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The proximal convoluted tubule (PCT) is a small tubular structure within the nephron of the kidney. The PCT connects Bowman's capsule with the proximal straight tubule, and it is essential for the reabsorption of water and solutes from filtrate within the nephron. The lining of the PCT contains many protein channels, which use both active and passive transport to move substances such as glucose and electrolytes across the tubule's lining and into the interstitial fluid for reabsorption. The epithelium of the PCT also prevents waste products from being reabsorbed into the bloodstream.
The kidney contains about 4 million functional units called nephrons, which filter the blood. Each nephron consists of the renal corpuscle, the proximal convoluted tubule, the loop of Henle and the distal convoluted tubule. As blood flows through the capillaries in the renal corpuscle, about 20 percent of the plasma water is secreted into Bowman's capsule. This plasma water, known as glomerular filtrate, passes into the PCT. Protein channels within the lining of the PCT reabsorb some of the water and solutes from the glomerular filtrate.
The proximal convoluted tubules are lined with epithelial cells. These cells are connected to each other with tight junctions that prevent waste products and other potentially harmful substances from passing between the cells and reaching the bloodstream. Most of the substances that undergo tubular reabsorption are polar molecules, so protein channels are necessary for transporting them across the cell membrane. Each substance that undergoes tubular reabsorption has protein channels uniquely suited for it.
Numerous tiny folds called microvilli in the PCT lining increase the surface area, which maximizes the space available for protein channels. Protein channels use adenosine triphosphate (ATP) to actively transport solutes that are traveling against their concentration gradients, and passively transports those moving along the gradient. Sodium, chloride, glucose, potassium and bicarbonate are some of the solutes transported from the glomerular filtrate, across the membrane of the tubule lining, and into the interstitial fluid. About 70 percent of sodium and water reabsorption and 100 percent of glucose and amino acid reabsorption take place in the proximal convoluted tubule.
The increase in the concentration of solutes in the interstitial fluid causes it to become more highly concentrated than the glomerular filtrate. The resulting osmotic pressure causes water to passively flow across the membrane and into the interstitial fluid. The water and solutes then pass into the peritubular capillaries and back into the bloodstream. Glomerular filtrate that has not been reabsorbed passes from the proximal convoluted tubule into the proximal straight tubule and into the loop of Henle and the distal convoluted tubule. The proximal convoluted tubule reabsorbs substances the body needs, prevents waste from re-entering the bloodstream and assists downstream nephron structures by passing along fluid with the composition and concentration they need to function properly.
What I'm getting out of this is that the proximal convoluted tubule helps separate the useful fluids from the waste water that will eventually become urine. It sounds like the "small intestine" of the kidney, if that analogy would be accurate.