Formation of urine

Question 1

What are the five major stages in the formation of urine?

Answer 1

1. Glomerulus (Filtration of blood), 2. Proximal tubule (Reabsorption of filtrate, Secretion into tubule), 3. Loop of Henle (Concentration of urine), 4. Distal tubule (Modification of urine), 5. Collecting duct (Final modification of urine)

Question 2

What is the primary function of the glomerulus?

Answer 2

Filtration of blood

Question 3

What occurs in the proximal tubule during urine formation?

Answer 3

Reabsorption of filtrate and secretion into the tubule

Question 4

What is the role of the loop of Henle in urine formation?

Answer 4

Concentration of urine

Question 5

What happens in the distal tubule during the formation of urine?

Answer 5

Modification of urine

Question 6

What is the final stage of urine formation?

Answer 6

Final modification of urine in the collecting duct

Question 7

What are the three pressures involved in glomerular filtration?

Answer 7

Pressure within glomerular capillary (PGC), Plasma protein pressure (GC), Pressure within Bowman’s capsule (PBC)

Question 8

Does autoregulation of renal blood flow persist in denervated kidneys and isolated perfused kidneys?

Answer 8

Yes, autoregulation persists in denervated kidneys and isolated perfused kidneys.

Question 9

What is the cause of autoregulation in renal blood flow?

Answer 9

Autoregulation is due to the myogenic response of renal arterioles to stretch (Starling’s Law) and the modulation of afferent and efferent arteriolar contraction and dilation by renal metabolites.

Question 10

How does the myogenic response contribute to autoregulation?

Answer 10

If blood pressure decreases, the renal artery and efferent arterioles automatically constrict to maintain a constant renal blood flow (RBF) and glomerular filtration rate (GFR).

Question 11

What are some examples of renal metabolites that modulate afferent and efferent arteriolar contraction and dilation?

Answer 11

Examples include adenosine and nitric oxide.

Question 12

Where does blood flow from the nephron go after water and solutes are reabsorbed from the tubule?

Answer 12

It is taken back into the peritubular vessels and vasarecta surrounding the tubule.

Question 13

How does a drop in filtration pressure (hypotension) affect glomerular filtration rate (GFR)?

Answer 13

A drop in filtration pressure causes a decrease in GFR.

Question 14

What is the response of the macula densa to a change in tubular sodium (Na+) levels?

Answer 14

The macula densa senses a change in tubular sodium levels.

Question 15

What happens when the macula densa detects a decrease in tubular sodium levels?

Answer 15

It stimulates the juxtaglomerular cells to release renin.

Question 16

What is the role of renin release in response to decreased tubular sodium levels?

Answer 16

Renin release leads to the generation of angiotensin II.

Question 17

What is the function of angiotensin II?

Answer 17

Angiotensin II is a vasoconstrictor, leading to an increase in blood pressure (BP).

Question 18

How does increased blood pressure (BP) affect glomerular filtration rate (GFR)?

Answer 18

Increased BP causes the filtration pressure to increase, and GFR returns to normal.

Question 19

What are the two processes depicted in the image?

Answer 19

Reabsorption (arrow out of lumen) and Secretion (arrow into lumen).

Question 20

Where does reabsorption occur in the nephron?

Answer 20

Reabsorption occurs when substances move out of the tubular lumen and into the surrounding capillaries.

Question 21

Where does secretion occur in the nephron?

Answer 21

Secretion occurs when substances move from the capillaries into the tubular lumen.

Question 22

What substances are almost completely reabsorbed in the PCT?

Answer 22

Glucose, amino acids, and a small amount of filtered proteins are almost completely reabsorbed in the PCT.

Question 23

What is the driving force for reabsorption in the PCT?

Answer 23

The driving force for reabsorption is Na+K+ATPase.

Question 24

What is the role of the Na+-K+-ATPase in sodium reabsorption from the PCT?

Answer 24

The Na+-K+-ATPase pumps out Na+ from cells into the blood against chemical and electrical gradients.

