15. Urinary System 2

Question 1

Explain reabsorption and secretion in the tubules - What happens to substances that are not reabsorbed and the ones that are in terms of concentration? (3)

Answer 1

1. Usually more than 99% of the water in the
   glomerular filtrate is reabsorbed as it
   passes through the tubules
2. Therefore dissolved substances that are not
   reabsorbed are greatly concentrated (e.g.
   creatinine)
3. Some dissolved substances are almost
   completely reabsorbed so their concentration
   decreases (e.g. glucose and amino acids)

Question 2

What kind of border is on the luminal surface and basal channels in base of proximal tubule? (1)

Answer 2

Brush border to increase surface area

Question 3

Steps of sodium reabsorption in the proximal tubule (8)

Answer 3

1. Sodium (Na⁺) is actively transported from tubular epithelium cells into the intercellular spaces.
2. Sodium then moves into the peritubular capillaries, reducing the intracellular Na⁺ concentration.
3. Active transport creates an electrochemical gradient, causing Na⁺ diffusion from the tubular lumen into the tubular epithelium.
4. For every Na⁺ reabsorbed, an anion (commonly Cl⁻) follows or a cation (commonly H⁺) is exchanged.
5. Secondary Active Transport: Glucose, amino acids co-transport with Na⁺ into the cell using carrier proteins at the brush border.
   - This is done against the concentration gradient for these other molecules
6. Countertransport: Na⁺ enters the cell while H⁺ is exchanged out of the cell (Na⁺/H⁺ antiport).
7. Water Reabsorption: Solute movement creates an osmotic gradient, leading to water reabsorption into peritubular capillaries.
8. Concentration of Urea: As water is reabsorbed, urea and other solutes become more concentrated, facilitating further reabsorption or excretion.

Question 4

Explain the proteins in the proximal tubule (1)

Answer 4

Pulled into the cells by endocytosis – then degraded in cells to amino acids which are moved into the peritubular space via facilitated diffusion

Question 5

Explain the small peptides in the proximal tubule (2)

Answer 5

1. Hydrolyzed in brush border cells to amino acids
2. The amino acids are reabsorbed via cotransport

Question 6

What is the loops of henle important for? (1)

Answer 6

Functions to reduce the volume of the
urine

Question 7

Describe the descending loop in the loops of Henle (4)

Answer 7

1. Highly permeable to water
2. Does not contain the mechanism to reabsorb Na+
3. Thus filtrate becomes more concentrated as it descends the loop – due to water leaving the loop
4. Reaches maximum concentration at the tip of the loop

Question 8

Describe the ascending loop in the loops of Henle (5)

Answer 8

1. Impermeable to water
2. Contains Na+ pumps which actively reabsorb Na+ into the medullary tissue and capillaries
3. Contains Cl- pumps
4. This is what creates the osmotic gradient that draws water out of the descending loop
5. By the time the filtrate reaches the top of the ascending loop it has the same concentration as when it entered but the volume is less

Question 9

What occurs in the distal convoluted tubule? How does this happen? (7)

Answer 9

1. Final adjustments are made to the chemical
   constituents of the urine
2. Sodium reabsorption
3. Potassium excretion
4. Reabsorption of water (Varies with the level of the hormone aldosterone)
5. Secretion of hydrogen ions
   - Increases if blood pH is low (acidic)
   - Stops if blood pH is high (alkaline)

Question 10

What occurs in the collecting ducts? (2)

Answer 10

1. Permeability to water
2. Varies with the level of the Antidiuretic hormone (ADH)

Question 11

What is the counter-current mechanism for concentrating urine? (2)

Answer 11

1. The countercurrent multiplier: the loops of Henle
2. The countercurrent exchanger: the vasa recta

Question 12

Explain this photo for the countercurrent multiplier and exchanger (2)

Answer 12

1. Urine can be less concentrated than the original filtrate
2. Occurs in the absence of ADH – the collecting duct is then impermeable to water – increased volume of urine excreted

Question 13

How does concentrated urine occur? (3)

Answer 13

1. Need to excrete solutes while conserving water
2. ADH causes the entire collecting duct to become highly permeable to water
3. Then as tubular fluid passes through the collecting duct water is pulled by osmosis into the concentrated medullary interstitium

Question 14

What factors stimulate/inhibit the release of ADH (5)

Answer 14

Stimulated by
1. Hypertonicity of hypothalamic ECF
2. Low blood pressure
3. Trauma, pain

Inhibited by
1. Cold
2. Alcohol

Question 15

Explain the structure juxtaglomerular apparatus (6)

Answer 15

1. Distal tubule (DCT) passes in the angle between the afferent (1) and efferent (2) arterioles.
2. The cells of the DCT that contact the arterioles are more dense than other DCT cells – called the macula densa (4)
3. The smooth muscle cells of the arterioles are swollen and contain dark granules where they contact the macula densa – called the juxtaglomerular cells (3)
4. Lacis and mesangial cells (5 and 6) fill the space between the DCT and the arterioles
   - Secrete prostaglandins
   - Act as macrophages.

Question 16

Explain the function of the juxtaglomerular apparatus (4)

Answer 16

1. Sensor/effector mechanism for regulation
   of blood volume
2. Juxtaglomerular cells – baroreceptors that
   monitor the afferent arteriole blood pressure: If blood pressure is low juxtaglomerular cells release renin
3. Macula densa cells – osmoreceptors that
   monitor Na+ osmolality: Low Na+ osmolality results in the release of renin
4. Sympathetic nerve activation: Cause renin release via beta-1 adrenoceptors

Question 17

Describe angiotensin II in the renin-angiotensin-aldosterone system

Answer 17

Angiotensin II
1. Constricts resistance vessels thereby increasing arterial blood pressure
2. Aldosterone from adrenal cortex
3. ADH from posterior pituitary
4. thirst centers in the brain

Question 18

Describe aldosterone in the renin-angiotensin-aldosterone system

Answer 18

1. Aldosterone (minceralcortoid steroid).
2. Action: Increases Na⁺/K⁺ movement, Na⁺/K⁺ pump activity.
3. Effect: Na⁺/water reabsorbed, K⁺/H⁺ excreted.
4. Regulates: Blood pressure, HCO₃⁻, pH.