How does the digestive system work with the excretory system

Learn key MCAT concepts about the digestive and excretory systems, plus practice questions and answers

Part 1: Introduction to the digestive and excretory systems

The body requires energy to function and maintain homeostasis. How does our body absorb the nutrients it needs and excrete the waste it produces? 

The digestive and excretory systems work together to break down the food we eat into absorbable nutrients and expel waste. A significant amount of biology questions on the MCAT directly cover content described in this guide—including the structure and function of the kidney. Having a thorough understanding of these systems is a definite advantage now and in your future medical career.

Throughout this guide, you will find several bolded terms that are important to understand and recall. At the end of this guide, you will also find several MCAT-style questions for you to test your knowledge. 

Let’s jump right in!

Part 2: Digestive system

a) Gastrointestinal anatomy

The digestive tract is a long, tube-like structure that runs through the entire torso and abdomen of the human body. Food that enters the digestive tract at the mouth undergoes a series of digestive steps as useful nutrients are absorbed into the bloodstream, and any unabsorbed nutrients exit as waste.

Believe it or not, digestion starts at the mouth. The mouth is responsible for both the mechanical and chemical digestion of food. Mastication, or chewing, is the mouth’s primary method of mechanical digestion. Mechanical digestion results in the breaking down of large particles of food into smaller pieces, increasing the total surface area. The increased surface area aids in chemical digestion performed by salivary enzymes. The salivary glands of the mouth produce enzymes, known as salivary amylase and salivary lipase, which begin to break down the chemical bonds of sugars and lipids in the food. As food doesn’t stay for very long in the mouth, the degree of digestion is quite limited but will continue further along the digestive tract.

Figure: Anatomy of the digestive tract.

In preparation for swallowing, the tongue rolls food into a ball-shaped mass known as a bolus. The bolus is then moved into the oropharynx, where muscles work to push the bolus out of the mouth.

During swallowing, food passes through the pharynx, a tube-shaped structure that is shared between the respiratory and digestive systems. To prevent food from entering the lungs, a flap-like structure known as the epiglottis automatically closes the opening of the larynx. As a result, food can safely travel down the pharynx and into the esophagus.

The esophagus connects the mouth to the stomach and has two sphincters: the upper and lower esophageal sphincters. (Recall that sphincters are simply ring-shaped muscles that can open and close by contracting.) These sphincters regulate the entrance of food into the esophagus and the stomach, respectively. 

Perhaps surprisingly, humans lack voluntary control of the esophagus. Only the upper third of the esophagus is composed of skeletal muscle, which allows us to swallow our food. Our food moves down the rest of the esophagus through wavelike movement created by the smooth muscle occupying the rest of the tube. This movement is known as peristalsis. The movement is assisted by the presence of saliva that has been mixed with the bolus, which serves an additional function as a lubricant.

As our food moves down the esophagus, it eventually reaches the stomach. The stomach is where much of the chemical digestion of food occurs. Several glands in the stomach lining secrete acid chemicals and other enzymes to initiate chemical digestion. 

• Hydrogen ions are secreted by parietal cells within the stomach.

• Pepsinogen is secreted by chief cells within the stomach. Pepsinogen is a zymogen, an inactive form of the enzyme pepsin that has to be cleaved with hydrogen ions to become active pepsin.

• Pepsin, the activated form of pepsinogen, is responsible for breaking proteins down into amino acids. 

• Gastrin is secreted by G-cells located within pyloric glands. Gastrin stimulates the secretion of hydrochloric acid (HCl). The stomach contains a large amount of hydrochloric acid, producing an acidic environment. 

• Mucus is secreted by goblet cells. This mucus protects the lining of the stomach and prevents the highly acidic environment from damaging the stomach itself.

Chief cells and parietal cells are found together in gastric glands. Assisted by the mechanical churning of muscles surrounding the stomach, the enzymes and acid secreted in the stomach digest food into chyme. Chyme is a thick, heterogeneous mixture of food and digestive enzymes that undergoes further digestion and absorption in the small intestine.

The resulting chyme travels to the small intestine. The small intestine is responsible for the digestion and absorption of nutrients. The small intestine consists of three different sequential sections: the duodenum, jejunum, and ileum. Alkaline juices produced by the pancreas, an accessory organ, are secreted into the duodenum and neutralize the highly acidic contents arriving from the stomach. Chyme initially travels through the duodenum, where the presence of chyme in the duodenum initiates the release of many enzymes from several accessory organs that are discussed later in this guide. These enzymes perform the bulk of chemical digestion as the food passes through the digestive tract. 

