The kidney's structure, functions, and what controls these functions.

The kidney is made up of nephrons, which are a kidney's functional units. These nephrons collect fluid filtered from the blood. The kidney connects to the renal artery, renal, vein, and ureter. Purified blood leaves the kidney using the renal vein, urine leaves using a ureter and the renal artery carries blood from the aorta to the kidney.

The nephron has a cup-shaped nephric capsule that surrounds a cluster of capillaries called the glomerulus. A good deal of fluid from the blood filters into the capsule. Large proteins and whole blood cells are left behind due to the fact that their too big to pass through the filters along with the plasma or blood fluid.

There are four main parts of the nephron tubule: the proximal convoluted tubule, the U-shaped loop of Henle, the distal convoluted tubule, and the collecting duct. A substantial amount of resorption takes place in the proximal convoluted tubule. The small proteins, glucose, and ions are returned to the blood by active transport. If the glucose in the filtrate, or filtered fluid, exceeds the kidney threshold level, some glucose will remain and appear in the urine. The loops of Henle permit the production of rather concentrated urine. The collecting duct, together with the loop of the Henle, plays a vital role in water balance.

The concentration of urine takes place in the collecting ducts although the process depends on the activities in the loops of Henle. The loops of Henle and the collecting ducts are in the medulla of the kidney. The other parts of the nephron are outside the medulla in the outer region, the cortex. There's a fluid in the medulla that contains an osmotic gradient in which solutes are steadily more concentrated in the direction away from the cortex. There are two kinds of solutes in the gradient: salt and urea. The loops of Henle create a salt gradient which actively transport salt ions out of the filtrate.

The actual concentration of the urine takes place in the collecting duct after the urine passes through the distal convoluted tubule from the loop of Henle. The urine leaves the collecting duct through the pelvis of the kidney and down the ureter to the urinary bladder.

Urine composition and rate of urine formation is regulated by the hormones vasopressin, aldosterone, and angiotensin, and the enzyme renin. The posterior pituitary gland in the brain releases vasopressin. Vasopressin increases resorption of water from the urine. When the body is losing water the body stimulates vasopressin secretion in result slowing down the loss of water through urine. The body detects water decrease either by lower blood volume (i.e. bleeding) or an increase in concentration of blood plasma (i.e. sweating). Very little water is resorbed from urine when there is an absence of vasopressin.

Water volume depends on the amount of salt; therefore the amount of salt in the body directly affects the volume and concentration of the blood. When the salt levels in the blood increase the adrenal gland in the kidney secretes a hormone called aldosterone. Aldosterone causes the body to resorb sodium by the distal tubule. The rate is determined by blood salt content. Since less salt is resorbed by the kidney the urines salt content increases therefore increasing urinary volume. When the sodium levels in the blood decrease the kidneys secrete and enzyme called renin. Renin converts the plasma protein angiotensinogen into the hormone angiotensin. Angiotensin causes the constriction of blood vessels to increase the aldosterone secretion. Then the aldosterone promotes the resorption of sodium by the kidney and the urine's salt content decreasing urinary volume.