6.1 Regulation of the Composition of Body Fluids

Types of fluid

Intracellular fluid / cytosol - inside the cells (2/3)

Extracellular fluid - outside the cells (1/3):

• Intravascular fluid (1/4) - blood plasma located within the blood vessels.

• Interstitial/intercellular/tissue fluid - between the cells.

• Transcellular fluid - in specific body regions, includes fluid in the brain, spinal cord, eyes, joints, and surrounding the heart.

Osmotic concentration

Water moves easily through plasma membranes, and so any difference in osmotic concentration between the intracellular fluid and the extracellular fluid does not last very long. If an imbalance in osmotic concentrations occurs in any tissue, osmosis normally restores the balance within seconds.

Osmotic pressure

The tendency of a solution to take in water. The greater the difference in osmotic concentrations between two solutions, the greater the osmotic pressure.

Fluid loss and gain

Fluid gain:

• Most fluid is obtained from water that is either taken in as a liquid or contained in food that is eaten.

• A small amount of fluid obtained as a by-product of chemical processes occurring within the cells.

Fluid loss:

• Via the kidneys,

• Through the skin,

• From the surface of the lungs,

• From the alimentary canal.

Excretion

The removal of waste products of metabolism from the boy, as may wastes are toxic and would be harmful to health if allowed to accumulate in the body fluids.

Organs that take part in excretion:

• LUNGS - excretion of carbon dioxide. Carbon dioxide and water are produced by all body cells during cellular respiration. The body cannot use carbon dioxide, so it is carried in the blood until it reaches the lungs, where it is excreted. Some water is also lost from the lungs in the form of water vapour as we exhale.

• SWEAT GLANDS - in the skin, secrete water containing by-products of metabolism such as salts, urea and lactic acid.

• ALIMENTARY CANAL - passes out bile pigments (the breakdown products of haemoglobin from red blood cells) that entered the small intestine with the bile. They leave the body with the faeces.

• KIDNEYS - the principle excretory organs. Responsible for maintaining a constant concentration of materials in the body fluids. One of the important wastes removed by the kidneys is urea, which is produced in the liver during the breakdown of proteins.

Kidneys

Only water loss from the kidneys can be regulated to achieve a constant concentration of dissolved substances in the body fluids. Thus, the kidneys are not just excretory organs, they also play a major role in regulating composition of body fluids.

• A tube, the ureter, leaves each kidney and drains into a muscular reservoir, the urinary bladder, which empties to the outside through another tube, the urethra.

• Each contains about 1.2 microscopic units called nephrons. These are the functional units of the kidney, carrying out the kidney’s role in excretion and water regulation.

Controlling water levels

Water is continually lost from the body in sweat, urine, faeces, and exhaled breath, and is normally balanced by water intake. However, during strenuous activity or extreme heat, this water loss can be quite high. As water is lost:

• Plasma becomes more concentrated and hence has a HIGH OSMOTIC PRESSURE.

• As a result, water moves from the interstitial/intercellular fluid into the plasma by osmosis.

• This makes the interstitial fluid more concentrated, and therefore water diffuses out of the cells, so the cells start to shrink from dehydration.

• When this happens, OSMORECEPTORS in the HYPOTHALAMUS detect the increase in osmotic pressure.

• A number of responses are then triggered that increase the water content, and hence, LOWER OSMOTIC PRESSURE.

Antidiuretic hormone

The level of active reabsorption of water is regulated by nervous and hormonal control of water and salt reabsorption in the kidneys. (Active reabsorption occurs in the distal convoluted tubule and collecting duct. Passive reabsorption using osmosis occurs in the proximal convoluted tubule and loop of Henle)

This level of active reabsorption is controlled by the hormone known as antidiuretic hormone (ADH):

• Produced by the hypothalamus → Released from the posterior lobe of the pituitary.

• Controls the permeability of the walls of the distal convoluted tubule and collecting duct.