After passing through the duodenum, the chyme makes its way to the jejunum and ileum for absorption. Characteristic of these regions are microvilli, which are small, finger-like structures that increase the surface area of the intestine to promote absorption. Here, the nutrients that are present in the digested mixture are absorbed into epithelial cells and transported into the capillaries for use. Most small molecules that aren’t hydrophobic, such as water, simple sugars, most vitamins, and amino acids, are simply absorbed through facilitated diffusion and secondary active transport into epithelial cells and then into capillaries.

For molecules that are hydrophobic, including the fat-soluble vitamins K, A, D, and E, special structures called lacteals exist to allow these molecules into the lymphatic system, which eventually empties into the left subclavian vein. For more information on the absorption and digestion of fats, be sure to refer to our guide on lipid and amino acid metabolism.

Finally, after passing through the small intestine, the mixture makes its way to the large intestine. The majority of nutrients—such as amino acids, carbohydrates, and vitamins—have been absorbed in the small intestine.

The large intestine is responsible for absorbing water and leftover salts. It also contains gut flora, colonies of beneficial bacteria that reside within the body and assist in the production and absorption of nutrients such as vitamin K. After passing through the large intestine, the only material that remains after absorption is waste, which is consolidated into fecal matter. 

Fecal matter is stored in the last segment of the large intestine. When the body is ready to eliminate waste, it is pushed through the rectum and exits the body through the anal sphincters.

The movement and peristalsis of material through the digestive tract is largely governed by the enteric nervous system: a subdivision of the autonomic nervous system. The nerve endings within the enteric nervous system enervate sections of the smooth muscle lining the digestive tract, thus governing the rate of digestion as food passes through the body.

b) Accessory organs

Although these accessory organs do not form any portion of the tubelike digestive tract, the liver, gallbladder, and pancreas are important accessory organs that aid in chemical digestion. 

Located in the upper-right quadrant of the abdomen, the liver plays many important roles in aiding digestion. One of the most important functions the liver serves is producing bile. Bile is a mixture composed of different salts, cholesterol, and bilirubin pigment. 

In addition to bile, the liver produces the protein albumin, which helps transport various hormones in our bloodstream. The liver is also responsible for processing and filtering toxins from the blood and for storing or creating energy stores. This is done by removing excess nutrients to store them as glycogen and triacylglycerols or providing more energy through the metabolic processes of gluconeogenesis and glycogenolysis. (For more information on these processes, be sure to refer to our guide on carbohydrate metabolism.)

Beneath the liver lies the gallbladder. As the site of bile storage, the gallbladder primarily functions to excrete bile into the small intestine when signaled. The peptide hormone cholecystokinin signals the gallbladder to expel its bile. Bile helps to emulsify fats. Note that this emulsification is not chemical digestion. Instead, it assists the mixing of oily, nonpolar fats with the aqueous, polar environment of the digestive juices.

The pancreas secretes enzymes that digest carbohydrates, proteins, and lipids. Specifically, the pancreas secretes pancreatic amylase, pancreatic peptidases, and pancreatic lipases, which enter the duodenum via a duct system in the pancreas. The production and secretion of these enzymes are promoted by cells known as acinar cells. 

Each of these accessory organs is considered to be exocrine in nature. In contrast to endocrine organs, which secrete substances to the interior of the body, exocrine glands secrete substances to the exterior of the body. Sweat glands and sebaceous glands are considered to be exocrine as they secrete substances to the skin. The liver, gallbladder, and pancreas are considered to be exocrine as they secrete substances to the digestive tract, which interacts with elements of the outside world through the food we eat.

The pancreas is unique in that it has both exocrine and endocrine functions. In addition to secreting digestive juices, the pancreas produces insulin which controls blood sugar levels. Insulin production is a small part of what the pancreas actually does. For more information on the endocrine function of the pancreas, be sure to refer to our guide on the endocrine system.

c) Enzymatic digestion and absorption

Enzymes play a large role in digestion. In the stomach, the enzyme pepsin helps break down amino acid bonds and increase the surface area of food. To complete enzymatic digestion, more enzymes are released in the duodenum with the help of accessory organs. We’ll briefly cover these enzymes.

