Kidney and Excretory System Flashcards

Always a favorite of MCAT test-makers, the excretory system can also be a major source of confusion. Use these cards to master filtration, reabsorption, and secretion, as well as hormone involvement and the complexity of the loop of Henle.

1
Q

What are the two main structural components of the kidney?

A

The cortex and the 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.

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2
Q

List four primary functions of the kidneys.

A
  • Regulation of blood pressure
  • Regulation of blood osmolarity
  • Regulation of blood pH
  • Excretion of soluble wastes
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3
Q

What is filtration?

A

Filtration is 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.”

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4
Q

In the context of the kidney, what is secretion?

A

Secretion is 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.

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5
Q

What is reabsorption?

A

Reabsorption is 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.

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6
Q

Why are non-waste molecules filtered into the glomerulus?

A

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.

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7
Q

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

A

These ions are secreted.

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

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8
Q

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

A

A variety of biologically useful molecules are reabsorbed in the proximal tubule. Examples include water, amino acids, glucose, and bicarbonate (HCO3-).

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9
Q

What does plasma osmolarity measure?

A

Plasma osmolarity 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.

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10
Q

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?

A

This describes the 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.

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11
Q

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

A

The osmolarity of human urine would be 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.

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12
Q

What is a portal system?

A

A portal system 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.

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13
Q

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

A

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.

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14
Q

What is the function of the renal artery?

A

The renal artery transports blood from the aorta to the kidney.

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

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15
Q

What is the function of the renal vein?

A

The renal vein transports blood away from the kidney and back toward the heart.

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

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16
Q

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

A

Antidiuretic hormone (ADH) and aldosterone regulate blood pressure.

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

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17
Q

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

A

Aldosterone and ADH are produced when blood pressure is low. 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.

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18
Q

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

A

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.

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19
Q

Which functions of a nephron occur in the renal cortex?

A

Filtration, reabsorption, and secretion all take place within the renal cortex.

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

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20
Q

Which functions of a nephron occur in the renal medulla?

A

Reabsorption and urine concentration take place within the renal medulla.

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.

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21
Q

What is the function of the afferent arterioles?

A

The afferent arterioles supply blood to the glomerulus.

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

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22
Q

Define:

glomerulus

A

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

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23
Q

What is the function of the efferent arterioles?

A

The efferent arterioles carry blood away from the glomerulus.

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

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24
Q

Define:

nephron

A

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

25
Q

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

A

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.

26
Q

Define:

Bowman’s capsule

A

Bowman’s capsule is a rounded structure at the beginning of the nephron. It collects the filtrate exiting the glomerulus.

27
Q

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

A

The proximal convoluted tubule (PCT) is located after the Bowman’s capsule and before the loop of Henle.

Most reabsorption and secretion occurs in the PCT.

28
Q

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

A

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

29
Q

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

A

Vitamins, glucose, and amino acids would be excreted.

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

30
Q

Define:

loop of Henle

A

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

31
Q

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

A

A longer loop of Henle 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.

32
Q

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

A

The descending limb 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.

33
Q

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

A

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.

34
Q

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

A

The ascending limb 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.

35
Q

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

A

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.

36
Q

Name the nephron structure that immediately precedes the collecting duct.

A

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

37
Q

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

A

The DCT 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.

38
Q

What signals trigger the distal tubule to increase reabsorption?

A

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.

39
Q

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

A

The PCT 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.

40
Q

What is the function of the collecting duct?

A

The collecting duct 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.

41
Q

What signals trigger the collecting duct to increase water reabsorption?

A

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.

42
Q

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

A

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.

43
Q

What anatomical structure connects each kidney to the bladder?

A

The ureter, a thin tube, transports urine from the kidney to the bladder.

The collecting ducts of the nephrons drain into the ureters.

44
Q

What anatomical structure stores urine prior to excretion?

A

The urinary bladder, a sac lined with smooth muscle, stores urine. Its epithelium can become compact to accomodate a larger volume.

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

45
Q

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

A

The urethra is the final tube through which urine is excreted.

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

46
Q

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

A

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.

47
Q

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

A

Aldosterone 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.

48
Q

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

A

Aldosterone concentration 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.

49
Q

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?

A

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.

50
Q

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

A

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.

51
Q

What effect does ADH secretion have on urine osmolarity?

A

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.

52
Q

Vasopressin is an alternative name for which hormone?

A

Vasopressin is another name for antidiuretic hormone, or ADH.

53
Q

List four differences between aldosterone and ADH.

A
  • Aldosterone promotes the reabsorption of Na+, which leads to water retention. ADH increases water reabsorption directly.
  • Aldosterone is a steroid hormone, while ADH is a peptide.
  • Aldosterone 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.
54
Q

How does excretion help maintain homeostasis?

A

Through excretion, 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.

55
Q

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

A

The kidneys 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.

56
Q

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

A

The kidneys 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.

57
Q

How are nitrogenous wastes removed from the body?

A

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.

58
Q

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

A

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.

59
Q

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

A

Diabetes is often characterized by the presence of glucose in the urine.

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.