which has the consistency of soup and is found in the small intestine, changes to the consistency of fecal matter as water and salts are reabsorbed during its passage through the small intestine.
During its movement through the large intestine, material that escaped digestion in the small intestine is acted on by bacteria
Bacteria in the large intestine are responsible for the synthesis of vitamin K needed for normal blood clotting and for the production of some of the B-complex vitamins
After they are formed, these vitamins are absorbed from the large intestine and enter the blood.
Although some absorption of water, salts, and vitamins occurs in the large intestine, this segment of the digestive tube is not as well suited for absorption as is the small intestine.
Salts, especially sodium, are absorbed by active transport, and water is moved into the blood by osmosis
No villi are present in the mucosa of the large intestine
Normal passage of material through the large intestine takes about 3 to 5 days
If the rate of passage of material quickens, the consistency of the stools or fecal material becomes more and more fluid, and diarrhea results
If the time of passage through the large intestine is prolonged beyond 5 days, the feces loses volume and becomes more solid because of excessive water absorption.
This reduces stimulation of the bowel emptying reflex, resulting in retention of feces, a condition called constipation
The subdivisions of the large intestine are listed below in the order in which food material or feces passes through them.
1. Cecum
2. Ascending colon
3. Transverse colon
4. Descending colon
5. Sigmoid colon
6. Rectum
7. Anal canal\
The cecum opening itself is about 5 or 6 cm (2 inches) above the beginning of the large intestine. Food residue in the cecum flows upward on the right side of the body in the ascending colon. The hepatic or right colic flexure is the bend be- tween the ascending colon and the transverse colon, which extends across the front of the abdomen from right to left. The splenic or left colic flexure marks he point where the descending colon turns down- ward on the left side of the abdomen. The sigmoid colon is the S-shaped segment that terminates in the rectum. The terminal portion of the rectum is called the anal canal, which ends at the external opening, or anus.
Two sphincter muscles stay contracted to keep the anus closed except during defecation
Smooth or involuntary muscle composes the inner anal sphincter, but striated, or voluntary, muscle composes the outer anal sphincter.
This anatomical fact sometimes becomes highly important from a practical standpoint. For example, often after a person has had a stroke, the voluntary anal sphincter at first becomes paralyzed. This means, of course, that the individual has no control at that time over bowel movements.
The peritoneum (pair-i-toh-NEE-um) is a large, moist, slippery sheet of serous membrane that lines the abdominal cavity and covers the organs located in it, including most of the digestive organs.
The parietal layer of the peritoneum lines the abdominal cavity
The visceral layer of the peritoneum forms the outer, or covering, layer of each abdominal organ.
The small space between the parietal and visceral layers is called the peritoneal space.
It contains just enough peritoneal fluid to keep both layers of the peritoneum moist and able to slide freely against each other during breathing and digestive movements
Organs outside of the peritoneum are said to be retroperitoneal
The vermiform appendix is, as the name implies, a worm- like, tubular structure.
Although it serves no important digestive function in humans, it contains lymphatic tissue and may play a minor role in the immunologic defense mechanisms of the body the appendix is directly attached to the cecum.
The appendix contains a blind, tube-like interior lumen that communicates with the lumen of the large intestine 3 cm (1 inch) below the opening of the ileocecal valve into the cecum
If the mucous lining of the appendix becomes inflamed, the resulting condition is the well-known affliction, appendicitis the appendix is very close to the rectal wall
For patients with suspected appendicitis, a physician often evaluates the appendix by performing a digital rectal examination.
The two most prominent extensions of the peritoneum are the mesentery and the greater omentum.
The mesentery (MEZ-en-tair-ee), an extension between the parietal and visceral layers of the peritoneum, is shaped like a giant, pleated fan
Its smaller edge attaches to the lumbar region of the posterior abdominal wall, and its long, loose outer edge encloses most of the small intestine, anchoring it to the posterior abdominal wall
The greater omentum is a pouch-like extension of the visceral peritoneum from the lower edge of the stomach, part of the duodenum, and the transverse colon
Shaped like a large apron, it hangs down over the intestines, and because spotty deposits of fat give it a lacy appearance, it has been nicknamed the lace apron
It usually envelops a badly inflamed appendix, walling it off from the rest of the abdominal organs.
Barium sulfate solution is referred to as a contrast medium that is radiopaque
This simply means that it does not permit passage of x-rays
Barium shows up in x-ray images as a high contrast white outline of the structure that it fills and is often used in x-ray examinations to help visualize the various hollow segments of the GI tract.
The barium solution has either been swallowed (barium swallow) or introduced into the lower segments of the tract by enema.
The barium swallow is used in upper GI x-ray studies to visualize the esophagus, stomach, and small intestines, the barium enema is used in lower GI x-ray studies to visualize the segments of the colon, rectum, and anal canal.
Digestion, a complex process that occurs in the alimentary canal, consists of physical and chemical changes that prepare food for absorption
Mechanical digestion breaks food into tiny particles, mixes them with digestive juices, moves them along the alimentary canal, and finally eliminates the digestive wastes from the body.