• High concentration of ADH in blood plasma → Tubules are very permeable to water, and thus water is able to leave the tubule and enter the surrounding capillary network (this outward flow of water from the fluid within the tubules reduces its volume and hence increases the concentration of materials remaining).

• Low concentration of ADH in plasma → Tubules are not very permeable to water, and little water is reabsorbed into the plasma of the blood (fluid within the tubules remains fairly dilute, as its volume is not reduced to any significant extent).

Osmoregulation by Antidiuretic hormone feedback loop

If water was lost through excess sweating, the following process would occur:

• Stimulus - Water concentration of the blood plasma decreases, increasing osmotic pressure of the blood.

• Receptors - Osmoreceptors in the hypothalamus are stimulated, detecting the increased osmotic pressure of the blood.

• Modulator - Hypothalamus stimulates the posterior lobe of the pituitary gland to release ADH (the effector) into the bloodstream.

• Effector - Antidiuretic hormone is carried all over the body by the blood, but it affects its target organs, which are the nephron tubules in the kidney. Permeability to water of the distal convoluted tubules and collecting ducts is increased.

• Response - More water is then reabsorbed into the blood plasma from the distal convoluting tubule and collecting duct.

• Feedback - Reabsorption of water increases the amount of water in the plasma, and so the osmotic pressure is decreased.

Aldosterone

A hormone that plays a part in the regulation of water output. Sometimes called the salt-retaining hormone. Is secreted by the adrenal cortex in response to a:

• Decrease in concentration of sodium ions in the blood,

• Decrease in blood volume,

• Decrease in blood pressure,

• Increase in the concentration of potassium ions in the blood.

Aldosterone acts on the distal convoluted tubules and collecting ducts to increase the amount of sodium ions reabsorbed into the bloodstream and the amount of potassium secreted in the urine. It achieves this through active transport using a sodium-potassium pump. For every three sodium ions reabsorbed, two potassium ions are secreted. There is thus a net movement of ions into the blood, leading to a subsequent transport of water into the blood via osmosis.

• Hypothalamus secretes Corticotropin Releasing Hormone which stimulates the anterior lobe of the pituitary.

• Anterior lobe of pituitary secretes Adrenocorticotropic hormone, which stimulates the Adrenal cortex.

• Adrenal cortex secretes aldosterone.

• Aldosterone acts on the renal tubules (Ascending loop of Henle, Distal convoluted tubule, and collecting duct) to increase amounts of sodium reabsorbed into the bloodstream, and increases the amount of potassium secreted in the urine.

• Water is also reabsorbed with sodium, hence blood volume is increased.

Thirst response

Water level of the body can be increased by taking in more fluid. Osmoreceptors are able to stimulate the thirst centre in the hypothalamus, prompting the person to drink water. This fluid is absorbed across the wall of the alimentary canal into the blood, decreasing the osmotic pressure.

• Stimulus - Water concentration of blood plasma decreases, osmotic pressure of the blood increases, mouth becomes dry.

• Receptor - Osmoreceptors in the thirst centre of the hypothalamus are stimulated as they detect the rising osmotic concentration of the blood.

• Modulator - Stimulation of the thirst centre in the hypothalamus makes the person feel thirsty.

• Effector - The conscious feeling of thirst stimulates the person to drink.

• Response - The fluid consumed is absorbed from the alimentary canal into the plasma in the blood.

• Feedback - Water leaves the blood, and the extracellular and intercellular fluids return to their normal concentrations.

Dehydration

• Occurs when water loss exceeds water intake.

• Symptoms become noticeable when a person has lost about 2% of their normal body water.

• Caused due to sweating, vomiting, diarrhoea, loss of thirst reflex as one gets older.

• Symptoms include severe thirst, low blood pressure, dizziness, and headache.

• If the condition remains untreated, the patient may become delirious, lose consciousness and die.

Water intoxication

• Also called water poisoning or hyperhydration

• Body fluids become diluted and cells take in extra water by osmosis

• Symptoms include light headedness, headache, vomiting and collapse.