2. Plasma volume and electrolyte homeostasis

Question 1

What is obligatory reabsorption in the PCT?

Answer 1

Approximately 65% of water is reabsorbed in the proximal convoluted tubule
- not regulated independently of resorption of solutes

Question 2

What substances do cells of the PCT secrete into the lumen?

Answer 2

• metabolites (bile salts, urate, prostaglandins)

- xenobiotics (externally administered molecules) e.g., penicillin, aspirin and morphine

Question 3

Why is penicillin effective in the treatment of UTI’s?

Answer 3

because it achieves high concentrations in the filtrate

Question 4

What events occur in the Descending loop of Henle?

Answer 4

Na+ and Cl- are not reabsorbed - region is impermeable to electrolytes
- concentrated in the lumen as water is pulled osmotically into the surrounding medullary interstitium

Question 5

What events occur in the ascending loop of Henle?

Answer 5

Na+, K+ and Cl- are actively transported out of the lumen

- water is not reabsorbed, region is impermeable to water

Question 6

How does the osmotic gradient change from the superficial to deep medulla?

Answer 6

Increasing which is used to reabsorb water from the collecting duct

Question 7

What ions are reabsorbed in the distal convoluted tubule?

Answer 7

Small amount of Na+
Cl- is co-transported
Impermeable to water
Ca2+ reabsorption occurs under the control of parathyroid hormone

Question 8

What ions and hormones are reabsorbed in the late distal tubule and collecting ducts?

Answer 8

• approx. 4% of Na+ is actively reabsorbed in this segment
• H+ and K+ are actively secreted
• aldosterone acts on the segment to increase Na+ absorption and K+ excretion
• water is only reabsorbed if ADH is present (vasopressin)

Question 9

Describe the net movement of water when plasma osmolality is low

Answer 9

Net movement of water into cells

Question 10

Describe the net movement of water when plasma osmolality is too high

Answer 10

Net movement of water out of cells

Question 11

Where is ADH/Vasopressin excreted from?

Answer 11

Posterior pituitary gland in response to increased plasma osmolality

Question 12

How is secretion of ADH regulated?

Answer 12

Osmoreceptors in supraoptic and paraventricular nuclei of hypothalamus

Question 13

How does ADH mediate an increase in extracellular fluid osmolality?

Answer 13

ADH released

• water is allowed to diffuse out of distal and collecting tubules
• increasing retention of water and makes urine more concentrated

Question 14

How does ADH make the collecting duct permeable to water?

Answer 14

Causes the insertion of water permeable channels into late distal convoluted tubule and collecting duct
- allows penetration of water

Question 15

Describe the haemodynamic control of ADH

Answer 15

Decrease in blood pressure or volume also stimulates ADH secretion
- blood pressure changes are sensed by baroreceptors in the aortic arch and carotid sinus which respond to stretch

Question 16

What cells release renin and why?

Answer 16

When sodium levels are low in the tubular fluid of the distal convoluted tubule, macula densa cells cause juxtaglomerular cells (granula cells) of afferent arteriole to secrete renin into blood

Question 17

What does renin mediate the conversion of?

Answer 17

Converts angiotensinogen into angiotensin I

Question 18

How is angiotensin I converted to angiotensin II?

Answer 18

ACE enzyme

- angiotensin converting enzyme

Question 19

What is the effect of angiotensin II on the adrenal cortex?

Answer 19

Acts on Zona glomerulosa cells of the adrenal cortex which secretes aldosterone into the blood

Question 20

What does aldosterone stimulate the synthesis of?

Answer 20

Synthesis and insertion of epithelial sodium channel (ENaC) and a sodium pump in distal tubule and collecting duct cells
- promotes Na+ reabsorption from tubular fluid and K+ secretion to tubular fluid

Question 21

How does the renin-angiotensin-aldosterone system contribute to the regulation of sodium balance?

Answer 21

All of the mechanisms work independently of each other to regulate osmolality of bodily fluids and extracellular fluid volume

Question 22

What is the atrial natriuretic peptide (ANP)?

Answer 22

Works opposite to the ADH system + RAA system

Question 23

How does ANP work?

Answer 23

• Increase in total body Na+ increases plasma volume
• Increased blood volume is sensed by atrial myocytes and stimulates release of ANP
• Inhibits secretion of renin, aldosterone and ADH
• Causes vasodilation thereby decreasing vascular resistance