5.3.3 Maintaining Water and Nitrogen Balance in the Body

In this lesson, we will explore how the body maintains water and nitrogen balance, with a focusWhat the writer draws attention to at a given moment (e.g., setting, character, detail). on the digestion and excretion of excess amino acids. We will also explore the effects of osmotic changes in body fluids, as well as the routes through which water, ions, and urea are lost from the body. We will examine the important role of the kidneys and the antidiuretic hormone (ADH) in maintaining water balance in the body, as well as the methods used to treat kidney failure, specifically organA structure made of different tissues working together to perform a specific function. transplant and kidney dialysis.

Osmotic Changes and Cellular Function

OsmosisThe movement of water across a partially permeable membrane from a dilute solution to a more concentrated solution. is the movement of water across a semipermeable membrane from an area of lower solute concentration to an area of higher solute concentration. Changes in body fluids' osmotic balance can have significant effects on cellular function.

Water Loss from the Body

• Lungs: During exhalation, water vapour is lost from the lungs, resulting in water loss from the body.
• Skin: Water, ions, and urea are lost through sweat glands in the skin. Sweating helps regulate body temperature and is a mechanism for water and waste elimination.

The kidneys play a crucial role in maintaining water and nitrogen balance in the body. Excess water, ions, and urea are removed through the kidneys via urine.

• Proper water balance is essential for cells to function efficiently. If cells lose or gain too much water through osmosis, it can disrupt cellular processes and lead to impaired function.
• Nitrogen is a waste product generated from the breakdown of proteins in the body. The liver converts ammonia, a toxic byproduct of protein metabolismChemical reactions within a cell allowing microorganisms to maintain themselves and multiply., into urea. Urea is then excreted by the kidneys, ensuring nitrogen balance is maintained.

Protein Digestion and Excess Amino Acids (HT only)

Proteins are essential nutrients found in our diet and are composed of amino acids. During protein digestion, proteins are broken down into individual amino acids in the digestive system. Excess amino acids in the body need to be processed and excreted to maintain nitrogen balance.

The Role of the Liver:

The liver is a vital organ responsible for various metabolic processes, including the conversionThe act of changing someone's religion. of excess amino acids into urea. Excess amino acids undergo a process called deaminationA chemical change in DNA where a cytosine base is converted into uracil, often due to hydrolysis, leading to sequencing errors. in the liver, where the amino group (-NH₂) is removed.

Ammonia and Urea (HT only)

Deamination of amino acids results in the formation of ammonia (NH₃), a toxic substance that needs to be eliminated promptly.

Ammonia is highly toxic to cells and can damage various organs if allowed to accumulate. To ensure safe excretion, the liver converts ammonia into urea, a much less toxic compound. Urea is a water-soluble waste product that can be safely transported in the bloodstream to the kidneys for excretion.

Urea is filtered by the kidneys and excreted from the body in urine. The kidneys play a crucial role in maintaining nitrogen balance by regulating the excretion of urea.

The Kidneys and Water Balance

The primary function of the kidneys is to filter waste products from the blood and maintain water balance in the body. Water balance refers to the regulation of the body's water content, ensuring that the right amount of water is retained or excreted to maintain proper hydration.

The kidneys filter the blood through tiny structures called nephrons, which are the functional units of the kidneys. Within the nephrons, blood is filtered, and the filtrate is then processed to selectively reabsorb useful substances and excrete waste products as urine.

• Filtration: Filtration occurs at the glomerulus, a network of tiny blood vessels within the nephron. Blood pressure forces water, ions, and small molecules out of the blood and into the nephron. This initial filtrate contains water, ions, glucose, and other substances present in the blood.
• Selective Reabsorption: After filtration, the filtrate passes through the renal tubules, where selective reabsorption takes place. Selective reabsorption refers to the process by which the kidneys reabsorb useful substances, including glucose, ions, and water, back into the bloodstream. The reabsorption of water is crucial for maintaining water balance in the body.

The kidneys regulate water balance by adjusting the amount of water that is reabsorbed or excreted in urine. Hormones, such as antidiuretic hormone (ADH), play a crucial role in regulating the reabsorption of water in the kidneys.

