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CSEC Biology: The Kidneys and Osmoregulation

Osmoregulation is the process by which the balance of salt and water (osmotic pressure or water potential) is maintained across the fluids of the body. This state of normalcy or balance of blood plasma concentration is known as homeostasis. This is necessary to prevent the unnecessary movement of water into or out of body cells.

If bodily fluids become too dilute, water enters the cells via osmosis, causing the cells to swell. They may burst or lyse as a result.

If bodily fluids become too concentrated, water leaves the cells via osmosis, causing the cells to shrink and become dehydrated. Metabolic reactions cannot occur in the absence of water, so the cells die if too much water leaves.

The body does not exist in isolation, though. There is a constant input of water and electrolytes into the system through food, drink and water produced by cells from respiration. Excess water, electrolytes, and wastes are transported to the kidneys and excreted in the form of urine, to the skin as sweat and in faeces, helping to maintain osmotic balance. Insufficient fluid intake results in fluid conservation by the kidneys.

Without a mechanism to regulate osmotic pressure, or when a disease damages this mechanism, toxic waste and water will accumulate, which can have dire consequences.


The kidneys are responsible for the regulation of the osmotic pressure/concentration of bodily fluids by controlling how much water is reabsorbed into the blood plasma during selective reabsorption. This determines how much water is lost in urine.


When body fluids are too dilute:

The osmoreceptor of the hypothalamus detects the decrease in fluid concentration and stimulates the pituitary gland to stop secreting ADH (Antidiuretic Hormone, also called Vasopressin). In the absence of ADH, the walls of the second convoluted tubules and collecting ducts remain nearly impermeable to water, and so very little water is reabsorbed into the blood. Thus, large quantities of dilute urine are produced, i.e. the urine excreted contains a lot of water.


When the body fluids are too concentrated:

The osmoreceptor of the hypothalamus detects the high concentration of the blood plasma and begins to produce ADH and stimulates the pituitary gland to secrete it. The blood carries ADH to the kidneys, where its presence causes the walls of the second convoluted tubule and collecting ducts to become more permeable to water. Much more water is reabsorbed from the filtrate back into the blood and very small quantities of concentrated urine are produced, i.e. the urine excreted contains very little water.


Kidney Failure and Dialysis

Kidney failure is when the nephrons stop working correctly, and are unable to remove waste from the blood and regulate the volume and composition of blood plasma and body fluids. Toxic wastes including urea build up in the blood, reaching potentially fatal levels. Kidney failure is treated by either a kidney transplant or dialysis.

In dialysis, blood from a vein flows through a dialysis machine and is then returned to the body. In the machine, blood is separated from dialysis fluid by a partially permeable membrane. Waste products pass from the blood into the dialysis fluid along with excess water and excess salts. Waste from the blood is removed and the concentration of the blood and body fluids are regulated. Most people suffering from kidney failure require dialysis around three four-hour sessions weekly.

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