Question

Describe how the kidney is important for osmoregulation in vertebrates. If you compared a kidney in a desert rodent to a rodent that lives in a more water-rich environment how might they differ?

Answer 1

The kidneys are the main osmoregulatory organs in mammalian systems. There function is to filter blood and maintain the osmolarity of body fluids at 300 mOsm. The nephron is the functional unit of the kidney, which actively filters blood and generates urine

There are three steps in the formation of urine

1.glomerular filtration, which occurs in the glomerulus

2.tubular reabsorption, which occurs in the renal tubules

3.tubular secretion, which also occurs in the renal tubules.

Glomerular Filtration

Due to high blood pressure and the “leaky” connections between the endothelial cells of the glomerular capillary network allow solutes to pass through easily. All solutes in the glomerular capillaries, except for macromolecules like proteins, pass through by passive diffusion. No energy is utilized in this process. Glomerular filtration rate (GFR) is the volume of glomerular filtrate formed per minute by the kidneys.

Tubular Reabsorption and Secretion

Tubular reabsorption occurs in the proximal convoluted tubule, a  part of the renal tubule. Almost all nutrients are reabsorbed back to the blood either by passive or active transport. Reabsorption of water and some key electrolytes influenced by hormones. Sodium (Na+) is the most abundant ion and most of it is reabsorbed by active transport. Because Na+ is actively transported out of the tubule, water follows it to even out the osmotic pressure.  Every solute has a limit of reabsorption and once the limit is reached the excess is not reabsorbed.

In the loop of Henle, different regions has different permeability The descending limb is permeable to water, not solutes; the opposite occurs for the ascending limb. Additionally, the loop of Henle in located the renal medulla, which is naturally high in salt concentration and tends to absorb water from the renal tubule and concentrate the filtrate. The osmotic gradient increases as it moves deeper into the medulla. Because two sides of the loop of Henle perform opposing functions, it acts as a countercurrent multiplier. The vasa recta around it acts as the countercurrent exchanger.

Once filtrate reaches the distal convoluted tubules(DCT), most of the urine and solutes have been reabsorbed. If the body requires additional water, then the remaining water can be reabsorbed at this point. Excretion of wastes occurs due to lack of complete reabsorption combined with tubular secretion. Undesirable products like metabolic wastes, urea, uric acid, and certain drugs, are excreted by tubular secretion. Most of the tubular secretion happens in the DCT, some occurs in the early part of the collecting duct also.  Kidneys also maintain an acid-base balance by secreting excess H+ ions.

Desert rodents do not readily find water, hence they must excrete very less amount of water. They are able to produce highly concentrated urine. They have juxtamedullary nephrons that is the Henle's loop of juxtamedullary nephron goes deep down into the medulla which helps in concentrating urine. Longer the Henle's loop, more amount of solute will be reabsorbed and hence more amount of water could be removed from filtrate.