Glomerular filtration, reabsorption, secretion and Loop of Henle

Question 1

What are the functions of the kidney?

Answer 1

Filtration

Secretion

Control fluid and electrolyte balance

Reabsorption of metabolites

Question 2

What is the mechanism of glomerular filtration?

Answer 2

1. Filtration occurs via hydrostatic forces favouring filtration and oncotic pressure forces favouring reabsorption (Starling’s forces)
2. Filterability across the glomerular filtration barrier is determined by molecular size, electrical charge and shape
3. Golden rule of circulation:
   
   1. A high resistance produces a high hydrostatic pressure upstream.
   2. A high resistance produces a low hydrostatic pressure downstream.
4. The afferent and efferent arterioles contribute to very high Pgc – (pressure of glomerular capillaries)
5. This high Pgc means that the oncotic pressure is always exceeded, and means ONLY filtration occurs
6. Pgc >> IIP (oncotic pressure), exceeds by 10mm Hg

Question 3

What is the mechanism of tubular reabsorption?

Answer 3

1. As 20% of plasma is filtered into Bowmans capsule, this means that the plasma protein concentration increases, therefore raising oncotic pressure.
2. As a result of the low pressure in the peritubular capillaries and the high oncotic pressure, Starling’s forces are in favour of reabsorption.
3. Most substances are reabsorbed by carrier mediated transport systems, e.g. glucose, amino acids, organic acids, sulphate and phosphate ions
4. Na+ is reabsorbed by active transport (Na+ pumps).

Question 4

What is the mechanism of tubular secretion?

Answer 4

1. Secretory processes move substances from the peritubular capillaries into the tubule lumen
2. Normal ECF K+ conc is 4mmoles/l, > 5.5mmoles/l = hyperkalaemia which leads to a decrease in resting membrane potential.
3. If K+ < 3.5mmoles/l = hypokalaemia leading to an increase in resting membrane potential.
4. K+ secretion is regulated by aldosterone hormone

Question 5

What are the 3 basic renal processes?

Answer 5

1) Filtration
2) Secretion
3) Reabsorption

Question 6

Why does only filtration occur at glomerular capillaries

Answer 6

1. Because the glomerular capillary pressure ALWAYS exceeds the oncotic pressure by around 10

Question 7

Why does only reabsorption occur at the peritubular capillaries?

Answer 7

1. Because the oncotic pressure is higher than the pressure in the peritubular capillaries.
2. High oncotic pressure is caused by the removal of 20% of plasma volume at the glomerular capillaries therefore increasing the plasma protein concentrations.

Question 8

What factors can influence glomerular filtration?

Answer 8

1. Radius of the afferent and efferent glomerular capillaries
2. The radius of the capillaries is controlled by:
   
   1. Sympathetic constriction of afferent and efferent arterioles
   2. Circulating catecholamines (primarily the constriction of afferent arterioles)
   3. Angiotension II leads to constriction of efferent arterioles at low plasma concs of angiotensin II, and constriction of both afferent and efferent at high angiotensin II plasma concs.

Question 9

What is meant by renal plasma threshold for glucose?

Answer 9

1. This is the level of glucose in the plasma above which, glucose would be excreted into the urine
2. The renal plasma threshold for glucose is 10mmol/l

Question 10

What is the significance of the active reabsorption of Na+ ions at the proximal tubule?

Answer 10

1. Reabsorption of Na+ is critical as it sets up a gradient for other ions to be reabsorbed and retained in the plasma.
2. Na+ conc in plasma is also important for maintaining carrier mediated transport of certain molecules like glucose, i.e. low Na+ conc. Results in decreased glucose transport

Question 11

How are K+ ions handled by the kidney?

Answer 11

1. K+ secretion is controlled by the aldosterone hormone
2. An increase in K+ plasma conc. Stimulates aldosterone release, which travels to the kidneys to stimulate an increase in renal tubule K+ secretion.

Question 12

What are the essential features of the Loop of Henle that enable them to act as counter current multipliers?

Answer 12

1. The ascending limb of Loop of Henle actively co-transports Na+ and Cl- out of the tubule lumen into the interstitium. The ascending limb is also impermeable to H20
2. The descending limb is freely permeable to H20 but mostly impermeable to NaCl ions.

Question 13

Why is it important to maintain K+ homeostasis?

Answer 13

1. K+ plasma conc is essential for life
2. Hyperkalaemia can lead to a decrease in the resting potential of excitable cells and eventually ventricular fibrillation and then death.
3. Hypokalaemia leads to an increase in the resting potential of excitable cells, which hyperpolarizes cardiac muscle, leading to arrhythmias and eventually death.

Question 14

Explain how hyperglycaemia in DM causes osmotic diuresis?

Answer 14

1. Hyperglycaemia is excess glucose in the blood.
2. As this blood passes through the kidneys, the excess glucose can block up the tubules of the kidneys.
3. The blocking of the tubules by glucose can result in a lack of water being reabsorbed in the descending loop of henle.
4. The lack of water reabsorption means that there is excess water in the tubules, and therefore the tubules leading to increasing volume of urine and increased frequency of passing urine.