Recall that the duodenum is filled with enzymes that aid in the digestion of chyme. Brush-border enzymes are present to help break down chains of carbohydrates into monomers the body can absorb. Some examples of brush-border enzymes are disaccharidases, such as maltase, lactase, and sucrase. These enzymes break down the sugars maltose, lactose, and sucrose into monomers. Along with disaccharidases, peptidases exist to break down proteins. Dipeptidases are responsible for breaking down dipeptides, while aminopeptidases remove the N-terminal of peptide chains. 

Arguably, the most important enzyme in the duodenum is enteropeptidase. Enteropeptidase is a peptidase that cleaves the inactive zymogen, trypsinogen, into trypsin. Trypsin is crucial in activating pancreatic enzymes that are released into the duodenum.

Aside from enzymes, there are two important hormones functioning in the duodenum: secretin and cholecystokinin. Both secretin and cholecystokinin are secreted in response to chyme. These hormones help release the bile and enzymes contained in the gallbladder and pancreas. Without these hormones, enzymes such as pancreatic amylase (responsible for digesting polysaccharides into disaccharides), pancreatic peptidases (trypsin, chymotrypsin, and carboxypeptidases A and B, which are all responsible for breaking down proteins), and pancreatic lipase (responsible for breaking down fats), would not be secreted. 

Part 3: Excretory system

a) Urine production

How is urine produced by our bodies? Urine production occurs through three major steps in the kidney: filtration, secretion, and absorption. These steps all take place in a structure called the nephron, a single working unit of a kidney. There may be as many as 1 million nephrons in each of your kidneys!

Filtration is the first step in urine production. As capillaries meet the nephron, part of the fluid is passed through to the first portion of the nephron, a structure known as Bowman’s capsule. The movement of the fluid is dictated by the competing hydrostatic and oncotic pressures known as Starling’s forces. The fluid that passes through Bowman’s capsule comes from our blood and contains a mix of molecules, including water and nutrients, which must be reclaimed by the body, and soluble nitrogenous waste products such as urea.

Filtration provides the starting fluid for urine production. The amount of fluid filtered depends on our body’s needs at the time of filtration. If we are well-hydrated more fluid will be filtered, but if less water is available in the bloodstream, the nephrons may encounter difficulty in maintaining homeostasis.

Secretion is the next step in urine production. The function of urine is to remove waste and excess nutrients the body doesn’t need, such as protein, sugars, and urea. Our kidneys are able to secrete the excess nutrients and waste our bodies don’t need directly into the nephrons where the fluid that will become urine is passing through. 

Absorption is the final step in urine production. Nutrients, such as glucose and vitamins, or water that were secreted or filtered into the urine, are reabsorbed by the kidney to maintain homeostasis. If our bodies possess more than enough nutrients, absorption may not be as necessary. However, absorption becomes vital in malnourished or starvation-state conditions. 

The end product of these three stages is urine, a collection of filtered liquid that contains waste products and excess nutrients. In reality, urine production is much more complicated and depends heavily on the structure of the nephron itself.

b) The nephron

The function of the nephron is highly dependent on its structure. At first glance, the structure of the nephron might be overwhelming. However, you will soon learn that each segment performs highly related functions. 

Figure: Anatomy of the nephron

The nephron begins with Bowman’s capsule, a cuplike structure where fluid filtrate is collected from the glomerulus (the collection of blood vessels from which fluid is displaced). The movement of fluid into Bowman’s capsule is dependent on Starling’s forces, or competing hydrostatic and oncotic pressures. The oncotic pressure of blood in the glomerulus is much higher than the Bowman’s capsule, causing fluid to want to move into the vessels. However, the hydrostatic pressure of fluid in the glomerulus is higher than the opposing hydrostatic pressure of Bowman’s capsule. Because the hydrostatic pressure is much higher than the opposing oncotic pressure, fluid moves into Bowman’s capsule.

From Bowman’s capsule, the fluid travels to the proximal convoluted tubule. The proximal convoluted tubule is the first site of absorption of nutrients, such as glucose, salts, vitamins, amino acids, and water, that the body wants to retain. Urea, hydrogen ions, and potassium ions continue to pass through the tubule.  

