Maintaining Water and Nitrogen Balance

Maintaining Water Levels 

Controlling the balance between water loss and gain in the body is therefore an essential bodily process. Humans ingest water through food and drink and also produce it as a by-product of respiration. Water is lost from the body uncontrollably in your breath when you exhale and also in sweat (along with some ions and urea). 

The kidneys tightly control water loss via the urine to ensure the body contains the optimum amount of water for cells to function.

Maintaining Nitrogen Levels

Humans ingest proteins in food that get digested into amino acids and absorbed into the blood. As amino acids and proteins cannot be stored in the body, any excess amino acids must be excreted. The body must first convert them into urea, which is a two step process:

1. The amino acids are deaminated in the liver to form ammonia. This means the nitrogen–containing part (amino part) of the molecule is removed from the rest of the molecule. 
2. Ammonia is toxic so the body immediately converts it into urea which can be excreted safely via the kidneys.

Kidney Function

The kidneys are responsible for removing waste products and excess molecules from the blood. This forms urine which is then stored in the bladder and released from the body through the urethra.

The kidneys begin by filtering the blood, removing urea (from broken down proteins), ions, glucose and excess water.

Then they undergo selective reabsorption which is where useful molecules like glucose, some ions and water diffuse back into the blood. This ensures that the body keeps the right amount of water and other molecules that is needs to function. 

Anti-Diuretic Hormone (ADH)

ADH is a hormone that increases the permeability of the walls of the kidney tubules to water. This means more water is reabsorbed into the blood and the water content of the blood increases. 

The amount of water in the blood is controlled by changing the amount of ADH that gets released, in a negative feedback cycle:

Kidney Failure

Kidneys can stop working due to damage or disease. This causes waste to build up in the body, the balance of water, glucose and ions to be disrupted and can even be fatal.  

Luckily, the body can survive with only one functioning kidney and advancements in treatments have made the condition more manageable. 

 

Dialysis is a common treatment for kidney failure.

The patient’s blood flows from an artery in the arm into a dialysis machine, where waste products get filtered out and the balance of ions and water is restored, before being returned to the body via a vein in the arm.

The patients blood is mixed with blood thinners to prevent clotting and aid the movement of the blood through the machine. 

Inside the dialysis machine, there is a partially permeable membrane that separates the blood from dialysis fluid. The dialysis fluid has a similar concentration of glucose, water and ions to the blood and no urea.

As the dialysis fluid flows in the opposite direction to the blood, concentration gradients are created and substances are exchanged between the two fluids. Urea, excess ions and excess water is removed from the blood and glucose remains in the blood for use in the body.

Disadvantages:

• It is a very time-consuming and restrictive process; patients have to have dialysis many times a week and each session can take several hours. 
• The equipment is very specialised and expensive.
• Diet must be tightly controlled between sessions. 
• Will be effective for a limited time before a transplant is required. 

 

Kidney transplants can cure kidney failure. 

Healthy kidneys from donors are transplanted into patients whose kidneys do not function well. 

Kidneys can be donated from the recently deceased or from living people with two kidneys who are willing to sacrifice one of theirs (this comes with some risks to the donor).

Transplanted kidneys are often rejected by the patients immune system but drugs can be given to prevent this.

Transplants are cheaper than dialysis in the long run and save patients a lot of time as they will no longer require dialysis.

Unfortunately, waiting lists for kidney transplants are very long and dialysis is often used to keep patients alive until one becomes available.