MCAT Biology Review

Chapter 9: The Digestive System

9.4 Absorption and Defecation

Absorption of nutrients primarily occurs in the small intestine, especially in the jejunum and ileum. The large intestine largely absorbs water.


The small intestine consists of three segments: the duodenum, the jejunum, and the ileum. As discussed previously, the duodenum is primarily involved in digestion. The jejunum and ileum are involved in the absorption of nutrients. The small intestine is lined with villi, which are small, fingerlike projections from the epithelial lining, as shown in Figure 9.6. Each villus has many microvilli, drastically increasing the surface area available for absorption. In addition, at the middle of each villus is both a capillary bed for the absorption of water-soluble nutrients and a lacteal, a lymphatic channel that takes up fats for transport into the lymphatic system.

Figure 9.6. Lining of the Small Intestine


Segments of the small intestine: Dow Jones Industrial

·        Duodenum

·        Jejunum

·        Ileum

Simple sugars, such as glucose, fructose, and galactose, and amino acids are absorbed by secondary active transport and facilitated diffusion into the epithelial cells lining the small intestine, as shown in Figure 9.7. Then, these substances move across the epithelial cells into the intestinal capillaries. Blood is constantly passing by the epithelial cells in the capillaries, carrying the carbohydrate and amino acid molecules away from the epithelial cells. This creates a concentration gradient such that the blood always has a lower concentration of monosaccharides and amino acids than inside the epithelial cells. Thus, simple carbohydrates and amino acids diffuse from the epithelial cells into the capillaries. The absorbed molecules then go to the liver via the hepatic portal circulation.

Figure 9.7. Absorprtion of Carbohydrates and Amino Acids in the Small Intestine

What about fats? Small fatty acids will follow the same process as carbohydrates and amino acids by diffusing directly into the intestinal capillaries. These fatty acids do not require transporters because they are nonpolar, so they can easily traverse the cellular membrane. Larger fats, glycerol, and cholesterol move separately into the intestinal cells but then reform into triglycerides, as shown in Figure 9.8. The triglycerides and esterified cholesterol molecules are packaged into insoluble chylomicrons. Rather than entering the bloodstream, chylomicrons enter the lymphatic circulation through lacteals, small vessels that form the beginning of the lymphatic system. These lacteals converge and enter the venous circulation through the thoracic duct in the base of the neck, which empties into the left subclavian vein.

Figure 9.8. Absorption of Lipids

Vitamins are also absorbed in the small intestine. Vitamins can be categorized as either fat-soluble or water-soluble. Because there are only four fat-soluble vitamins (A, D, E, and K), these can be easily memorized. All other vitamins (B complex and C) are water-soluble. Fat-soluble vitamins dissolve directly into chylomicrons to enter the body. Failure to digest and absorb fat properly, which can be due to pathologies in the liver, gallbladder, pancreas, or small intestine, may lead to deficiencies of fat-soluble vitamins. The water-soluble vitamins are absorbed, along with water, amino acids, and carbohydrates, across the endothelial cells of the small intestine, passing directly into the plasma.

In addition to fats, carbohydrates, amino acids, and vitamins, the small intestine also absorbs water. Much of the water in chyme is actually the result of secretions. The average person may consume up to two liters of fluid per day, but secretions into the upper gastrointestinal tract may total up to seven liters of fluid per day. In order to maintain proper fluid levels within the body, much of this fluid must be reabsorbed by osmosis. As solutes are absorbed into the bloodstream, water is drawn with it, eventually reaching the capillaries. Water passes not only transcellularly(across the cell membrane), but also paracellularly (squeezing between cells) to reach the blood.


The final part of the gastrointestinal tract is the large intestine. It is primarily involved in water absorption. The large intestine has a larger diameter but shorter length than the small intestine. It is divided into three major sections: the cecum, the colon, and the rectum. The cecum is simply an outpocketing that accepts fluid exiting the small intestine through the ileocecal valve and is the site of the appendix. The appendix is a small fingerlike projection that was once thought to be vestigial, although recent evidence suggests that it may have a role in warding off certain bacterial infections and repopulating the large intestine with normal flora after episodes of diarrhea. Inflammation of the appendix (appendicitis) is a surgical emergency; in fact, it is the most common reason for an unscheduled surgery in the United States.

The colon itself is divided into the ascending, transverse, descending, and sigmoid colons. Its main function is to absorb water and salts (such as sodium chloride) from the undigested material left over from the small intestine. The small intestine actually absorbs much more water than the colon, so the colon primarily concentrates the remaining material to form feces. Too little or too much water absorption can cause diarrhea or constipation, respectively.


Note the similarity between the muscles that control voiding of urine and feces. In both cases, there is an internal sphincter under autonomic control (internal urethral sphincter and internal anal sphincter) and an external sphincter under somatic control (external urethral sphincter and external anal sphincter). Urination is discussed in Chapter 10 of MCAT Biology Review.

Finally, the rectum serves as a storage site for feces, which consists of indigestible material, water, bacteria (E. coli and others), and certain digestive secretions that aren’t reabsorbed (enzymes and some bile). The anus is the opening through which wastes are eliminated and consists of two sphincters: the internal and external anal sphincters. The external sphincter is under voluntary control (somatic), but the internal sphincter is under involuntary control (autonomic).

The large intestine—and even the small intestine—is home to many different species of bacteria. In fact, 30 percent of the dry matter in stool consists of bacteria. Most of these bacteria are anaerobes, but the cecum is also home to many aerobic bacteria. This relationship is symbiotic: the bacteria are provided with a steady source of food, and the byproducts produced by the bacteria are beneficial to humans. For example, bacteria in the gut produce vitamin K, which is essential for the production of clotting factors, and biotin (vitamin B7), which is a coenzyme for many metabolic enzymes.

MCAT Concept Check 9.4:

Before you move on, assess your understanding of the material with these questions.

1.    What are the two circulatory vessels in a villus? What biomolecules are absorbed into each?



2.    What are the four fat-soluble vitamins?

3.    What are the three sections of the small intestine, in order? What are the three sections of the large intestine, in order?

·        Small intestine: , , 

·        Large intestine: , , 

4.    Vibrio cholera causes a severe infection in the intestines, leading to massive volumes of watery diarrhea—up to 20 liters per day. Given these symptoms, does cholera likely impact the small intestine or the large intestine?