The Role of ADH in Water Balance (HT only)

ADH, also known as vasopressin, is a hormone produced by the hypothalamus and released by the pituitary gland. Its primary function is to regulate the water balance in the body by controlling the reabsorption of water in the kidney tubules.

ADH and Permeability of Kidney Tubules:

The permeability of the kidney tubules refers to their ability to allow substances, including water, to pass through. ADH acts on the cells of the kidney tubules, specifically the collecting ducts, to increase their permeability to water.

Release of ADH:

ADH is released by the pituitary gland when the body's water concentration is too high or when the blood becomes too concentrated. High blood osmolarity or low blood volume triggers the release of ADH to help conserve water.

Effect of ADH on Kidney Tubules:

When ADH is present, it binds to receptors on the cells of the kidney tubules, causing an increase in the permeability of these tubules to water. This increased permeability allows more water to be reabsorbed back into the bloodstream instead of being excreted in urine.

Water Reabsorption and Concentrated Urine:

As ADH increases water reabsorption, it leads to the production of concentrated urine. When water is conserved, the urine becomes more concentrated with a lower volume, helping to maintain water balance in the body.

Negative Feedback:

The regulation of ADH secretion operates through negative feedback. When the body's water concentration is too high, ADH is released to increase water reabsorption. Once water balance is restored, ADH secretion is reduced.

Treating Kidney Failure

Kidney failure is a condition in which the kidneys are unable to effectively filter waste products and maintain proper water and electrolyte balance in the body. Two common methods for treating kidney failure are organ transplant and kidney dialysis.

Organ Transplant

Organ transplant involves replacing a damaged or non-functioning kidney with a healthy kidney from a donor. The donated kidney is typically obtained from a deceased or living donor who is a suitable match for the recipient. Organ transplant offers the possibility of restoring normal kidney function and eliminating the need for ongoing treatment.

Advantages of Organ Transplant:

• Restores normal kidney function and eliminates the need for dialysis.
• Improves overall health and quality of life for the recipient.
• Provides a long-term solution for kidney failure.

Disadvantages of Organ Transplant:

• Limited availability of donor organs.
• RiskThe chance that a decision could lead to loss, failure, or negative consequences. of rejection, where the recipient's immune system attacks the transplanted kidney.
• The need for lifelong immunosuppressive medications to prevent rejection.

Kidney Dialysis

Kidney dialysis is a mechanical process that helps to filter and purify the blood when the kidneys are unable to do so adequately. There are two main types of kidney dialysis: hemodialysis and peritoneal dialysis.

Hemodialysis:

Hemodialysis involves using a machine called a dialyzer to filter the blood. During hemodialysis, the patient's blood is pumped through the dialyzer, where it is cleaned and excess fluid and waste products are removed. The purified blood is then returned to the patient's body.

Peritoneal Dialysis:

Peritoneal dialysis uses the lining of the abdominal cavity (peritoneum) as a natural filter. A special fluid called dialysate is introduced into the abdominal cavity through a catheter. The dialysate absorbs waste products and excess fluid from the bloodstream, and after a prescribed dwell time, it is drained out.

Advantages of Kidney Dialysis:

• Can be done at home or in a dialysis centre, providing flexibility for patients.
• Immediate relief from symptoms associated with kidney failure.
• No need for immunosuppressive medications.

Disadvantages of Kidney Dialysis:

• Frequent treatment sessions, typically three times a week for hemodialysis.
• Dietary restrictions and fluid control.
• Potential complications, such as infection or damage to the access site.

Conclusion

Maintaining water and nitrogen balance in the body is crucial for cellular function and overall health. Proper water balance is necessary for cells to function efficiently, and the production and excretion of urea help maintain nitrogen balance. Through the processes of filtration and selective reabsorption, the kidneys ensure that water and essential substances, such as glucose and ions, are reabsorbed back into the bloodstream while waste products are excreted as urine. ADH plays a crucial role in maintaining water balance by controlling the permeability of kidney tubules to water, and treating kidney failure can be achieved through organ transplant or kidney dialysis.