Section 4

Question 2

What are the two steps involved in the process of tubular reabsorption?

Answer 2

Reabsorption starts with either active or passive movement of substances from the tubule into the interstitial space.

Reabsorption continues with passive movement of substances from the interstitial space back into the bloodstream.

Question 3

T or F: The glomerular filtrate that enters the tubules is identical to plasma with the exception of plasma proteins

Answer 3

True. That is, there is no selectivity to glomerular filtration.

Tubular reabsorption includes the processes by which water and other necessary solutes are returned to the plasma, while allowing waste products to remain in the filtrate

Question 4

What is transepithelial transport?

Answer 4

Transepithelial transport, also known as transcellular transport, is the movement of solutes across an epithelial cell layer through the cell, from the luminal membrane facing the tubule lumen to the basolateral membrane facing the interstitial fluid.

Question 5

Why can’t substances entering an epithelial cell be transported to neighboring cells?

Answer 5

Substances entering an epithelial cell cannot be transported to neighboring cells because the membranes of neighboring epithelial cells are not in contact, except where there are tight junctions connecting them. Thus, substances must move through the cell into the interstitial space.

Question 6

What are the 5 steps of transepithelial transport?

Answer 6

1. The substance must cross the luminal membrane.
2. The substance must pass through the cytosol.
3. The substance must cross the basolateral membrane.
4. It must diffuse through the interstitial fluid.
5. It must cross the capillary wall to enter the plasma.

The transport can be either passive or active

Question 7

What are the different locations of Na+ reabsorption within the kidney?

Answer 7

Proximal tubule: Reabsorbs 76% of Na+, critical for reabsorption of glucose, amino acids, water, Cl-, and urea.

Ascending limb of the loop of Henle: Absorbs 25% of total reabsorbed Na+, essential for urine concentration or dilution.

Distal and collecting tubules: Collectively reabsorb 8% Na+, under hormonal control, crucial for regulating extracellular fluid volume and secretion of K+ and H+.

Question 8

How is Na+ reabsorption facilitated across the _________ membrane of epithelial cells in the kidney tubules?

Answer 8

Na+ reabsorption across the basolateral membrane involves active transport mediated by the Na+-K+ ATPase pump.

Question 9

Describe the mechanism of passive transport of Na+ across the luminal membrane in the proximal tubule and the collecting duct.

Answer 9

In the proximal tubule, Na+ crosses through a cotransport carrier along with organic nutrients, using the concentration gradient established by the Na+-K+ ATPase pump (secondary active transport).

In contrast, in the collecting duct, Na+ passively enters the epithelial cells through a Na+ channel.

Question 10

Which hormonal system is primarily involved in the regulation of Na+?

Answer 10

The renin-angiotensin-aldosterone system (RAAS) is the most important hormonal system in Na+ regulation.

Question 11

What triggers the secretion of renin by granular cells in the juxtaglomerular apparatus? (3)

Answer 11

Three primary triggers for renin secretion are:

1.Granular cells’ detection of a drop in blood pressure (secrete renin)

2. Increased sympathetic activity (triggers granular cells to secrete renin),
3. Decreased luminal Na+ sensed by macula densa cells (triggers granular cells to secrete renin)

Question 12

Describe the role of granular cells in renin secretion.

Answer 12

Granular cells secrete renin into the blood in response to triggers such as low blood pressure, increased sympathetic activity, and decreased luminal Na+ levels (sensed by macula densa cells).

Question 13

What is the significance of Na+ reabsorption in the proximal tubule and loop of Henle?

Answer 13

In the proximal tubule and loop of Henle, a constant percentage of filtered Na+ is reabsorbed, independent of total Na+ levels in body fluids.

Question 14

What is the role of renin in the renin-angiotensin-aldosterone system (RAAS)?

Answer 14

Renin acts as an enzyme to convert angiotensinogen into angiotensin I.

Question 15

How is angiotensin I converted into angiotensin II?

Answer 15

angiotensin I is converted to angiotensin II by the enzyme angiotensin-converting enzyme (ACE), primarily in the lungs.

Question 16

What is the function of angiotensin II in the RAAS system?

Answer 16

Angiotensin II stimulates the release of aldosterone from the adrenal cortex.

Question 17

What hormone causes an increase in Na+ reabsorption in the distal and collecting tubules?

Answer 17

Aldosterone causes an increase in Na+ reabsorption in the distal and collecting tubules.

Question 18

Describe the sequence of events in the RAAS system from renin secretion to aldosterone action.

Answer 18

The sequence is as follows:

Renin secretion →
Conversion of angiotensinogen to angiotensin I →
Conversion of angiotensin I to angiotensin II →
Stimulation of aldosterone release →
Increase in Na+ reabsorption in the distal and collecting tubules.

Question 19

What is angiotensinogen?

Answer 19

A protein made in the liver that is present at high concentrations in the plasma.

Question 20

What effect does aldosterone have on tubular epithelial cells?

