Which structure of the kidney functions to reabsorb nutrients from the filtrate?

The proximal convoluted tubule

Explanation:

The proximal and distal convoluted tubules are coiled to increase the surface area for reabsorption. The filtrate from the glomerulus moves into the proximal convoluted tubule where selective reabsorption occurs. Here all glucose and 80% of salts are actively reabsorbed. Reabsorption of amino acids and vitamins also takes place here by active transport and diffusion and water by osmosis. The cells forming the tubules have numerous mitochondria which provide energy for active transport.

the proximal convoluted tubule

Explanation:

Each kidney contains about 4 million functional units, the nephrons. In each nephron a filtrate is made of the blood from which the urine is formed and which is used to remove waste substances. Broadly speaking, each nephron is composed of the renal glomerulus, Bowman's capsule, the proximal tubule, Hanle's handle, the distal tubule and the collecting tubules.

In the proximal tubule, a straight part can be distinguished, called a proximal right tubule or a straight pars, and a contoured part, the corresponding part with the proximal contoured tubule or convoluted pars (hence it is also known as a proximal convoluted tubule). The TCP is the portion of the proximal tubule located closest to the Bowman capsule and connects the Bowman capsule with the proximal right tubule, which in turn connects with the Henle handle. On a larger scale, TCP is located in the renal cortex while the straight part descends through the medullary rays to the medulla.

The proximal contoured tubule plays a fundamental role in the organism's homeostasis, playing an active role in the reabsorption of water, electrolytes and various organic compounds. This reabsorption is performed by active transport and passive transport through numerous protein channels of the epithelial surface. It occurs in all portions of the proximal tubule but is especially important in TCP. TCP also secretes various substances, including some organic acids, nitrogen bases and some waste substances that do not leak into the glomerulus, for example some medications.

The glomerular filtrate is collected in the Bowman capsule and goes directly to TCP where between 40 and 60% of the filtrate is reabsorbed. The filtrate substances with the greatest nutritional importance, especially glucose and amino acids, are almost completely reabsorbed by active co-transporters. Potassium is reabsrove in 65%, urea in 50% and phosphate and citrate around 80%.

Water and salt (ClNa) are reabsorbed in the proximal contoured tubule in a variable way, around 60%, depending on the conditions in which the organism is found, since water and ClNa are key in the regulation of body water volume Sodium is mainly reabsorbed in the proximal contoured tubule while chlorine is mainly reabsorbed in the straight pars, especially in the S3 segment, by passive diffusion.