Kidney and Excretory System

Question 1

What are the two main structural components of the kidney?

Answer 1

1. cortex
2. medulla

The cortex, or outer layer of the kidney, contains the proximal and distal convoluted tubules of the nephrons. The medulla, or inner region, contains the loops of Henle.

Question 2

List four primary functions of the kidneys.

Answer 2

• Regulation of blood pressure
• Regulation of blood osmolarity
• Regulation of blood pH
• Excretion of soluble wastes

Question 3

What is filtration?

Answer 3

The process by which fluid, molecules, and ions leave the blood. Filtration is promoted by hydrostatic pressure.

The material filtered into the glomerulus is called the “filtrate.”

Question 4

In the context of the kidney, what is secretion?

Answer 4

The process by which ions or molecules move from the blood into the nephron, joining the filtrate. These substances will then be excreted.

Secretion of protons and waste products occurs at the proximal and distal convoluted tubules.

Question 5

What is reabsorption?

Answer 5

The process by which water, molecules, and ions are transported from the filtrate back into the blood. As a result, these compounds are not excreted from the body.

Reabsorption occurs in all parts of the nephron.

Question 6

Why are non-waste molecules filtered into the glomerulus?

Answer 6

Filtration is not a specific process. Many biologically useful molecules, if they are sufficiently small, can pass into the glomerulus.

Important molecules that enter the filtrate, including glucose and amino acids, are simply reabsorbed from the nephron. Note that proteins are too large to be filtered.

Question 7

How do the kidneys eliminate extra ions that are not filtered directly into the glomerulus?

Answer 7

secreted

Both K⁺ and H⁺ are secreted in the proximal and distal tubules. H⁺ secretion is a vital part of pH balance.

Question 8

Name four molecules or ions that are reabsorbed in the proximal tubule.

Answer 8

1. water
2. amino acids
3. glucose
4. bicarbonate (HCO₃^-)

Question 9

What does plasma osmolarity measure?

Answer 9

It refers to the concentration of solute in the blood. High osmolarity signifies a large amount of solute and a relatively low amount of water.

In the blood, osmolarity is largely determined by the concentration of Na⁺ and Cl^- ions.

Question 10

In the loop of Henle, fluid flow in the two adjacent limbs is opposite; this contributes to a concentration gradient that increases from cortex to medulla. What term best relates to this phenomenon?

Answer 10

nephron’s countercurrent multiplier system

Sodium pumps in the ascending limb trigger a progressive increase in the amount of solute in the medulla. The unique structure of the loop causes this effect to compound or “multiply” over time.

Question 11

How would the osmolarity of human urine likely compare to that of a camel?

Answer 11

lower

High osmolarity occurs when a large amount of solute is present in a certain volume of fluid. Since camels must conserve water, their urine would be very concentrated.

Question 12

What is a portal system?

Answer 12

It is a circulatory structure in which one capillary bed drains into another. The capillaries surrounding each nephron are similarly arranged.

In humans, the hypophyseal and hepatic portal systems are two other examples to know.

Question 13

Name the two adjacent capillary beds that exchange material with the nephron.

Answer 13

The glomerulus and peritubular capillaries exchange fluid and solutes with the nephron.

The glomerulus, a dense web of capillaries, provides the filtrate that enters the nephron. Peritubular capillaries secrete and reabsorb substances to and from the tubules.

Question 14

What is the function of the renal artery?

Answer 14

It transports blood from the aorta to the kidney.

Within the kidney, the renal artery splits into arterioles, which provide blood to the glomeruli.

Question 15

What is the function of the renal vein?

Answer 15

It transports blood away from the kidney and back toward the heart.

Specifically, peritubular capillaries give rise to venules, which drain into the renal vein.

Question 16

What physiological condition is regulated by the action of ADH and aldosterone?

Answer 16

blood pressure

Important differences between these hormones exist, but both act on the nephrons of the kidney, eventually resulting in greater retention of water.

Question 17

Which hormones act on the nephron in response to low blood pressure?

