The urinary system

​​In a nutshell

The urinary system maintains water balance, removes excess substances absorbed from food and removes waste products from metabolism. Our kidneys play a key role in water absorption and this is controlled by the antidiuretic hormone. If our kidneys do not work correctly, this can lead to kidney failure. Kidney failure can be treated by dialysis or cured with a kidney transplant. 

Water balance

Our daily water intake can vary, but the amount of water in our body must stay constant. Maintaining the correct water balance is essential for good health. Water balance is important because the cell's water content is essential for the chemical reactions that happen there. 

A decrease in the water potential of the body fluids will lead to body cells losing water and shrinking. If the water potential increases, the body cells will take up water. Our cells are affected by an improper water balance, but our bodies show symptoms of inadequate amounts of water. This is known as dehydration. 

Structure of the urinary system

​There is also a muscle located above the urethra that keeps the exit of the bladder closed until we decide to urinate.​

Kidneys

The kidneys make urine by taking waste products out of your blood. These substances are filtered out of the blood in a process known as filtration. Useful substances like glucose, some ions and the correct amount of water are absorbed back into the blood in a process called selective reabsorption. 

Each kidney contains approximately one million tubes called nephrons, these nephrons make urine. Large molecules such as proteins cannot filter into the nephron, so they stay in the blood. Below is the structure of a nephron.

Urea- Higher tier only

Proteins cannot be stored in the body, therefore any excess amino acids are converted into fats and carbohydrates which can be stored. This happens in the liver in a process known as deamination.

Ammonia is a waste product from this process. Ammonia is toxic so it is converted to urea in the liver. Urea is transported to the liver where is it filtered out of the blood and excreted out of the body in urine. 

Ions

Ions such as sodium are taken into the body in food and then reabsorbed in the blood. If the ion or water content of the body is wrong, this could disrupt the balance of ions and water so too much or too little water is taken into the cells by osmosis. Having the incorrect amount of water can damage the cells and they might not work as well as normal. 

Some ions are lost in sweat, the amount of ions lost is not regulated and this is why the right balance of ions much be maintained by the kidneys. The right amount is absorbed into the blood after filtration and the rest is removed in the urine. 

Water

The body has to balance water coming in from food and drinking and water coming out from sweating, urinating and breathing out. We cannot control how much water we lose, so the amount of water has to be balanced by the amount we consume and the amount removed by the kidneys. 

Note: The following section is for students studying the higher tier specification.

Antidiuretic hormone (ADH)

Definition

ADH is a hormone released by the pituitary gland into the bloodstream. It controls the amount of water reabsorbed from the nephrons. 

It does this by increasing the permeability of cell membranes in the collecting duct of the nephron so that more water is reabsorbed from the urine by osmosis. This regulates the water content of the blood and is an example of osmoregulation. 

The brain monitors the water content of the blood. If the receptor detects the water content is too high, the pituitary gland releases less ADH so the collecting duct is less permeable and less water is reabsorbed from the kidney tubules, causing a large volume of dilute urine to be produced.  

If the brain receptor detects the water content is too low, the pituitary gland releases more ADH so the collecting ducts are more permeable and more water is reabsorbed from the kidney tubules. This causes a small volume of concentrated urine to be produced. This is another example of negative feedback. 

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Kidney failure

Kidney failure happens when the kidney stops working properly, so excess water, mineral ions and urea build up in the body.  Eventually, this leads to death. However, people with kidney failure can be kept alive by having dialysis treatment or a kidney transplant. 

Dialysis

Dialysis machines filter blood. It has to be done regularly (every $$2-3\ days$$​) to keep the concentration of dissolved substances at normal levels and to remove waste substances. Each session takes $$3-4\ hours$$​ and comes with serious risks such as blood clots and infections. It is also an expensive procedure for the NHS but it gives a patient with kidney failure extra time whilst they wait for a kidney transplant.

In a dialysis machine, the person's blood flows between partially permeable membranes that are surrounded by dialysis fluid. The membranes are permeable to ions and waste substances, but not big molecules like proteins. 

The dialysis fluid has the same concentration of dissolved ions and glucose as healthy blood to ensure useful dissolved ions and glucose won't be lost from the blood during dialysis. Only waste substances like urea and excess ions and water can diffuse across the barrier. 

Kidney transplants

Healthy kidneys from people who have died suddenly are transplanted into kidney failure patients and connected to their blood circulation. This is the only cure for kidney failure. Kidneys can also be transplanted from people who are still alive, as we have two, but there is a small risk for the donor. 

There is also a risk that the transplanted kidney can be rejected by the patient's immune system. This is because antigens on the transplanted kidney cells are different from the antigens on the patient's cells. The patient's immune system then produces antibodies that attack the transplanted kidney and reject it. 

To prevent this, the patient is treated with drugs that reduce the effects of the immune system known as immunosuppressants, but this increases their risk of infections. Another way to reduce the risk of rejection is by matching antigens on the donor organ to the host's cells.

Transplants are cheaper in the long run than dialysis and prevent patients from spending hours at the hospital to receive dialysis. However, there are long waiting lists.