Answer 20

Aldosterone increases the insertion of Na+ channels in the luminal membrane and Na+-K+-ATPase carriers in the basolateral membrane of tubular epithelial cells.

Therefore, aldosterone enhances Na+ retention by increasing the passive flow of Na+ out of the tubular fluid.

Question 21

What is the consequence of increased Na+ retention due to aldosterone?

Answer 21

Increased Na+ retention leads to increased water retention because water follows Na+, ultimately resulting in increased arterial blood pressure.

Question 22

What are the actions of atrial natriuretic peptide (ANP)?

Answer 22

OPPOSITE OF ALDOSTERONE (reduces Na+ load and bp):
1. ANP directly inhibits Na+ reabsorption in the distal tubules, leading to more Na+ excretion in the urine.
2. ANP inhibits both renin and aldosterone secretion.
3. ANP dilates the afferent arterioles and increases glomerular filtration rate (GFR), resulting in increased salt and water excretion.

Question 23

What stimulates the release of atrial natriuretic peptide (ANP)?

Answer 23

Stretch receptors in the left atrium, aortic arch, and carotid sinus stimulate the release of ANP in response to increased blood volume or venous return.

Question 24

What is the tubular, or transport, maximum (Tm)? What is it determined by?

Answer 24

The tubular maximum (Tm) is the maximum rate at which a substance can be actively reabsorbed across the tubule wall. It is determined by the number of carrier proteins in the plasma membrane.

Question 25

What happens if the concentration of a substance in the tubular fluid exceeds its Tm?

Answer 25

If the concentration of a substance in the tubular fluid exceeds its tubular maximum (Tm), the excess will be excreted in the urine.

• The renal threshold is the plasma concentration at which the Tm for a substance is exceeded, leading to its excretion in the urine.

Question 26

How does the kidney regulate the plasma concentration of phosphate?

Answer 26

The kidney regulates the plasma concentration of phosphate by setting the renal threshold for phosphate at the same level as the normal plasma concentration. When the plasma concentration of phosphate rises after eating a phosphate-rich meal, the excess phosphate above the normal concentration is excreted in the urine, restoring the plasma phosphate concentration to normal.

Hormones can also alter the renal threshold for phosphate to modulate its reabsorption based on the body’s needs.

Question 27

How does a fall in plasma phosphate concentration affect plasma calcium levels and PTH secretion?

Answer 27

A fall in plasma phosphate concentration increases plasma calcium levels, which directly suppresses PTH secretion. Reduced PTH levels lead to increased phosphate reabsorption by the kidneys, helping to return plasma phosphate concentration toward normal.

Also, a fall in plasma increases activation of vitamin D in the kidney which promotes absorption in the intestine.

Question 28

T or F: Glucose plasma concentrations are regulated by the kidneys

Answer 28

False. Glucose is small enough that it is freely filterable and the concentration in Bowman’s capsule filtrate is the same as it is in the plasma

Question 29

How is the filtered load of a substance calculated in the kidneys?

Answer 29

Filtered load = plasma concentration x GFR

Question 30

At what plasma concentration will glucose start appearing in the urine?

Answer 30

When the filtered load of glucose exceeds its normal Tm (375 mg/min)

Question 31

Diabetes mellitus causes increased glucose levels in the blood. This, in turn, can cause increased levels of glucose in the urine.

Using what you have learned thus far about the kidneys’ ability to filter glucose, explain why this increase in glucose in the urine exists

Answer 31

Ordinarily, urine contains no glucose because the kidneys are able to reabsorb it all. Bowman’s capsule collects the filtrate that the glomerulus forms which includes urea, electrolytes, and glucose. The filtrate then
passes into the proximal tubule to be reabsorbed.

Proximal tubule, however, can only reabsorb a limited amount of glucose. When blood glucose levels exceed about 160-180 m g/100 m L, the proximal tubule is overwhelmed and begins to excrete glucose into the urine

Question 32

How is water reabsorbed in the kidneys?

Answer 32

Water is passively reabsorbed along the tubule as it follows sodium. Approximately 65% is reabsorbed in the proximal tubule, 15% in the loop of Henle, and 20% in the distal and collecting tubules.

Water flows through water channels called aquaporins, with those in the proximal tubule always open allowing the flow of water by osmosis.

Question 33

How is chloride reabsorbed in the kidneys?

Answer 33

Chloride is not directly regulated by the kidneys. It moves between epithelial cells, following the reabsorption of sodium, as it goes down its electrochemical gradient. (The amount of chloride reabsorbed is determined by the amount of sodium reabsorbed.)

Question 34

What is the fate of urea in the kidneys?

Answer 34

Urea, a waste product from protein breakdown, is reabsorbed in significant amounts in the kidneys. Its concentration increases three-fold as fluid moves through the proximal tubule, leading to passive reabsorption. About 40-50% of plasma urea is filtered and excreted from the body with each pass through the nephron.

Blood urea nitrogen (BUN) is historically used as a measure of renal failure, as during renal failure, less urea is excreted, leading to its accumulation in the plasma.