Answer 17

Aldosterone and ADH

Through different mechanisms, both increase the nephron’s reabsorption of water, which raises blood volume.

Aldosterone is a steroid hormone, while ADH is a peptide.

Question 18

Does aldosterone, ADH, or neither hormone act on the nephron in response to high blood pressure?

Answer 18

Neither aldosterone nor ADH activity is triggered by high blood pressure.

Be careful! ADH and aldosterone function to alleviate low, not high, pressure. When pressure is high, these hormones are not produced and excess water is not reabsorbed. This concept appears often on the MCAT.

Question 19

Which functions of a nephron occur in the renal cortex?

Answer 19

1. Filtration
2. Reabsorption
3. Secretion

The cortex contains the glomerulus and the convoluted tubules of the nephron.

Question 20

Which functions of a nephron occur in the renal medulla?

Answer 20

1. Reabsorption
2. Urine concentration

The renal medulla contains the loop of Henle and the collecting duct. The loop of Henle reabsorbs both water and sodium. It also maintains the concentration gradient that allows urine to be concentrated in the collecting duct.

Question 21

What is the function of the afferent arterioles?

Answer 21

It supplies blood to the glomerulus.

Remember that afferent vessels bring blood toward an organ, while efferent vessels carry blood away.

Question 22

Define:

glomerulus

Answer 22

It is a ball of capillaries through which fluid, molecules, and ions filter into the nephron at Bowman’s capsule.

Question 23

What is the function of the efferent arterioles?

Answer 23

It carries blood away from the glomerulus.

Through vasodilation and vasoconstriction, the efferent arterioles function in the maintenance of glomerular filtration rate.

Question 24

Define:

nephron

Answer 24

It is the main functional unit of the kidney. It is responsible for filtration, secretion, and reabsorption.

Question 25

Trace the path of filtrate through the nephron, starting from the glomerulus and ending at the collecting duct.

Answer 25

• Fluid, molecules, and ions enter Bowman’s capsule from the glomerulus.
• The filtrate then enters the proximal convoluted tubule and descends the loop of Henle.
• After ascending the loop of Henle, the filtrate enters the distal convoluted tubule.
• Finally, the filtrate enters the collecting duct.

Question 26

Define:

Bowman’s capsule

Answer 26

A rounded structure at the beginning of the nephron. It collects the filtrate exiting the glomerulus.

Question 27

Name the nephron structure that immediately follows the Bowman’s capsule.

Answer 27

Proximal Convoluted Tubule

PCT

Most reabsorption and secretion occurs in the PCT.

Question 28

What is the benefit of the “convoluted” structure of the proximal convoluted tubule?

Answer 28

It provides a high surface area for effective secretion and reabsorption of substances.

Question 29

If a drug was administered to inhibit reabsorption in the PCT, which useful substances would be excreted?

Answer 29

• vitamins
• glucose
• amino acids

As biologically useful molecules, these substances are usually completely reabsorbed. Note that water and some ions are also reabsorbed in the proximal convoluted tubule.

Question 30

Define:

loop of Henle

Answer 30

It is a “U-shaped” structure that follows the proximal convoluted tubule. It generates a concentration gradient that assists the collecting duct in water reabsorption.

Question 31

How would a longer loop of Henle affect the osmolarity of an organism’s urine?

Answer 31

It would facilitate the production of urine with high osmolarity (more concentrated).

A longer loop can generate a larger gradient through the countercurrent multiplier system. This gradient then draws more water out of the collecting duct through osmosis.

Question 32

What occurs in the descending limb of the loop of Henle?

Answer 32

It is the site of water reabsorption.

The descending limb is permeable to water but not to solutes. As a result, the filtrate becomes more concentrated as water is reabsorbed down the length of the limb.

Question 33

The descending limb is permeable only to water. What functional benefit does this provide?

Answer 33

Water can passively flow out of the descending limb instead of requiring energy.

A high solute concentration is present in the deeper regions of the medulla. The water permeability of the descending branch allows water to flow out, down its gradient, without ions moving inward.

