Other Digestive Physiology - MCAT Biological and Biochemical Foundations of Living Systems

The correct answer shows the proper order, and you should pay special attention to the names of the two sphincters. The cardiac sphincter is by the heart, and comes before the stomach. The pyloric sphincter is at the bottom of the belly and comes after the stomach. Food does not pass through the larynx, which is used in breathing and speaking.

Calcium plays an important role in many physiological functions, including signal transduction, muscle contraction, and bone formation. Of all of the processes listed in the answer choices, calcium is least directly related to biotin and vitamin K production. Biotin and vitamin K are synthesized by gut flora.

The majority of food absorption occurs in the later two-thirds of the small intestine, the jejunum and the ileum. Water and water-soluble vitamins, however, are absorbed in the large intestine.

The digestive system begins in the mouth with fragmentation. Mucous helps roll the food into a bolus and salivary amylase begins the process of chemical breakdown.

Lipid digestion begins to occur in the mouth because of the enzyme lipase. Saliva in the mouth incorporates both lipase, to begin lipid digestion, and amylase, to being carbohydrate digestion. Lipids are also broken down later in the digestive pathway by lipase in the small intestine. Protein digestion does not begin until food enters the stomach.

The colon extracts water and salts while forming feces. The colon also absorbs some nutrients that made it through the small intestine without being absorbed, but its main job is water and salt absorbtion.

Fat-soluble vitamins include A, D, E, and K. Water-soluble vitamins include B and C.

Triglycerides and cholesterol do not go straight into the venous circulation. These fats are made into structures called chylomicrons and are processed through lacteals into the lymphatic system.

While carbohydrates and proteins are digested and absorbed directly into the bloodstream, dietary fats are packaged into chylomicrons and moved into the lacteal. The lymphatic system then transports the dietary fats to the large veins of the body via the thoracic duct. The majority of fats are absorbed by this process, but a smaller minority of shorter fatty acids are able to float freely into the bloodstream.

The esophagus contains both striated (skeletal) muscle and smooth muscle. The two types of muscle, therefore, allow two distinct functions. Skeletal muscle, in the upper part of the esophagus, allows for voluntary control of swallowing; thus, when a bolus of food has been broken down by chewing, a person can consciously make a decision to begin the swallowing process. Once in the esophagus and past the striated muscle of the upper esophagus, the muscular control is smooth muscle. Smooth muscle is autonomically regulated, meaning that no voluntary control is needed to continue swallowing. Once the food enters the esophagus, the smooth muscle will propel it downwards towards the stomach. The mix of striated and smooth muscle allows for voluntary control of beginning to swallow, but automatic control for swallow completion.

We are told that bile salts can interact with both lipids and water in order to aid in the digestion and absorption of fats ingested in meals; thus, the bile salts are both hydrophobic (interact with the lipids) and hydrophilic (interact with water and polar molecules), giving them the definition of amphiphilic. An amphiphobic molecule would interact with neither polar, nor nonpolar molecules.

Vitamins A, D, E, and K (ADEK) are fat soluble vitamins, and require the ingestion and absorption of fat in order to maintain adequate stores of the vitamins in the body. Any problems with bile salt function or absorption of bile salt and fat miscelles in the small intestine can lead to a deficiency in vitamins A, D, E, and K. All other vitamins can be absorbed without the help of fat.

The parasympathetic nervous system, controlled in the gastrointestinal system by the vagus and pubic nerves, serves to decrease the rate of digestion. Rhythmic contractions of the small intestine and large intestine are slowed under parasympathetic control, leading to decreased digestion and absorption of food contents. This represents the "rest and digest" portion of the parasympathetic nervous system, in contrast to the "fight or flight" portion of the sympathetic nervous system.

The parasympathetic nervous system slows digestion, allowing for more absorption of nutrients. The sympathetic response increases the rate of flow through the digestive tract, limiting the amount of absorption that can occur.

The primary purpose of the colon is to reabsorb water and salt (NaCl) in order to dehydrate the stool into a compact form for defecation. On average, the colon reabsorbs 1.5L of water per day and moves excreted sodium from the stool back into systemic circulation for further use. Inhibition of sodium reabsorption by the cholera toxin is what leads to severe dehydration and diarrhea seen in the disease.

This question asks us to understand osmolarity and osmosis. Osmosis is the property of water whereby it moves from a high concentration of water to a low concentration of water, while osmolarity refers to the tendency of solutes to affect the draw of water from one area to another. In more practical terms, this means that water moves from areas of low solute concentration to areas of high solute concentration. Fiber is not digestible by humans, thus it cannot be absorbed by the gastrointestinal enterocytes. Additionally, because fiber cannot be absorbed, it cannot affect the absorption of other compounds in the gut to a great extent. Fiber does, however, increase the number of solute particles in the gut, and so draws water from the circulation and tissues via osmosis. Increasing the flow of water into the lumen will add more water to stool and help relieve constipation.

The interstitial cells of Cajal link the parasympathetic nervous system with the smooth muscle of the gastrointestinal system. They are responsible for the rhythmic contractions of the gastrointestinal system that propel digested food, and are found in the stomach, small intestine, and large intestine. Interference with their "pacemaker-like" function can lead to constipation and gastroparesis (failure of the gastrointestinal system to contract).

Absorption of fats, iron, and proteins occurs in the small intestine with the aid of microvilli.

Conjugation of the bile salts, made by the liver and stored by the gallbladder, enhances the ability of fat micelles to dissolve in water (hydrophilicity), prevents the bile salts from being trapped in the gastrointestinal lumen, and prevents pancreatic enzymes, like lipase and amylase, from degrading the bile salts. In phase II metabolism in the liver, conjugation increases the ability of a substance to be dissolved in water, which promotes aqueous solubility and excretion.

The gallbladder stores and concentrates bile from the liver. Bile is used to emulsify fats, assisting digestion in the small intestine. A person whose gallbladder has been removed should restrict fat consumption, as they will have a limited ability to digest fatty compounds. Triglycerides are a form of fat.

The colon consists of a few main parts, including the ascending colon. The ascending colon is the first region of the large intestine, connecting it to the small intestine. The ascending colon transitions into the transverse colon, which then becomes the descending colon. The descending colon transitions into the muscular sigmoidal colon, which terminates in the rectum.