Water and Nitrogen Balance

This section explains water and nitrogen balance, including excretion, the kidneys and their function and the structure and function of nephrons. 

Excretion

Excretion is the process of removing waste products produced by the body’s metabolism. It is essential for maintaining homeostasis and preventing harmful substances from accumulating. The main excretory products include:

• Urea – A toxic waste product formed from the breakdown of excess amino acids in the liver. It is removed from the body in urine via the kidneys.
• Sweat – Produced by sweat glands in the skin, containing water, salts, and a small amount of urea.
• Carbon Dioxide (CO₂) – A waste product of respiration excreted through the lungs.
• Water – Lost through urine, sweat, and breathing to help regulate body fluid levels.

The Kidneys and their Function

The kidneys are vital organs responsible for filtering waste from the blood and maintaining water and ion balance. Each kidney contains millions of tiny filtering units called nephrons (kidney tubules) that perform this function.

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Structure and Function of the Nephrons

• Ultrafiltration – Blood enters the kidney under high pressure, forcing small molecules such as water, urea, glucose, and salts into the nephron. Larger molecules like proteins and blood cells remain in the blood.
• Selective Reabsorption – Useful substances such as glucose, some ions, and water are reabsorbed back into the bloodstream.
• Urine Formation – The remaining urea, excess salts, and water form urine, which is transported to the bladder for excretion.

How Urea is Formed

Urea is produced in the liver through a process called deamination:

• Protein Digestion – When we consume more protein than needed, excess amino acids cannot be stored by the body.
• Deamination – The liver removes the nitrogen-containing part of amino acids, converting it into ammonia.
• Urea Formation – Ammonia is highly toxic, so it is converted into urea, a less harmful substance, which is then transported in the blood to the kidneys for excretion.

Kidney Failure

If the kidneys stop functioning properly, waste products like urea build up in the blood, leading to serious health issues. Kidney failure can be treated using:

• Dialysis – A machine removes waste, excess salts, and water from the blood, acting as an artificial kidney.
• Kidney Transplant – A healthy kidney from a donor replaces the failing kidney, allowing normal function to resume. Patients must take immunosuppressant drugs to prevent rejection.

Maintaining a balance of water and nitrogenous waste is crucial for survival, and the kidneys play a key role in this process. Understanding these mechanisms is essential for studying homeostasis and human physiology in Biology.