Chewing, or mastication; swallowing, or deglutition (deg-loo-TISH- un); peristalsis (Figure 15-3); and defecation are the main processes of mechanical digestion
Chemical digestion breaks down large, non-absorbable food molecules into smaller, absorbable molecules— molecules that are able to pass through the intestinal mucosa into blood and lymph
Chemical digestion consists of numerous chemical reactions catalyzed by enzymes in saliva, gastric juice, pancreatic juice, and intestinal juice.
Enzymes are specialized protein molecules that act as catalysts they speed up specific chemical reactions without themselves being changed or consumed during the reaction process.
During chemical digestion, certain enzymes very selectively speed up the breakdown of specific nutrient molecules and no others.
Enzymes responsible for speeding up the breakdown of fats, for example, have no effect on carbohydrates or proteins.
The names of many enzymes end with the suffix -ase combined with the word that describes the type of substance involved in the chemical reaction
Lipase, for ex- ample, is a fat-digesting enzyme that acts on lipids (fats) and protease enzymes serve to break down protein nutrients into smaller molecules. the breakdown process is called hydrolysis -an important type of chemical reaction during hydrolysis, enzymes speed up reactions that add water (hydro) to chemically break up or split (lysis) larger molecules into smaller molecules
Very little digestion of carbohydrates (starches and sugars) occurs before food reaches the small intestine
Salivary amylase usually has little time to do its work because so many of us swallow our food so fast.
Gastric juice contains no carbohydrate-digesting enzymes.
But after the food reaches the small intestine, pancreatic and intestinal juice enzymes digest the starches and sugars
A pancreatic enzyme (amy- lase) starts the process by breaking down polysaccharides such as starches into disaccharides (double sugars).
Three intestinal enzymes—maltase, sucrase, and lactase—digest disaccharides by changing them into monosaccharides (simple sugars)
Maltase digests maltose (malt sugar), sucrase digests sucrose (ordinary cane sugar), lactase digests lactose (milk sugar).
The end products of carbohydrate digestion are the monosaccharides; the most abundant is glucose
Protein digestion starts in the stomach.
Pepsin, an enzyme in the gastric juice, causes the giant protein molecules to break up into somewhat simpler compounds
Pepsinogen, a component of gastric juice, is converted into active pepsin enzyme by hydrochloric acid (also in gastric juice)
In the intestine, other enzymes (trypsin in the pancreatic juice and peptidases in the intestinal juice) finish the job of protein digestion
Every protein molecule is made up of many amino acids joined together
When enzymes have split up the large protein molecule into its separate amino ac- ids, protein digestion is completed.
Hence the end product of protein digestion is amino acids
For obvious reasons, the amino acids are also referred to as protein building blocks.
Very little carbohydrate and fat digestion occurs before food reaches the small intestine
Most fats are undigested until after emulsification by bile in the duodenum (that is, fat droplets are broken into very small droplets).
After this takes place, pancreatic lipase splits up the fat molecules into fatty acids and glycerol (glycerin)
The end products of fat digestion, then, are fatty acids and glycerol.
When carbohydrate digestion has been completed, starches (polysaccharides) and double sugars (disaccharides) have been changed mainly to glucose, a simple sugar (mono- saccharide).
The end products of protein digestion, on the other hand, are amino acids.
Fatty acid and glycerol are the end products of fat digestion
After food is digested, it is absorbed; that is, it moves through the mucous membrane lining of the small intestine into the blood and lymph
In other words, food absorption is the process by which molecules of amino acids, glucose, fatty acids, and glycerol go from the inside of the intestines into the circulating fluids of the body
Absorption of foods is just as essential as digestion of foods.
As long as food stays in the intestines, it cannot nourish the millions of cells that compose all other parts of the body.
Their lives depend on the absorption of digested food and its transportation to them by the circulating blood.
Many important minerals, such as sodium, are actively transported through the intestinal mucosa.
Water follows by osmosis
Other nutrients such as monosaccharides and amino acids are also actively transported through the intestinal mucosa and diffuse into the blood of capillaries in the intestinal villi.
Fatty acids and glycerol diffuse into the absorptive cells of the GI tract and then are secreted into the lymphatic vessels or lacteals found in intestinal villi.
The so called “water soluble vitamins” (vitamin C and the B vitamins) are dissolved in water and absorbed primarily from the small intestine
The “fat soluble vitamins” (vitamins A, D, E, and K) are absorbed along with the end products of fat digestion in the small intestine and then pass into the lacteals.
Bacterial action in the colon also produces some vitamin K that is absorbed through the lining of the large intestine.
Structural adaptations of the digestive tube, including folds in the lining mucosa, villi, and microvilli, increase the absorptive surface and the efficiency and speed of absorption and transfer of materials from the intestinal lumen to body fluids.
The plicae (folds) have villi, the villi have microvilli, and even the microvilli have bumps that cannot be seen in the figure. Thus, the absorptive surface area of the small intestine is al- most limitless.