Title: The Urinary System
Key words: Excretion, homeostasis, urine, urea, kidney, ureter, bladder, nephron, renin/angiotensin system, filtration, reabsorption, sodium, aldosterone
Date: Aug 2000
Category: 6. The Body
Author: DJE Candlish
The Urinary System
The urinary system has three overall functions:
The waste products involved are mainly the nitrogen-containing (nitrogenous) byproducts of cellular and protein metabolism, urea and uric acid. These substances would become toxic if they were not removed from the body.
The system itself consists of two kidneys, two ureters, the bladder and the urethra.
The kidneys are located on either side of the spine, protected by the lower ribs and a layer of fatty tissue. Anatomically, they are divided into the cortex, medulla and pelvis, but the basic functional unit of the kidney is the nephron (see figure X). This has two components, the glomerulus, which 'filters' the blood and the tubule, which regulates water and electrolyte balance.
The glomeruli are located in the renal cortex, but the tubules descend into the medulla. Each tubule consists of a proximal tubule, the loop of Henlé with ascending and descending limbs, the distal tubule and collecting tubule.
The kidney has four specific functions:
Filtration takes place in the glomerular capillaries. Here, blood pressure is high, as the renal artery connects directly to the capillary bed. This high pressure forces small molecules, like glucose, amino acids, salts (electrolytes) and nitrogenous waste through the capillary walls into the Bowman's capsule and then on to the renal tubules. Large molecules and cells remain in the blood. In the average human, the kidneys filter around 170 litres of fluid a day, almost all of which is reabsorbed. Normal urine output is around 1-2 litres daily.
The cells of the renal tubule remove any excess of substances such as urea, uric acid and creatinine from the blood. These are actively secreted into the lumen of the tubules and so into the urine to remove them from the body. The levels of these substances in the blood are kept fairly constant by this process.
The renal tubules also reabsorb various substances, including glucose, water, amino acids and essential salts, to maintain the normal balance and avoid loss into the urine.
More water is reabsorbed in the collecting tubule if the blood is too concentrated. The remaining water and waste products enter the pelvis of the kidney as urine, which is passed down the ureters to the bladder.
The ureters are muscular tubes which connect the kidneys to the bladder and can 'push' urine down by peristalsis or rhythmic muscle contractions, just like swallowing food.
The bladder is a flexible storage organ for urine. The bladder stores urine until it is full and then the sphincter or valve around the urethra relaxes, allowing the urine to be passed out of the body. Stretch receptors in the bladder wall indicate when it is full and trigger the desire to urinate. Urine leaves the body via the urethra. In women, the urethra is relatively short, so bladder infections due to bacteria ascending the urethra are quite common.
The kidneys and homeostasis
To function normally, the cells in the body need a regulated environment, within a narrow range of normal values for pressure, temperature, pH (acidity or alkalinity) and chemical concentration. The kidneys play a part in homeostasis, the overall process that maintains these values, by eliminating nitrogenous waste and excess electrolytes, water and acid ions.
When the body contains excess water, the kidneys produce more dilute urine and when the body is dehydrated, the urine is made more concentrated. A number of factors may be involved in altering urine concentration.
When the body loses a lot of water, due to vomiting or diarrhoea, for example, the pituitary gland in the brain releases anti-diuretic hormone (ADH). This increases the permeability to water of the distal tubules and collecting ducts. The amount of water reabsorbed is increased and the urine becomes more concentrated, until the normal balance is reached. At this point, the release of ADH is inhibited.
The renin/angiotensin system
Renin is an enzyme produced by the kidneys which plays a role in blood pressure regulation. In the blood stream, renin activates angiotensin, producing angiotensin I. Next, this is converted to angiotensin II by an enzyme present in both the lungs and kidneys.
Angiotensin II is a potent vasoconstrictor (makes blood vessels constrict) which can reduce blood flow in the renal artery and capillaries, reducing urine output. Angiotensin II also stimulates the release of aldosterone from the adrenal glands above the kidneys, causing a rise in blood pressure. Aldosterone increases the reabsorption of sodium and water by the kidneys, effectively increasing blood volume.
Normally, any increase in blood volume tends to cause a compensatory increase in sodium excretion in the urine and a decrease in blood volume has the opposite effect.