The main parts of a nephron are shown in Figure 1-19.
- Ultrafiltration occurs in the malpighian corpuscle so that blood plasma passes from the capillary glomerulus to the Bowman's capsule.
Large molecules cannot leave the blood because the pores of the filter are too small but, in the proximal convoluted tubule, sodium chloride together with useful substances such as glucose, amino acids, proteins and ascorbic acid may be absorbed back into the blood stream.
- The descending and ascending loops of Henle dip into an osmotic gradient in the fluid between the cells which is maintained by sodium ion movements. When urine eventuAlly flows down the collecting tubule, water may be lost from the tubule into the surrounding tissue (since the concentration is higher outside the tubule near the bottom of the loops).
The final concentration of the urine is controlled by antidiuretic hormone (ADH) from the posterior lobe of the pituitary gland beneath the brain.
- ADH makes the wall of the collecting tubule more permeable, and water can leave more readily by osmosis.
- Thirsty animals produce a lot of ADH and only a small volume of urine. Their urine is more concentrated.
- The distal convoluted tubule is the principal site of acidification of the urine, and this is where potassium, hydrogen and ammonium ions enter the urine. Nitrogen is excreted from the body as urea.