T10. URINE FORMATION II

Question 1

What processes are involved in the conversion of glomerular filtrate to urine?

Answer 1

tubular reabsorption, tubular secretion, and water conservation

Question 2

Where does tubular reabsorption and secretion occur?

Answer 2

Across the proximal convoluted tubule (PCT) and distal convoluted tubule (DCT)

Question 3

What are the two main routes for tubular reabsorption in the PCT?

Answer 3

Transcellular route and paracellular route

Question 4

What is the transcellular route of reabsorption in the PCT?

Answer 4

Substances pass through the cytoplasm of PCT epithelial cells

Question 5

What is the paracellular route of reabsorption in the PCT?

Answer 5

Substances pass through leaky junctions between PCT cells, allowing water and solutes to be reabsorbed via solvent drag

Question 6

What structural features enable the PCT to reabsorb substances efficiently?

Answer 6

Prominent microvilli, great length, and abundant mitochondria

Question 7

How much of the glomerular filtrate is reabsorbed by the PCT?

Answer 7

About 65%

Question 8

What is the role of sodium reabsorption in the PCT?

Answer 8

Creates osmotic and electrical gradients that drive the reabsorption of water and other solutes

Question 9

What transport proteins are involved in sodium uptake in the PCT?

Answer 9

Symports with glucose, amino acids, or lactate; Na+/H+ antiports

Question 10

Do sodium symports consume ATP?

Answer 10

No, sodium-transporting symports in the apical membrane do not consume ATP

Question 11

How is intracellular sodium prevented from accumulating in PCT cells?

Answer 11

Na+/K+ ATPase pumps sodium into extracellular fluid using active transport

Question 12

How is sodium returned to the bloodstream after being pumped out of PCT cells?

Answer 12

It is picked up by peritubular capillaries

Question 13

How do chloride ions move in the PCT?

Answer 13

They follow sodium by electrical attraction

Question 14

How do potassium, magnesium, and phosphate ions move in the PCT?

Answer 14

They diffuse through the paracellular route with water

Question 15

How are small proteins and peptide hormones reabsorbed in the PCT?

Answer 15

By pinocytosis

Question 16

How is urea reabsorbed in the PCT?

Answer 16

It diffuses with water through the epithelium; 40–60% is reabsorbed

Question 17

How much urea do the kidneys remove from the blood?

Answer 17

Half

Question 18

How does water reabsorption occur in the PCT?

Answer 18

Water follows solutes by osmosis through paracellular and transcellular routes using aquaporins

Question 19

How much water is reabsorbed in the PCT?

Answer 19

Two-thirds of 180 L/day

Question 20

What makes tubule cells and tissue fluid hypertonic in the PCT?

Answer 20

Reabsorption of salt and organic solutes

Question 21

How is glucose reabsorbed in the nephron?

Answer 21

Completely reabsorbed in the PCT via sodium-glucose transport (SGLT), facilitated diffusion (GLUT2), and simple diffusion

Question 22

What is the transport maximum (Tm)?

Answer 22

The limit to how much solute can be reabsorbed, determined by the number of transport proteins

Question 23

What happens when solute concentration exceeds Tm?

Answer 23

Excess solutes appear in the urine

Question 24

What is the Tm for glucose?

Answer 24

Approximately 220 mg/dL; above this results in glycosuria

Question 25

Which other solutes have a transport maximum?

Answer 25

Phosphate, sulfate, amino acids, uric acid, lactic acid

Question 26

What drives uptake into the peritubular capillaries?

Answer 26

Osmosis and solvent drag, influenced by high interstitial fluid pressure, low blood hydrostatic pressure, and high colloid osmotic pressure

Question 27

What is the primary function of the nephron loop?

Answer 27

Generate salinity gradient for urine concentration, water conservation, and electrolyte reabsorption

Question 28

How much Na+, K+, and Cl- is reabsorbed by the thick segment of the nephron loop?

Answer 28

About 25%

Question 29

Why doesn't water follow ions in the thick segment of the nephron loop?

Answer 29

Because the thick segment is impermeable to water

Question 30

What is the osmolarity of the tubular fluid entering the DCT?

Answer 30

Very dilute

Question 31

How much water is reabsorbed in the nephron loop?

Answer 31

Approximately 20% of the water

Question 32

How much is fluid volume reduced by the end of the proximal tubule?