Question 25

What follows Na+ by diffusion in the process of sodium reabsorption from the PCT?

Answer 25

Cl- follows Na+ by diffusion, and phosphate and sulfate are co-transported with Na+.

Question 26

How is glucose transported into the PCT cell?

Answer 26

Glucose is co-transported into the PCT cell with Na+.

Question 27

What drives water reabsorption in the PCT?

Answer 27

Active transport of Na+ out of PCT cells drives the reabsorption of 65% of filtered water in the PCT.

Question 28

What channels are involved in the transcellular route of water reabsorption in the PCT?

Answer 28

Aquaporin (AQP) channels, specifically AQP1 and AQP3/4, located on the apical and basolateral surfaces of PCT cells.

Question 29

How does water reabsorption occur along the nephron?

Answer 29

Water reabsorption occurs passively by osmosis, following the movement of sodium.

Question 30

How are amino acids (AAs) reabsorbed from the PCT?

Answer 30

There are seven independent transport processes for the reabsorption of AAs from the PCT, which depend on the type of amino acid.

Question 31

How are proteins handled in the PCT?

Answer 31

Very little protein passes through the glomerulus. Proteins that are filtered are reabsorbed from the proximal tubule (PT) via receptor-mediated endocytosis. The presence of proteins in the urine often indicates a sign of glomerular damage.

Question 32

Why are some substances and drugs not filtered at the glomerulus?

Answer 32

Some substances and drugs cannot be filtered at the glomerulus due to their size or protein binding.

Question 33

What are the two types of specialized pumps in the PCT that can transport compounds from the plasma into the nephron?

Answer 33

The two types are organic acid pumps (e.g., uric acid, diuretics, antibiotics like penicillin) and organic base pumps (e.g., creatinine, procainamide).

Question 34

Where does reabsorption primarily occur in the nephron?

Answer 34

Reabsorption primarily occurs in the proximal tubule.

Question 35

What is the function of the Loop of Henle (LOH) in the nephron?

Answer 35

The LOH is responsible for the further modification of tubular fluid and the recovery of fluid and solutes from the glomerular filtrate.

Question 36

What are the two stages of the process in the Loop of Henle?

Answer 36

(1) Reabsorption of water in the descending limb (D), and (2) Reabsorption of Na+ and Cl- in the ascending limb (A).

Question 37

Which type of nephrons is the process in the Loop of Henle more important for?

Answer 37

The process is more important for juxtamedullary nephrons, which have longer loops of Henle.

Question 38

What is the permeability of the thin descending limb of the Loop of Henle to water and salts?

Answer 38

The thin descending limb is freely permeable to water via Aquaporin-1 channels but does not involve active transport of salts (e.g., Na+, Cl-).

Question 39

What is the permeability and transport mechanism in the thick ascending limb of the Loop of Henle?

Answer 39

The thick ascending limb is impermeable to water but has specialized Na+/K+/2Cl- (NKCC2) co-transporters, which facilitate the reabsorption of Na+, K+, and Cl-.

Question 40

What is the osmolality of the fluid entering the Loop of Henle from the proximal tubule? And how does it change throughout the loop?

Answer 40

The fluid entering the LOH from the proximal tubule is isotonic (300 mOsm), but as water is reabsorbed in the descending limb and solutes (e.g., Na+, Cl-) are pumped out in the ascending limb, the filtrate becomes hypertonic (very concentrated, 1,200 mOsm) at the tip of the LOH and then hypotonic (150 mOsm) at the end.

Question 41

Does water reabsorption occur in the ascending limb of the Loop of Henle?

Answer 41

No, water reabsorption does not occur in the ascending limb.

Question 42

What is the purpose of countercurrent multiplication in the Loop of Henle?

Answer 42

Countercurrent multiplication creates a large osmotic gradient within the medulla, which is facilitated by Na+/K+/2Cl- co-transport in the ascending limb of the LOH.