After passing through the proximal convoluted tubule, the filtrate moves through the loop of Henle. The loop of Henle has two distinct sections: a descending and ascending limb. The descending loop of Henle is known to be permeable to water absorption and impermeable to ion absorption. In fact, water absorption increases as the loop descends deeper into the kidney and the tissue becomes hypertonic. The ascending loop of Henle is quite different from the descending loop. Instead of being permeable to water, the ascending loop is only permeable to salts. The ascending loop provides another opportunity for the body to absorb any salts it might need.

At the loop of Henle, water is absorbed into the vasa recta, a series of capillaries intertwined with the hairpin-like loop. This absorption occurs at a surprisingly efficient rate thanks to the countercurrent exchange system. The countercurrent exchange system relies on the opposing concentration gradients through the loop of Henle and blood in the vasa recta, which allows water to leave the descending loop of Henle at a higher rate due to the increasing concentrations of ions in the vasa recta. The opposing directions of flow maintain a hypertonic state near the descending limb, allowing efficient water absorption by the kidney. 

After the loop of Henle, the increasingly concentrated filtrate passes through the distal convoluted tubule. The distal convoluted tube is another site of sodium absorption. As sodium is absorbed, water can follow, reducing the output of urine. The sodium permeability of the distal convoluted tube is under the control of the hormone aldosterone. Aldosterone and another hormone, antidiuretic hormone, also control water absorption in the collecting duct. The collecting duct is the nephron’s last opportunity to absorb water. The remaining filtrate at the end of the nephron is concentrated urine, full of waste and excess nutrients that need to be excreted. 

The filtrate from the collecting ducts of multiple nephrons will merge together to form the ureters, which carry urine from both kidneys into the bladder. Urine is stored in the bladder until it can be successfully eliminated through the urethra.

The vast majority of the length of each nephron is housed within the medulla, or inner portion, of the kidney. The medulla of each kidney is surrounded by the cortex, which houses the glomerulus of each nephron. The outermost layer of the kidney is known as the renal capsule, a thick and fibrous covering that protects the organ from traumatic damage.

c) Endocrine control

Maintaining homeostasis within the human body is the result of many complex interactions between body systems. Thus, we’d expect that other body systems affect and rely on the excretory system. One such system is the endocrine system. Two hormones, aldosterone and antidiuretic hormone, play an especially important role in balancing water and ion absorption. 

Aldosterone is a steroid hormone secreted by the adrenal cortex. The renin-angiotensin pathway functions to stimulate aldosterone release. The renin-angiotensin-aldosterone system, or RAAS, responds to decreased blood pressure. 

• Decreased blood pressure stimulates the kidneys to release renin from juxtaglomerular cells. Renin is an enzyme that cleaves angiotensinogen into angiotensin I. 

• Angiotensin I is converted into angiotensin II. Angiotensin II is the key molecule that stimulates aldosterone release. 

• Aldosterone exerts its effects on the distal convoluted tube and collecting duct to increase their sodium absorption and subsequent water absorption, as well as potassium and hydrogen ion excretion. 

• Aldosterone indirectly increases water absorption, increasing the amount of water that is present in the bloodstream. Raising this blood volume subsequently raises blood pressure. 

Antidiuretic hormone (ADH) is similar, but its effects and activation follow a much simpler mechanism. Antidiuretic hormone is a peptide hormone secreted by the posterior pituitary gland in response to high blood osmolarity. Antidiuretic hormone signals the body to absorb more water to reduce blood osmolarity. Its effects are targeted at the collecting duct. As a result, the collecting duct becomes more permeable to water, and more water is reabsorbed from the urine.

The kidneys also work to regulate the pH of the blood.  The pH of blood must be regulated. If blood is too acidic or basic, it has negative consequences on our bodies. Recall that the respiratory system performs this function through the bicarbonate buffer system:

Figure: The bicarbonate buffer system.

The excretory system contributes to this control by dictating what molecules are excreted or absorbed by the body. If the pH of blood is too low, the excretory system reabsorbs bicarbonate and excretes hydrogen ions. Similarly, if the pH of blood is too high, the excretory system reabsorbs hydrogen ions while excreting bicarbonate. Because the excretory system can control the levels of these two molecules, it plays a unique role in maintaining blood homeostasis.

As a result, the kidneys perform an important function in maintaining the homeostatic equilibrium of the entire body. By selectively filtering or retaining molecules in the body, the kidneys also perform osmoregulation, the maintenance of proper electrolyte concentrations within the body. Osmoregulation is achieved through the selective reabsorption of the primary solute, water, along with retention or excretion of solutes such as amino acids, glucose, and other salt ions.

