Osmoregulation

Osmoregulation maintains the correct balance between the water and solute contents of the body fluids. Osmoregulation is mainly controlled by the kidneys and skin

• Amount of water: too little water causes dehydration of the cells, and too much water causes the cells to burst • Concentration of solutes: e.g. salts, glucose. As it determines the osmolarity (the number of solute particles dissolved in the one litre of water)

About 99% of the body’s water is returned from the kidneys into the bloodstream. Under normal conditions, most of it is reabsorbed from the proximal tubule and the loop of henle. When fluid intake is low, the kidney conserves water by forming more concentrated urine in the following way: - sodium ions are actively pumped out of the loop of Henle - this results in a steep concentration gradient between the hypotonic filtrate in the distal and collecting ducts and the surrounding hypertonic tissue fluid - this causes water to move by osmosis from the filtrate into the blood So more water is conserved and less urine is formed

Antidiuretic hormones (ADH) is a hormone secreted by the hypothalamus. Anti- opposed to or preventing, diuretic: causing increased output of urine. Special cells called osmoreceptors in the hypothalamus monitor he osmolarity of the blood. These cells are stimulated when osmolarity increases because of dehydration. E.g. through sweating. This results in the stimulation of the: • thirst centre in the hypothalamus, causing more water to be drunk • pituitary gland, causing it to release ADH

ADH dissolves in the blood plasma and is carried all over the body. When it reaches its target organ, the kidney, it causes the pores of the collecting ducts to enlarge, making them more permeable to water. Results in: • more water moving by osmosis from the filtrate in the collecting ducts, into the blood in the capillaries • less, but more concentrated urine being excreted

Aldosterone is a steroid hormone produced by the adrenal glands, that are situated at the top of each kidney. Aldosterone controls the concentration of sodium ions in the body. • under normal conditions, 65% of the sodium in the renal filtrate is reabsorbed from the proximal tubules and another 25% from the loop of Henle • there are more sodium ions than any other ions in the fluids of the body. Regulating the sodium ion balance is on of the most important functions of the kidneys

The secretion of aldosterone is triggered when: • blood levels of sodium are low, causing a drop in osmolarity • blood volume is low, creating low blood pressure Aldosterone causes: • the reabsorption of sodium ions from the filtrate returning them to the bloodstream • water to follow the sodium ions, increasing the blood volume and therefore blood pressure