Question

Please briefly discuss each of these topics

1. Describe the filtration membrane, its three parts and their characteristics.

2. Describe the pressures that promote and oppose glomerular filtration.

3. What is GFR? Discuss the three mechanisms that regulate /influence glomerular filtration rate.

4. Discuss tubular reabsorption in the proximal convoluted tubule. What is the importance of sodium reabsorption as far as reabsorption of other solutes is concerned?

5. What are the substances that are secreted during the process of tubular secretion?

Answer 1

1) Glomerular filtration membrane - it is a membrane that is used to separate two fluids , in this case - the blood in capillaries and the fluid in bowan's capsule.

• 1) Endothelium of capillary- It is perforted with number of holes , which is known as fenestrae. These endothelial cell lining has a negative charge.
• 2)Basement membrane - it is the layer just below the endothelium and consists of collagen and proteoglycan meshwork, so as to have large space between them for fluid filtrate. the proteoglycan have strong negative electric charge.
• 3)Podocytes- which are the epithelial cells , which forms outer boundary of filtration membrane. They are discontinous as they have long foot like processes, which encircle outer surface of capillaries. These foot processes interdigitate with adajacent foot processes causing the formation of large gaps in between them- which is the filtration slit. Podocytes also have negative charge.

2) Filtration at glomerulus is governed by 4 starling forces:

1) Glomerular hydrostatic pressure- favours filtration - 60mmHg

2) Glomerular capillary colloid osmotic pressure- opposes filtration - 32mmHg

3) Bowans capsule hydrostatic pressure - opposes filtration - 18mmHg

4) Bowans capsule colloid osmotic pressure- favours filtration. - 0mmHg( is considered as 0, due to the absence or very low amount of protein in bowans space)

3)

• The rate at which glomerular filtrate is formed per minute is called glomerular filtration rate.
• Normally - 125mL/min or 180L/day
• GFR is regulated by;
• 1) Intrinsic feedback control of kidney- Autoregulation
• GFR are RBF remains relatively constant even there is large change, which occur in mean systemic arterial pressure. If mean arterial pressure is between 80-180, change in GFR will be less than 10%. This is because of intrinsic mechanism of kidney, which is independent of outside nerve supply. It is known as autoregulation.
• 2) Symphathetic Nervous System
• Strong activation of symphathetic nerves of kidney, results in contraction of renal arterioles and there by decrease renal blood flow and GFR. A moderate to mild stimulation of sympathetic nerves has little influence on renal blood flow and GFR
• 3) Hormone and Vasoactive substances;
• They either constrict or dilate afferent and efferent arteriole, which cause change in GFR and renal blood flow
• a) Norepinephrine and epinephrine- constriction, GFR decreases.
• b)Endothelin - decrease GFR
• c)EDRF - prevents excessive vasoconstriction, increase GFR
• d) Prostaglandin and Bradykinnin - increase GFR
• e)Agiotensin II- Prevent decrease in GFR

4) Transportation of water and solute from tubular fluid to PCT is highly selective. Substances are abosorbed through tubular epithelial layer to renal interstitium from there, absorbed to PCT, and finally to blood. Reabsorption can takes place through the cells known as transcellular pathway or can be between the cells. In PCT, along wiith water K, Mg, Cl are also reabsorbed. Reabsorption can be active or passive.

Sodium reabsorption - 65% is reabsorbed in PCT, through 3 pathways:-

a)sodium co transport, - Seen in first half of PCT along with glucose aminoacids and other solutes

b)sodium-hydrogen counter transport, - Seen in first half of PCT

c) chloride driven sodium transport. - distal portion of PCT, occurs through paracellular pathway.

Sodium is a important extracellular electrolyte and can influence blood pressure and blood volume than any other solutes.