The Human Excretory System
The urinary system is made-up of the kidneys, ureters, bladder, and urethra. The nephron, an evolutionary modification of the nephridium, is the kidney's functional unit. Waste is filtered from the blood and collected as urine in each kidney. Urine leaves the kidneys by ureters, and collects in the bladder. The bladder can distend to store urine that eventually leaves through the urethra.
Human excretory system and the details of the kidney. Images from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates and WH Freeman , used with permission.
The Nephron
The nephron consists of a cup-shaped capsule containing capillaries and the glomerulus, and a long renal tube. Blood flows into the kidney through the renal artery, which branches into capillaries associated with the glomerulus. Arterial pressure causes water and solutes from the blood to filter into the capsule. Fluid flows through the proximal tubule, which include theloop of Henle, and then into the distal tubule. The distal tubule empties into a collecting duct. Fluids and solutes are returned to the capillaries that surround the nephron tubule.
The nephron has three functions:
Glomerular filtration of water and solutes from the blood.
Tubular reabsorption of water and conserved molecules back into the blood.
Tubular secretion of ions and other waste products from surrounding capillaries into the distal tubule.
Nephrons filter 125 ml of body fluid per minute; filtering the entire body fluid component 16 times each day. In a 24 hour period nephrons produce 180 liters of filtrate, of which 178.5 liters are reabsorbed. The remaining 1.5 liters forms urine.
Urine Production
Filtration in the glomerulus and nephron capsule.
Reabsorption in the proximal tubule.
Tubular secretion in the Loop of Henle.
Components of The Nephron
Glomerulus: mechanically filters blood
Bowman's Capsule: mechanically filters blood
Proximal Convoluted Tubule: Reabsorbs 75% of the water, salts, glucose, and amino acids
Loop of Henle: Countercurrent exchange, which maintains the concentration gradient
Distal Convoluted Tubule: Tubular secretion of H ions, potassium, and certain drugs.
Kidney Stones
In some cases, excess wastes crystallize as kidney stones. They grow and can become a painful irritant that may require surgery or ultrasound treatments. Some stones are small enough to be forced into the urethra, others are the size of huge, massive boulders (or so I am told).
Kidney Function
Kidneys perform a number of homeostatic functions:
Maintain volume of extracellular fluid
Maintain ionic balance in extracellular fluid
Maintain pH and osmotic concentration of the extracellular fluid.
Hormone Control of Water and Salt
Water reabsorption is controlled by the antidiuretic hormone (ADH) in negative feedback. ADH is released from the pituitary gland in the brain. Dropping levels of fluid in the blood signal the hypothalamus to cause the pituitary to release ADH into the blood. ADH acts to increase water absorption in the kidneys. This puts more water back in the blood, increasing the concentration of the urine. When too much fluid is present in the blood, sensors in the heart signal the hypothalamus to cause a reduction of the amounts of ADH in the blood. This increases the amount of water absorbed by the kidneys, producing large quantities of a more dilute urine.
Aldosterone, a hormone secreted by the kidneys, regulates the transfer of sodium from the nephron to the blood. When sodium levels in the blood fall, aldosterone is released into the blood, causing more sodium to pass from the nephron to the blood. This causes water to flow into the blood by osmosis. Renin is released into the blood to control aldosterone.
Excrete toxic metabolic by-products such as urea, ammonia, and uric acid.
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