Explain the terms ultrafiltration and selective absorption.
Ultrafiltration-Ultrafiltration involves filtration of the blood which takes place in the glomerulus. The blood containing urea from the afferent arteriole enters the glomerulus under high pressure. The high pressure is created because the efferent arteriole is narrower than the afferent arteriole. The high pressure causes the liquid part of the blood to filter out from the glomerulus into the renal tubule. This filtrate is known as 'glomerular filtrate'. Glomerular filtrate consists of water, urea, salts, glucose, and other plasma solutes. Blood corpuscles, proteins, and other large molecules remain behind in the glomerulus. Therefore the blood which is carried away by the efferent arteriole is relatively thick.
The glomerular filtrate is an extremely dilute solution containing a lot of usable materials like water, urea, salts, glucose, and other plasma solutes. When the filtrate reaches the proximal convoluted tubule, sodium ions and chloride ions, glucose, amino acids, water, and some vitamins move back into the blood. But their re-absorption is only to the extent that the normal concentration of the blond is not disturbed. This is called selective absorption.