- THE ANATOMY OF THE URINARY SYSTEM
This is the system which is used to get rid of urea and other waste products from the body. These are carried to the urinary system by the bloodstream, filtered out and excreted as urine. It is a very important system because it is the only way that waste material can be removed from the body. The waste material which is left in the large intestine at the end of the process of digestion, and which is passed out through the anus, is material that has not been digested. It is the ‘left overs’, and does not contain any material passed back into the intestines from the body.
The urinary system is made up of:
- 2 kidneys
- 2 ureters
- The bladder and
- The urethra
Their arrangement is shown in the diagram below:
These are located in the abdominal cavity, halfway down the back, one on either side of the spinal column or backbone. They are quite large organs, a sheep’s kidney being about the size of one’s fist, and as you can see from the diagram above, they are shaped like a bean. They are well supplied with blood vessels, carrying blood to the kidneys by the renal artery, and away by the renal vein. There is also a complex system of tubes called nephrons which collect the urine as it is formed. These all lead into collecting ducts which empty into the kidney pelvis that joins the ureter. The kidney is divided
into the cortex on the outside, and the medulla on the inside. The two diagrams below show the blood system and the system of nephrons in the kidney, but remember that these diagrams are greatly simplified.
Figure 1: The Kidney
These are muscular tubes which carry urine from the kidneys to the bladder. As the ureter enters the bladder a valve prevents the urine from flowing in the wrong direction.
This is a hollow organ with muscular walls which can expand or contract depending on how much urine it holds. It is fed by the two ureters leading from the kidneys and acts as a reservoir for urine. When it is full, the urine is released through the urethra which leads to the penis in males and the vagina in females. The urine is then passed out of the body.
- 2. THE PHYSIOLOGY OF THE URINARY SYSTEM
The filtering out of waste products from the blood, the formation of urine and the re-absorption of water takes place in the nephrons of the kidneys. Small arteries leading from the renal artery divide up to form a tuft of capillaries called a glomerulus.
This is surrounded by a capsule known as Bowman’s capsule which is hollow and leads into a winding tube called the convoluted tubule. This, in turn, connects up to a U-shaped tube called the Loop of Henle, which leads to another set of convoluted tubules and into a collecting duct connecting up to the ureter. Look at the large diagram on the next page and you will see all these parts clearly marked.
Figure 2: The Kidney Nephron
Blood coming into the glomerulus capillaries is under high pressure carrying the waste products from the rest of the bloodstream. Due to the high pressure, the waste products are forced from the blood into the hollow tube of Bowman’s Capsule; this is done by hydrostatic pressure. This liquid, known as a filtrate, then passes through the convoluted tubule, around the Loop of Henle, through the other set of convoluted tubules and into the collecting duct. Once it enters the collecting duct it has become urine. However, during the passage of the filtrate through the various tubes, anything still required by the body, such as glucose and salts, is re-absorbed back into the bloodstream with nearly all the water. This re-absorption is mainly done by osmosis. In humans, about 180 litres of fluid is filtered into the Bowman’s Capsule every day, but, under normal circumstances, man expels only 1 litre of fluid as urine each day. About 99% of the fluid forced into the tube system is re-absorbed, the water passing back into the blood as it passes through the Loop of Henle. Waste products are not re- absorbed, and as the filtrate travels around the system, it becomes more and more concentrated until it is classed as urine. Urine carries all the waste nitrogen from the body, together with some waste salts.
Figure 3: A Kidney Nephron showing how water, glucose and salts pass between the Nephron and the bloodstream
The amount of water re-absorbed back into the blood is controlled by a hormone called the anti- diuretic hormone or A.D.H. If the blood becomes too concentrated with salts and glucose, A.D.H. is released increasing the amount of water passing from the filtrate into the blood. This continues until the concentration of salts in the blood has reached the right level.