Question 34

What occurs in the ascending limb of the loop of Henle?

Answer 34

It is the site of active Na⁺ transport.

The ascending limb is permeable to salt and not water. Sodium ions are pumped out of the limb, decreasing the osmolarity of the filtrate inside.

Question 35

The ascending limb is permeable only to salt. What functional benefit does this provide?

Answer 35

Water is prevented from reentering the loop of Henle.

Salt must actively be pumped out of the ascending loop to achieve the proper concentration gradient within the kidney. If water followed, this process would be disrupted.

Question 36

Name the nephron structure that immediately precedes the collecting duct.

Answer 36

The distal convoluted tubule is located between the ascending limb of the loop of Henle and the collecting duct.

Question 37

What is the function of the distal convoluted tubule (DCT)?

Answer 37

It fine-tunes the reabsorption and secretion already performed by the proximal convoluted tubule.

Specifically, the DCT is involved in K⁺ and H⁺ secretion as well as plasma calcium regulation.

Question 38

What signals trigger the distal tubule to increase reabsorption?

Answer 38

Elevated plasma osmolarity and low blood volume, both markers of low blood pressure, trigger the release of aldosterone from the adrenal glands. Aldosterone signals the DCT to reabsorb more Na⁺. Water follows the Na⁺ and is thus reabsorbed.

Increased water reabsorption raises blood volume and thus blood pressure.

Question 39

The proximal tubule is more specialized for reabsorption than the distal tubule. What structural feature of the PCT reflects this role?

Answer 39

It is lined with a brush border, or epithelium covered in microvilli. As in the small intestine, this structure maximizes surface area.

The epithelium of the DCT does not possess a brush border.

Question 40

What is the function of the collecting duct?

Answer 40

It carries the filtrate to the ureter. It mainly functions to concentrate the urine when necessary.

Urine concentration is promoted by the hormone ADH and assisted by the concentration gradient maintained by the loop of Henle.

Question 41

What signals trigger the collecting duct to increase water reabsorption?

Answer 41

Low blood pressure triggers the release of antidiuretic hormone (ADH) from the posterior pituitary.

ADH causes the collecting duct to become permeable to water, facilitating passive reabsorption.

Increased water reabsorption raises blood volume and thus blood pressure.

Question 42

How would dehydration affect the osmolarity of an individual’s urine?

Answer 42

A dehydrated person’s urine would have a high osmolarity. In other words, it would be concentrated.

Dehydration, or water loss, results in low blood volume. This triggers ADH and aldosterone to increase the reabsorption of water in the nephron. Urine that contains less water compared to solute will have a higher osmolarity.

Question 43

What anatomical structure connects each kidney to the bladder?

Answer 43

ureter

A thin tube, transports urine from the kidney to the bladder.

The collecting ducts of the nephrons drain into the ureters.

Question 44

What anatomical structure stores urine prior to excretion?

Answer 44

urinary bladder

A sac lined with smooth muscle, stores urine. Its epithelium can become compact to accommodate a larger volume.

Note that this structure is distinct from the gall bladder, which functions in digestion.

Question 45

What anatomical structure connects to the bladder and carries urine out of the body?

Answer 45

urethra

It is the final tube through which urine is excreted.

In males, the urethra also functions in reproduction as a pathway for semen during ejaculation.

Question 46

A patient suffers from an impaired adrenal cortex. His kidney function may be altered due to the effects of which hormone?

Answer 46

aldosterone

A mineralocorticoid, may affect this patient’s kidneys. Since this hormone is released by the adrenal cortex, impairment of that structure could cause improper aldosterone release.

Aldosterone is a steroid, like the other products of the adrenal cortex. For the MCAT, this can be remembered by recalling their broad classification: corticosteroids.

Question 47

Describe the location and mechanism of aldosterone’s action on the nephron.

Answer 47

It acts on the distal convoluted tubule in response to low blood pressure. It directly facilitates the reabsorption of sodium ions.

