Human Body Fluids

Body Fluids

The human body is about 60% water by weight. Body fluids are blood plasma, lymph, interstitial fluid (tissue fluid) which consist mainly water. They contain many different substances including nutrients, gases, water, hormones, and inorganic salts, acids and bases. All the chemical reactions in the body take place in a watery medium.

The body fluid is distributed in two principal compartments. The intracellular compartment and the extracellular compartments. Two-thirds of the body fluid is found within cells called intracellular fluid. The remaining is outside the cells in the extracullar compartment called the extracellular fluid (ECF).

The movement of fluid from one compartment to another depends on blood pressure and osmotic concentration, decided by plasma proteins.

Water Balance

Fluid intake must equal fluid output, so that the total amount of fluid in the body remains constant. Water is taken in through the alimentary tract and a small amount is formed by the metabolic processes. Water is excreted in expired air and in faeces, through the skin as sweat and mainly as urine.

The balance between fluid intake and output is controlled by the kidneys. Dehydration occurs when fluid output is greater than the intake. e.g. through vomiting, sweating, diarrhea. The Anti-diuretic Hormone released into the blood by the posterior pituitary gland regulates the kidneys to reabsorb more water.

Electrolyte balance and fluid balance are interdependent. Electrolytes are compounds such as inorganic salts, acids and bases that form ions in solution. The electrolyte composition varies among body fluids in different compartments. The ECF contains high levels of sodium ion. Potassium ion concentration is higher in the ECF. Sodium ion concentration is adjusted mainly by regulating the amount of water in the body, as well as, by the hormone aldosterone secreted by adrenal cortex. Potassium ions are also important in maintaining fluid volume and helps to regulate acid-base levels.

Acid-Base balance

Acid-base balance depends on the concentration of hydrogen ions and its regulations is critical to health. pH is a measurement of the H⁺ ion concentration of a solution. A neutral pH is 7. Lower pH indicates acidity. Higher pH indicates alkalinity.

Blood is slightly alkaline at pH 7.4. Change in pH can affect rate of chemical reactions and structure and function of proteins.

The term acidosis refers to any condition in which the hydrogen ion concentration of plasma is elevated above normal. There are 2 types of acidosis:

Metabolic acidosis : This can be due to excessive loss of bicarbonate ion as in diarrhea or large accumulation of lactic acid as in exercise.

Respiratory acidosis : This develops when CO₂ is produced more rapidly than it is excreted by the lungs. This occurs during respiratory diseases.

The term alkalosis is any condition where the hydrogen ion concentration is below the normal range. There are 2 types of alkalosis.

Metabolic alkalosis : When there is loss of hydrochloric acid from stomach as in excessive vomiting it can result in muscle twitches and convulsions.

Respiratory alkalosis : This occurs when the respiratory system excretes CO₂ more quickly than it is produced. Hyperventilation of lungs due to high altitude or stress can cause this. The kidneys help regulate the pH by excreting or conserving certain ions.

Renal failure : This can be acute renal failure / chronic renal failure.

Acute Renal failure : There is a sudden and severe reduction in the glomerular filtration rate and function of the kidney due to traumatic injury to normal kidneys, such as loss of blood, loss of fluid as in burns, diarrhoea etc.

Chronic renal failure : This occurs when the nephrons are progressively and irreversibly damaged and can be the end result of kidney stones or high BP or Diabetes mellitus.

Kidney ( Renal Calculi) : Calculi form in the kidneys and bladder when urinary constituents normally in solution are precipitated. The solutes involved are oxalates, phosphates, urates and uric acid. They are more common in males and after 30 years of age. Dehydration, Infection, hyperparathyroidism etc can lead to stone formation.