Acknowledgements: Nandan Patel

Part 4: High-yield terms

Mechanical digestion: results in the breaking down of large particles of food into smaller pieces with no disruption to chemical bonds

Epiglottis: a flaplike structure that closes the opening of the larynx when swallowing

Peristalsis: wavelike movements created by smooth muscles that assist in movement

Chyme: a thick, heterogeneous mixture of food and digestive enzymes that leaves the stomach and undergoes further digestion and absorption in the small intestine

Microvilli: small, finger-like structures that increase the surface area of the intestine to promote absorption

Fat-soluble vitamins: vitamins K, A, D, and E

Lacteals: blind endings of the lymph system that extend into the small intestine and absorb hydrophobic nutrients

Bile: mixture composed of different salts, cholesterol, and bilirubin pigment that serves to emulsify fats

Gallbladder: an accessory organ that primarily functions to store and excrete bile into the small intestine 

Acinar cells: cell type in the duodenum that secretes pancreatic amylase, pancreatic peptidases, and pancreatic lipases

Nephron: a single working unit of a kidney

Countercurrent exchange system: opposing gradients of water and ion concentration created by the loop of Henle and vasa recta, which creates an efficient system of water and ion reabsorption

Aldosterone: a steroid hormone secreted by the adrenal cortex in response to low blood pressure

Antidiuretic hormone (ADH): a peptide hormone secreted by the posterior pituitary gland in response to high blood osmolarity

Part 5: Passage-based questions and answers

Doctors are examining a patient who has come in complaining of fatigue, decreased urination, and nausea. A comprehensive physical exam shows no sign of physical trauma or illness that could be causing the patient’s symptoms. A blood panel of the patient shows slightly elevated levels of potassium and hydrogen ions. The patient insists that they’ve been eating sufficiently the past couple of days while drinking 12 glasses of water each day. A urine sample taken from the patient appears to be dark yellow. 

Suspecting an endocrine malfunction, doctors order several diagnostic exams. The results show that the patient’s adrenal cortex appears to be secreting abnormally high levels of mineralocorticoid hormones. 

Question 1: What is one potential consequence of an elevated concentration of hydrogen ions in the blood?

A) Decreased blood pressure

B) Increased blood pressure

C) Increased blood pH

D) Decreased blood pH

Question 2: Why might the patient’s urine be dark yellow?

A) The patient ingested yellow pigment.

B) The patient is well hydrated.

C) The patient’s body is absorbing water, concentrating the urine.

D) The patient’s body is excreting ions to preserve water.

Question 3: Which part of the nephron is most directly affected by the endocrine dysfunction?

A) Bowman’s capsule, because the adrenal cortex secretes antidiuretic hormone to promote liquid filtration

B) The loop of Henle, because the adrenal cortex secretes antidiuretic hormone to promote liquid filtration

C) The distal convoluted tube, because the adrenal cortex secretes aldosterone to promote water absorption

D) The collecting tube, because the adrenal cortex secretes antidiuretic hormone to promote water absorption

Question 4: Which of the following would also be observed due to malfunction of the adrenal glands?

A) Decreased vision

B) Decreased respiratory rate

C) Increased parasympathetic nervous system activity

D) Increased sympathetic nervous system activity

Question 5: What is one viable course of treatment for this patient?

A) Provide the patient with more fluids

B) Administer an antibody to bind to the hormone the adrenal cortex secretes

C) Administer antidiuretic hormone

D) The patient cannot be treated

Answer key for passage-based questions

1. Answer choice D is correct. Increased levels of hydrogen ions in the blood lead to an increasingly acidic environment, which leads to a decrease in blood pH (choice D is correct). An increasingly basic environment leads to an increase in blood pH (choice C is incorrect). The concentration of hydrogen ions in the blood should not directly affect blood pressure (choices A and B are incorrect). 

2. Answer choice C is correct. Dark yellow urine is a sign that the urine is concentrated due to high amounts of water reabsorption in the kidneys (choice C is correct). The passage states that the patient has been drinking copious amounts of water (choice B is incorrect). There is no evidence that the yellow color is due to dye ingestion (choice A is incorrect). Water tends to “follow” the flow of ions. If ions are excreted, water will follow, preventing the body from preserving it (choice D is incorrect).

