Where In The Nephron Does Most Solute Reabsorption Occur?

Filtration, reabsorption, and secretion take place in the process of producing urine.The physiologic goal is to change the composition of the blood plasma, which, in turn, allows only waste to be eliminated as urine.I discussed filtrate formation in the last section.We will now examine how nutrients are selectively returned to the blood and how urine is regulated.

Reabsorption

In a day, up to 180 liters of fluid pass through nephrons of the kidney, such a volume must be reabsorbed.The proximal convoluted tubule, loop of Henle, distal convoluted tubule, and to a lesser extent, collecting ducts undergo reabsorption.

Nephrons reabsorb specific solutes and water differently depending on the portion of the tissue.The majority of reabsorption and secretion occurs passively due to concentration gradients, but the amount of water reabsorbed and lost is closely regulated.It is usually recovered in the proximal convoluted tubule, the loop of Henle, and the distal convoluted tubule.Around 10 percent (about 18 L) reaches the collecting ducts.Antidiuretic hormone and aldosterone regulate how much water is retained in urine.As a result of antidiuretic hormone, the collecting ducts can recover almost all of the water that passes through them, in the case of dehydration, or almost none of the water if the person is overhydrated.

1. Figure.Nephron secretion sites and reabsorption sites.There are arrows pointing away from the tubules, indicating substances that return to the blood.An arrow pointing to a tubule indicates that additional substances are being removed from the blood and moved into the filtrate.

Mechanisms of Recovery

In order for substances to move across membranes, they need to go through simple diffusion, facilitated diffusion, active transport, secondary active transport, and osmosis.

Substances diffuse up their concentration gradients from a higher to a lower concentration.No energy is required and the substance only needs to be soluble.

The process of facilitated diffusion is similar to simple diffusion in that it moves substances down concentration gradients.In contrast, it requires specific membrane transporters and channel proteins to move.It is important to note that facilitated diffusion occurs when glucose is transported or, in some circumstances, Na+ ions are moved.It is possible for two substances to share the same channel protein port in case of facilitated diffusion; these are known as symports and antiports.Across the cell membrane, symport mechanisms move two or more substances in the same direction, while antiport mechanisms move two or more substances in opposite directions.

.A membrane transporter must contain an appropriately shaped binding pocket to allow a substance to be transported.Na+ and K+ are transported actively by the Na+/K+ pump out and into the cell, respectively.A lower ion moves in the opposite direction to a higher ion as it is concentrated.

Symport and antiport can utilize concentration gradients maintained by ATP pumps.We call this a secondary active transport mechanism.Na+ ATPase pumps located on the cell's basilar membrane may keep Na+ constantly flowing out of the cell, maintaining a strong electrochemical gradient. .As a result, glucose diffuses from the basal membrane into the interstitial space, where it is subsequently absorbed in peritubular capillaries.

To maintain equilibrium, the nephron must reabsorb Ca2+, Na+, glucose, and amino acids.In addition, waste products like urea, K+, ammonia (NH3), creatinine, and some drugs are secreted into the filtrate.Both the lungs and kidneys regulate the acid-base balance of the body: The lungs remove H+ from the body, while the kidneys secrete or reabsorb H+ and HCO3–.About 50 percent of urea is passively reabsorbed by the proximal convoluted tubule.←.It triggers the insertion of urea transporters and aquaporin channel proteins.

Reabsorption and Secretion in the Proximal Convoluted Tubule