Answer 32

By one-third

Question 33

What is the function of the descending loop of Henle?

Answer 33

Permeable to water but not salt; water is reabsorbed into interstitial fluid and picked up by capillaries

Question 34

What happens to fluid concentration in the descending limb?

Answer 34

It becomes more concentrated as it descends

Question 35

What is the role of the ascending limb of Henle?

Answer 35

Impermeable to water; actively transports Na+, Cl−, and K+ into interstitial fluid

Question 36

How is salt transported in the ascending limb?

Answer 36

By Na+, Cl−, K+ co-transport, Na+ active pumping, and passive K+ diffusion

Question 37

What is the osmotic environment in deeper medulla?

Answer 37

Increasingly concentrated interstitial fluid

Question 38

What is tubular secretion?

Answer 38

The process of moving substances from the capillary blood into the tubular fluid

Question 39

What are two major functions of tubular secretion?

Answer 39

Waste removal and acid-base balance

Question 40

What wastes are removed via tubular secretion?

Answer 40

Urea, uric acid, ammonia, catecholamines, prostaglandins, creatinine, drugs

Question 41

How does tubular secretion contribute to acid-base balance?

Answer 41

By secreting hydrogen and bicarbonate ions

Question 42

How much water and salt does the fluid in the DCT contain?

Answer 42

About 20% of the water and 7% of the salts from glomerular filtrate

Question 43

What regulates reabsorption in the DCT and collecting duct?

Answer 43

Hormones: aldosterone, ANP, ADH, and parathyroid hormone

Question 44

What are the two types of cells in the DCT and collecting duct?

Answer 44

Principal cells and intercalated cells

Question 45

What do principal cells do?

Answer 45

Reabsorb Na+, Cl−, and water; secrete K+; respond to hormones

Question 46

What do intercalated cells do?

Answer 46

Reabsorb K+ and bicarbonate; secrete H+; maintain acid-base balance

Question 47

What is the permeability of the first portion of the DCT?

Answer 47

Almost impermeable to water and urea; active transport of Na+, Cl−, K+, and Mg2+

Question 48

What is the permeability of the collecting duct?

Answer 48

Also impermeable to urea (except medullary portion); principal and intercalated cells function here

Question 49

What is the role of urea in the countercurrent system?

Answer 49

Urea diffuses from the collecting duct into interstitial fluid and back into the loop of Henle, recycling to maintain concentration gradient

Question 50

What is aldosterone?

Answer 50

A steroid hormone secreted by adrenal cortex in response to low Na+, high K+, or low BP

Question 51

What triggers aldosterone secretion?

Answer 51

Low Na+, high K+, or low BP → renin → angiotensin II → adrenal cortex → aldosterone

Question 52

Where does aldosterone act?

Answer 52

Thick segment of nephron loop, DCT, and cortical collecting duct

Question 53

What does aldosterone do?

Answer 53

Stimulates Na+ reabsorption, K+ secretion; water and Cl− follow Na+

Question 54

What are the effects of aldosterone?

Answer 54

Retains NaCl and water, increases blood volume and pressure, reduces urine volume, increases K+ concentration in urine

Question 55

What is atrial natriuretic peptide (ANP)?

Answer 55

A hormone secreted by atria in response to high BP

Question 56

What are the four actions of ANP?

Answer 56

Dilates afferent arteriole, constricts efferent arteriole (↑ GFR), inhibits renin, aldosterone, ADH, and NaCl reabsorption

Question 57

What is the effect of ANP on urine production?

Answer 57

Promotes salt and water excretion to reduce blood volume and pressure

Question 58

What is antidiuretic hormone (ADH)?

Answer 58

Hormone from posterior pituitary released in response to dehydration and high blood osmolarity

Question 59

How does ADH affect the kidney?

Answer 59

Makes collecting duct more permeable to water → water reabsorbed into bloodstream

Question 60

How much filtrate is reabsorbed in the PCT?

Answer 60

About 65%

Question 61

How much filtrate is reabsorbed in the nephron loop?

Answer 61

About 25%

Question 62

How much is reabsorbed in the DCT and collecting duct?

Answer 62

Variable amounts depending on hormone levels, especially aldosterone and ANP

Question 63

What else do the tubules do besides reabsorption?

Answer 63

Extract drugs, wastes, and some solutes from blood and secrete them into tubular fluid