The urinary system
The urinary system — the kidneys, ureters, urinary bladder and urethra — preserves homeostasis by adjusting the composition of the circulating blood. The basic unit of the kidney is the nephron, in which urine is formed by filtration, resorption and secretion and whose loop of Henle allows the urine to be concentrated or diluted, and the urine drains from the kidney to the exterior.
What the system does
The urinary system, made up of the kidneys, ureters, urinary bladder and urethra, preserves homeostasis by adjusting the composition of the circulating blood: it conserves water, excretes metabolic wastes, reabsorbs useful compounds, and regulates pH and the electrolyte profile. The integument, the respiratory system and the digestive system assist it in excreting wastes and regulating the water and electrolyte composition of body fluids, but the urinary system has the dominant role, through its influence on the composition of the circulating blood.
The kidneys
The kidneys are located just above the waistline, and the right one is generally a little lower than the left because of the size and position of the liver. They are attached under the muscles of the back, and a heavy cushion of fat surrounds each one and helps hold it in place.
The nephron
The nephron is the basic structural and functional unit of the kidney. It is composed of the glomerulus and the renal tubule, which empty into a collecting tubule, a small tributary of a collecting duct. Its parts are the renal capsule, the proximal convoluted tubule, the descending and ascending limbs of the loop of Henle, and the distal convoluted tubule. The glomerulus is a network or cluster of blood capillaries inside Bowman's capsule, the cup-shaped top of the nephron.
Other important structures of the nephron include the proximal and distal convoluted tubules and the loop of Henle. The proximal convoluted tubule actively reabsorbs nutrients, plasma proteins and electrolytes from the filtrate. The loop of Henle has descending and ascending limbs, and the ascending limb delivers the urine to the distal convoluted tubule. The distal convoluted tubule opens into a collecting tubule, which joins its neighbours to form a collecting duct.
Urine formation
Urine is formed by three processes that occur in successive parts of the nephron.
Filtration is a continuous process in the glomerulus. Glomerular blood pressure causes water and dissolved substances to filter out of the glomeruli into Bowman's capsule. The glomerulus is the main site of filtration of wastes from the blood.
Resorption is the movement of substances from the renal tubules into the blood in the peritubular capillaries. The substances reabsorbed include water, nutrients and various ions.
Secretion is the movement of substances into the urine in the distal and collecting tubules from blood in the peritubular capillaries. Hydrogen ions, potassium ions and certain drugs are secreted by active transport, while ammonia is secreted by diffusion.
Concentration and dilution of urine
The loop of Henle has two parts, a thin descending limb and a thick ascending limb. The purpose of this structure is to allow both concentration and dilution of the urine, through the influence of hormones on distal structures.
In the thick ascending limb, which is impermeable to water, a sodium-potassium-chloride pump actively extracts these electrolytes from the tubular fluid. The descending limb, by contrast, is permeable. Because sodium and chloride are actively pumped out, the fluid arriving at the distal tubule is hypotonic, or dilute. The electrolytes in the interstitial tissue raise its osmolality, and water flows along the osmotic gradient from the descending limb into the interstitium. The fluid left in the tubule becomes hyperosmolar relative to normal plasma.
This mechanism is the countercurrent multiplier system, and its result is threefold. First, the high concentration of sodium and chloride, and of urea, in the medullary interstitium makes that part of the kidney hyperosmolar. Second, the fluid delivered to the distal convoluted tubule is hypotonic, so as it passes down through that tubule and the collecting duct it is exposed to very high osmolar pressures in the surrounding tissues. Third, extracellular fluid volume depends on the amount of sodium in the body, so the kidney must be capable of conserving sodium. If extracellular volume drops, a complex series of neurohormonal interactions leads to the release of aldosterone, which makes the collecting ducts permeable to sodium, and the sodium is absorbed.
The ureters, bladder and urethra
The ureters are narrow, long tubes whose expanded upper ends are located inside each kidney, and they drain urine from the renal pelvis to the urinary bladder. The bladder is an elastic muscular organ capable of great expansion, and it stores urine before voiding. The urethra is a narrow, short tube from the bladder to the exterior. It is the passageway by which urine leaves the bladder and, in men, also the passageway by which sperm and semen leave the body.