3. Answer choice C is correct. Recall that the adrenal cortex secretes aldosterone, which is considered to be a mineralocorticoid. Aldosterone acts on the distal convoluted tube and collecting tube to promote water absorption (choice C is correct). Answer choices A and B can be ruled out. The adrenal cortex secretes aldosterone—not antidiuretic hormone, which is produced by the posterior pituitary gland (choice D is incorrect).

4. Answer choice D is correct. In addition to aldosterone, the adrenal glands are responsible for secreting norepinephrine and epinephrine. Both of these hormones are involved in the fight-or-flight response, or stimulation of the sympathetic nervous system (choice D is correct). 

5. Answer choice B is correct. It appears that the patient’s kidneys continually absorb water due to increased aldosterone secretion by the adrenal cortex. One solution to this dysfunction is to inhibit aldosterone from binding and promote water retention. The best way to do that is to neutralize aldosterone by having an antibody prevent it from binding (choice B is correct). Providing the patient with more fluids or administering antidiuretic hormone would increase water retention in the patient without addressing the root cause of water retention (choices A and C are incorrect).

Part 6: Standalone questions and answers

Question 1: Which of the following organs produces bile?

A) Liver

B) Stomach

C) Pancreas

D) Gallbladder

Question 2: Where and how does mechanical digestion occur, and why is it important?

A) Stomach via pepsin; to break down food into nutrients that can be absorbed

B) Mouth via mastication; increases the surface area of food for digestion

C) Esophagus via peristalsis; prepares food for chemical digestion

D) Small Intestine via enzyme digestion; increases the surface area of food for digestion

Question 3: Which of the following is not a primary function of the liver?

A) Filtering toxins such as ammonia

B) Glycogenolysis and gluconeogenesis

C) Removing urea from the bloodstream

D) Albumin production

Question 4: Doctors are studying a patient who shows signs of incomplete digestion. Tests show that many of the carbohydrates, proteins, and lipids in the patient’s chyme are not digested after passing through the duodenum. Which of the following could be a cause of the problem?

A) Incomplete mechanical digestion

B) Pancreatic dysfunction in insulin production

C) Overdigestion by pepsin

D) Failure of enteropeptidase secretion

Question 5: How does the absorption of vitamins and minerals vary?

A) All types of nutrients are absorbed into the bloodstream

B) All types of nutrients are absorbed into the lymphatic system

C) Vitamin K is absorbed through lacteals while vitamin C is absorbed through microvilli

D) Vitamin B is absorbed through lacteals while vitamin E is absorbed through microvilli

Answer key for standalone questions

1. Answer choice A is correct. The liver is an accessory organ to the digestive tract that produces bile (choice A is correct). The gallbladder is a small pouchlike structure that is responsible for storing and secreting bile but does not significantly contribute to its production (choice D is incorrect). The stomach and pancreas release a different set of enzymes that contribute to chemical digestion (choices B and C are incorrect). 

2. Answer choice B is correct. Mechanical digestion primarily occurs in the mouth (choice B is correct). A small amount of mechanical digestion occurs in the stomach due to churning; however, the stomach’s primary role is to contribute gastric juices and create semifluid chyme (choice A is incorrect).

3. Answer choice C is correct. The liver plays many important roles in the body. The liver filters many toxins that are in the bloodstream (choice A is incorrect). However, the nephrons are responsible for filtering urea from the body (choice C is correct). The liver is also responsible for maintaining nutrients through glycogenolysis and gluconeogenesis, as well as producing albumin (choices B and D are incorrect).

4. Answer choice D is correct. Complete digestion of carbohydrates, proteins, and lipids in chyme is reliant on pancreatic enzymes (choice D is correct). The pancreas is responsible for insulin production, but insulin is not involved in extracellular nutrient digestion (choice B is incorrect). Pancreatic enzyme secretion and activation in the duodenum are dependent on trypsin activity. Trypsin is activated by enteropeptidase. Thus, a failure of enteropeptidase secretion would prevent pancreatic enzymes from functioning, creating the described symptoms.

5. Answer choice C is correct. Hydrophilic nutrients can easily pass through into the bloodstream, while hydrophobic nutrients must take a special path. The fat-soluble vitamins K, A, D, and E must be absorbed through the lacteals and into the lymphatic system (choice C is correct). All other vitamins, including vitamins C and B, are hydrophilic and can be absorbed directly into the bloodstream through microvilli (choice D is incorrect).