Indirectly, aldosterone also increases the reabsorption of water, which passively follows Na⁺ out of the distal convoluted tubule. This water retention raises blood volume, increasing the blood pressure.

Question 48

How does the concentration of aldosterone change in response to increased secretion of renin?

Answer 48

increases

Renin, an enzyme, is secreted from the kidneys and indirectly leads to aldosterone release. This series of reactions, known as the renin-angiotensin system, is often a target of blood pressure medication.

Question 49

An adenoma, or noncancerous tumor, is putting pressure on a patient’s posterior pituitary gland. Her kidney function may be altered due to the effects of which hormone?

Answer 49

Antidiuretic hormone, or ADH, may affect this patient’s kidneys.

This peptide hormone is stored in and released from the posterior pituitary.

Note that ADH is produced in the hypothalamus, not the posterior pituitary. It is transported to this gland after synthesis.

Question 50

Describe the location and mechanism of ADH’s action on the nephron.

Answer 50

ADH acts on the collecting duct and distal tubule in response to low blood pressure. It directly facilitates the reabsorption of water, thus increasing blood volume and pressure.

Specifically, ADH increases the number of aquaporins, or water channels, in the cells that line this part of the nephron.

Question 51

What effect does ADH secretion have on urine osmolarity?

Answer 51

Urine osmolarity, or solute concentration, increases.

ADH causes the increased reabsorption of water in the distal convoluted tubule and the collecting duct. More water retention leads to concentrated urine.

Question 52

Vasopressin is an alternative name for which hormone?

Answer 52

antidiuretic hormone

(ADH)

Question 53

List four differences between aldosterone and ADH.

Answer 53

• It promotes the reabsorption of Na⁺, which leads to water retention. ADH increases water reabsorption directly.
• It is a steroid hormone, while ADH is a peptide.
• It is released from the adrenal cortex, while ADH is secreted from the posterior pituitary.
• The primary stimulus for aldosterone release is reduced blood flow to the kidney, while the trigger for ADH release is elevated plasma osmolarity.

Question 54

How does excretion help maintain homeostasis?

Answer 54

The body can regulate blood pressure, osmolarity, pH, and waste products.

Blood pressure is regulated by varying the excretion of water and Na⁺. Osmolarity is lowered through the excretion of ions; in the case of H⁺ and HCO₃^-, this also alters pH. Excreting urea eliminates waste product.

Question 55

How would the kidneys react to a plasma pH of 7.32?

Answer 55

They will secrete H⁺.

Normal plasma pH is around 7.4, and acidosis occurs at levels below 7.35. Thus, the pH in this example is too acidic, and protons must be secreted.

Question 56

How would the kidneys react to a plasma pH of 7.47?

Answer 56

They will secrete a larger fraction of HCO₃^-, resulting in its elimination from the body.

Normal plasma pH is around 7.4, and alkalosis occurs at levels above 7.45. Thus, the pH in this example is too basic. Bicarbonate, or HCO₃^-, is an important base; its excretion will make blood more acidic.

Question 57

How are nitrogenous wastes removed from the body?

Answer 57

The nitrogen from amino acids is converted to ammonia, then urea. Urea is filtered in the nephron before being concentrated and excreted as urine.

Urea, below, is a soluble, nontoxic form of ammonia. Its excretion in the urine is a simple disposal method for the products of protein catabolism.

Question 58

If blood and proteins appear in the urine, which structure may be damaged?

Answer 58

The glomerulus may be impaired.

Ordinarily, the glomerulus cannot filter proteins and blood cells into Bowman’s capsule due to their large size. Glomerular damage could impact this size-based filtration, resulting in the appearance of blood cells and proteins in the urine.

Question 59

If glucose appears in the urine, which condition may be the cause?

Answer 59

Diabetes

Normally, the proximal convoluted tubule reabsorbs virtually all of the glucose present in the filtrate. However, diabetes results in chronically elevated blood glucose levels. When these levels exceed the maximum amount that can be reabsorbed, glucose will be excreted.