When the body needs to conserve water, the kidneys excrete a hypertonic urine. what do the terms isotonic and hypertonic mean? since the fluid that is to become urine begins as plasma (an isotonic solution), what must happen to change it to a hypertonic urine?

Isotonic - In an isotonic solution the solvent and the solute are equal in concentration.

Hypertonic - contains more solvent than the solute. In cases of urine, it means there is more water concentration compared to solutes such as glucose, Na ions, Cl ions or Urea.

Process of changing isotonic solution (plasma) to hypertonic solution

Blood enters the kidney through the renal artery which separates many times segmental arteries to interlobular arteries to arcuate arteries to interlobular arteries to afferent arteries which deliver blood to nephron. Afferent arterioles has a greater diameter than the efferent (carries blood from the nephron) hence forcing some components of blood out of glomerular capillaries such as water, salt, glucose, urea are passed to Bowmans capsule and are known as glomerular filtrate (contains about 20% of blood plasma) glomerular filtrate is passed from renal capsule to renal tube. Blood cells and plasma proteins are not filtered out because of their large size.

Reabsorption

Tubular reabsorption occurs through three mechanisms

Diffusion-movement of molecules/ions particles from an area of high concentration to an area of low concentration

active transport - a movement of molecules from a low concentrated area to a high concentrated area against a concentration gradient by use of energy

Osmosis-movement of water molecules from of hypotonic concentration to hypertonic concentration through a semi-permeable membrane.

Water and glucose are reabsorbed in the proximal convoluted tubule. There is a high concentration of sodium ions in the filtrate hence it moves from tubular fluids to the proximal convoluted tubule. Other substances that are reabsorbed include glucose, amino acids, lactic acids and bicarbonate ions. 80% of water is reabsorbed through diffusion.

In the loop of Henle, the remaining water, dissolved salts and urea passes through the descending loop of Henle (permeable to water and less permeable to Na, Cl, and urea) this being so it increases the osmotic gradient hence the solutes are reabsorbed by the ascending loop of Henle (permeable to Cl and Na ions impermeable to water)

In the distal convoluted tubule the anti-diuretic hormone makes cells to be more permeable to water hence allows more water to go back to the blood.