How the kidneys filter blood and make urine

The urinary system has four organs: two kidneys, two ureters, one urinary bladder, and one urethra. The kidneys sit against the back wall of the abdomen, behind the peritoneum (retroperitoneal), on either side of the spine. Each has an outer renal cortex and an inner renal medulla, which contains cone-shaped renal pyramids.

Urine made in the kidney drains into cup-shaped calyces, which merge into the funnel-shaped renal pelvis. From there:

1. The ureter carries urine to the bladder. It does not just drip downhill: smooth muscle in its wall pushes urine along by peristalsis. Each ureter enters the bladder wall at a slant, which acts as a one-way valve against backflow.
2. The urinary bladder stores urine. Its stretchy lining is transitional epithelium, and its wall muscle is the detrusor muscle.
3. The urethra carries urine out of the body. The female urethra is only about 4 cm long, which helps explain why urinary tract infections are more common in females. The male urethra averages about 20 cm and passes through the prostate.

Two sphincters control voiding. The internal urethral sphincter is smooth muscle and involuntary. The external urethral sphincter is skeletal muscle and under voluntary control.

The nephron is the kidney's functional unit, and each kidney has about 1.3 million of them.

Blood arrives through the afferent arteriole, passes through the glomerulus, and leaves through the efferent arteriole, which feeds the capillaries around the tubule. Those capillaries pick up everything the tubule reabsorbs.

Filtration happens at the glomerulus. Blood pressure pushes water and small dissolved substances (salts, glucose, amino acids) through a filter into Bowman's capsule. Blood cells and large proteins are too big, so they normally stay in the blood. In some kidney disorders, too much protein slips through this filter and shows up in the urine.

Reabsorption moves useful substances from the tubule back into the blood. The kidneys filter up to about 180 liters a day but reabsorb about 99 percent of it, so you make only about 1 to 2 liters of urine. The PCT does most of this work: when sodium is pumped out, water follows by osmosis.

Secretion moves extra substances from the blood into the tubule so they leave in urine, such as potassium, hydrogen ions, ammonia, creatinine, and some drugs. Secreting or reabsorbing hydrogen ions and bicarbonate is how the kidneys help control blood pH.

Application: glucose transporters in the PCT have a top speed. If blood glucose climbs very high, as in uncontrolled diabetes mellitus, the transporters are overwhelmed and glucose spills into the urine.

Antidiuretic hormone (ADH) comes from the posterior pituitary. When the blood becomes too concentrated, as with dehydration, ADH rises and inserts water channels called aquaporins into collecting duct cells. More water returns to the blood, so urine becomes smaller in volume and more concentrated. Without ADH, a large volume of dilute urine results.

Aldosterone comes from the adrenal cortex. It makes the DCT and collecting ducts reabsorb more sodium, and water follows the sodium, which raises blood volume and blood pressure. At the same time it increases potassium loss in urine.

A drop in blood pressure or fluid volume triggers this chain. The kidney releases renin, which leads to angiotensin II. Angiotensin II constricts blood vessels and signals the adrenal cortex to release aldosterone.

Beyond making urine, the kidneys help control blood pressure (through renin). They also stimulate red blood cell production (through the hormone erythropoietin, EPO). And they convert vitamin D to its active form, which supports calcium absorption.