Question 43

What does countercurrent multiplication permit in the Loop of Henle?

Answer 43

Countercurrent multiplication permits the passive reabsorption of water from the tubular fluid in the descending limb of the LOH.

Question 44

How does urea affect osmolality in the nephron?

Answer 44

Urea is freely filtered at the glomerulus and undergoes some reabsorption in the proximal tubule. However, the LOH and distal tubule are relatively impermeable to urea. Urea is absorbed from the inner medullary collecting duct and is secreted into the thick ascending limb. It can also diffuse out of the collecting duct into the medulla down its concentration gradient.

Question 45

What is the significance of urea in osmoregulation?

Answer 45

Urea contributes to the establishment of the osmotic gradient within the medulla and plays a role in the concentration of urine.

Question 46

What is the main function of the Loop of Henle?

Answer 46

The main function of the Loop of Henle is to facilitate the reabsorption of water and solutes, establish an osmotic gradient within the medulla, and concentrate urine.

Question 47

What occurs in the distal tubule (DCT) in terms of ion transport?

Answer 47

Active absorption and secretion occur in the DCT. Na+ and Cl- ions are actively reabsorbed from the tubular fluid in exchange for K+ or H+ ions secreted into the tubular fluid.

Question 48

Which cells are involved in the exchange of Na+ and K+ in the late DCT and early collecting duct?

Answer 48

Specialized principal cells are involved in the exchange of Na+ and K+ in the late DCT and early collecting duct. These cells are sensitive to aldosterone.

Question 49

Which cells are involved in the exchange of Na+ and H+ in the DCT and early collecting duct?

Answer 49

Specialized alpha intercalated cells are involved in the exchange of Na+ and H+ in the DCT and early collecting duct. There are subtypes of intercalated cells, namely alpha and beta intercalated cells.

Question 50

What is the function of α-intercalated cells in the DCT and collecting duct?

Answer 50

α-Intercalated cells secrete acid (H+) via H+/Na+ or H+/K+ exchange, involving ATPase or H+ATPase. They also reabsorb bicarbonate (HCO3-), which is the main buffer in the body.

Question 51

What is the function of β-intercalated cells in the DCT and collecting duct?

Answer 51

β-Intercalated cells secrete bicarbonate (HCO3-) via Pendrin and reabsorb acid (H+). They play a role in acid-base regulation.

Question 52

What is the role of carbonic anhydrase in the kidney tubules?

Answer 52

Carbonic anhydrase is found in many places in the body, including the lungs and kidney tubules. It catalyzes the conversion of CO2 and H2O to H2CO3 (carbonic acid), and the reversible conversion of HCO3- (bicarbonate) and H+.

Question 53

What is the relative permeability of the collecting duct (CD) to water and solutes?

Answer 53

The CD is relatively impermeable to the movement of water and solutes.

Question 54

What hormone increases the permeability of the collecting duct and when is it released?

Answer 54

Antidiuretic hormone (ADH) increases the permeability of the collecting duct when necessary. It is released from the posterior pituitary gland following hypothalamic input.

Question 55

How does ADH act in the collecting duct to regulate water balance?

Answer 55

ADH acts on vasopressin V2 receptors on principal cells in the DCT and collecting duct cells, leading to the activation of intracellular Aquaporin-2 (AQP2) water channels and the reabsorption of water.

Question 56

What happens to the collecting duct under maximal circulating ADH levels?

Answer 56

Under maximal circulating ADH levels (e.g., severe dehydration), the collecting duct becomes permeable to water due to maximal insertion of AQP2, allowing water reabsorption. It can reabsorb up to 66% of the water entering the collecting duct.

Question 57

What happens to the collecting duct in the absence of circulating ADH?

Answer 57

In the absence of circulating ADH, the collecting duct wall becomes impermeable to water due to the absence of AQP2. As a result, a large volume of water is excreted (up to 30L/day). This condition is associated with diabetes insipidus and can be treated using synthetic ADH.