The main product of nitrogen excretion in humans is urea. It is synthesized in the liver from ammonia in a series of chemical reactions known as the urea cycle.
The excretions produced in our metabolism are eliminated by various organs, including the skin, lungs and especially the kidneys. Bile pigments, produced in the liver, are eliminated along with the feces, giving them the characteristic brown color.
Located below the diaphragm, near the back wall of the abdomen, the kidneys are the size of a closed fist and resemble a bean grain. Each of them when opened longitudinally, has a peripheral region, the renal cortex, and another more internal, the kidney marrow.
In the renal cortex are the functional units of the kidneys, the nephrons. Each nephron is a long, bent tubule with an initial cup-like portion, the Bowman's capsule. In each kidney there are more than 1 million nephrons.
The continuation of the capsule is the proximal contorted tubule, followed by henle handle (or thin segment) and a tubule twisted distal. This last portion ends in a collecting duct (or straight collecting tubule), where the distal tubules of the other nephrons end.
The urine formed in the nephrons flows through the collecting tubules towards the renal pelvis and from it into the ureters. The urine then descends into the bladder, which is capable of storing up to 800 ml of urine. Bladder emptying occurs with the flow of urine along a channel, the urethra, which runs down the penis or opens in the region in front of the vaginal opening.
The blood to be filtered enters the kidney through the renal artery. This artery undergoes several branches and its terminal branches, the afferent arterioles, originate renal glomeruli (also known as Malpighi's glomeruli) that penetrate Bowman's capsules. Each glomerulus is a network of highly branched capillaries through which the blood undergoes numerous branches and gives rise to a network of capillaries that surrounds the renal tubules and Henle's loop.
All capillaries will eventually flow into venules that, merging with each other, will form the renal vein, which will allow blood to return to the inferior vena cava that goes to the heart.
The blood volume that passes through the human kidneys is 1,200 mL / min.