Question

-Understand the order of blood flow and filtrate flow in the kidneys. How do molecules enter the glomerulus, where are they filtered, where are they reabsorbed, and how are they excreted from the body?

-What factors regulate glomerular filtration rate (increase and decrease it)? How can urine formation be increased? What is a diuretic?

-How do freshwater fish regulate blood volume and salts? How do saltwater fish regulate blood volume and salts?

-How does the body respond to blood loss? (hormones, nervous system, cardiac output and contractility, heart rate, blood vessels) What causes death from hemorrhagic shock?

Answer 1

Proximal Convoluted tubule:

The main site for regulation of pH of urine and also for secretion and reabsorption.

Around two-third of water and NaCl are reabsorbed from PCT.

90% reabsorption of HCO3- ions occurs from the filtrate into the interstitial fluid.

There is also controlled secretion of H+ ions from the interstitial fluid to the tubule.

All the nutrients are reabsorbed completely from the PCT.

NH3+ is secreted into the lumen of the tubule to maintain the pH of urine or to counter the effects of secretion of H+ ions.

Descending Limb of Henle:

Its transport epithelium is rich in aquaporins and thus water is reabsorbed from this segment. However, this segment is virtually impermeable for salts.

The osmolarity of the interstitial fluid increases gradually and greater from cortex to medulla.

At the same time, NaCl concentration increases in the filtrate as water departs by osmosis.

Ascending limb of Henle:

The concentration reaches its peak at the elbow of LOH (1200 mOsm/L) and now the filtrate starts to move towards the cortex via ascending limb.

The thin segment of the ascending limb helps in the reabsorption of NaCl and thus the concentration starts to decrease in the ascending limb.

As the filtrate reaches into the thick segment, NaCl is absorbed actively. No water comes out from the ascending limb due to the absence of aquaporins.

By losing salts without water makes the filtrate more dilute as it reaches again into the cortex.

Distal convoluted tubule:

The NaCl is actively reabsorbed here and the water comes out into the interstitial fluid passively.

This portion is also important for the maintenance of pH as HCO3- ions are actively reabsorbed.

Cortical collecting duct:

This part is virtually impermeable to water. However, under the high concentration of ADH, this becomes permeable to water due to the fusion of aquaporins. Therefore, the permeability with respect to water is variable.

Under the influence of aldosterone, Na+ would be reabsorbed from the collecting duct, while potassium is secreted into the collecting duct.

The medullary portion of the collecting duct:

Its permeability to water is controlled by ADH. High levels of ADH causes reabsorption of water into the interstitial fluid.

This section is permeable to urea. (the only portion of nephron permeable to urea)

NaCl along with urea act as major osmolytes that maintain the concentration of interstitial fluid at 1200 mOsm/L in the lower medullary portion.