Question

Consider this an essay on how the kidney functions! Terrestrial animals can concentrate their urine by establishing an osmotic gradient in the kidney. Describe how the osmotic gradient is established in the kidney and how it is used to concentrate the urine in response to hormonal signaling. A diagram would help with your answer.

Answer 1

The osmotic gradient is established in the kidney to concentrate the urine in terrestrial animals and this mechanism is also called as Counter current mechanism. This mechanism takes place as the response of ADH (Antidiuretic Hormone). This ADH also known as Vasopressin.

Regulation of concentration of urine: As the loss of water occur, the water concentration decreases and thus the ADH releases from the posterior pituitary lobe. This hormone increases the reabsorption of water in the distal convoluted tubule and the collecting duct as they become more permeable to the water. Less filtration and more reabsorption of water and less discharge of hypertonic urine raises the water concentration in the blood and other body fluids.

As the body fluids become more diluted, then ADH releasing is stopped. Excess of water signals the posterior pituitary lobe to stop the release of ADH. Deficiency of this hormone lowers the permeability of the cells of the distal convoluted tubule and the collecting duct thus increases the reabsorption of water. More filtration and less reabsorption of water produce abundant dilute urine, so the water concentration of the urine becomes normal.

Loss of water   Serum tonicity increases ADH releases Increased tubular reabsorption of water dilution of body fluids.

Excess of water Inhibition of ADH release water permeability of cells decreases more filtration decreases the water concentration of body fluids.