3. Formation of Urine

3.1 Main processes involved in the production of urine

(a) Ultrafiltration

(b) Selective reabsorption

3.2 Ultrafiltration

Above - Process of ultrafiltration where the high hydrostatic pressure forces the glomerular filtrate through the glomerulus. The basement membrane that surrounds the blood capillary serves as a filter for small substances - water, glucose, amino acids and urea to pass through. The arrows indicate ultrafiltration occurring at the glomeruli and Bowman's capsule.

• • As blood flows through the glomerulus, high blood pressure causes small molecules, eg glucose, amino acids, water, nitrogenous waste – urea and creatinine, water and mineral salts to be forced out of the blood vessels.

  • High blood pressure is generated by:

1. The larger diameter of afferent arteriole than the efferent arteriole.

• • Basement membrane which surrounds the glomerular blood capillaries serves as a partially permeable filter that allows the small molecules to pass through. Large molecules – red blood cells, white blood cells, platelets and plasma proteins are retained within the blood capillaries.

  • The resulting filtrate containing glucose, amino acids, water, nitrogenous waste and mineral salts that leaves the glomerulus and enters the Bowman’s capsule is known as the glomerular filtrate.

3.3 Selective reabsorption

3.3.1 Structure of the epithelial cells of Proximal Convoluted Tubule

Adaptations of epithelial cells lining the Proximal Convoluted Tubule:

1. Epithelial cells bear microvilli to increase surface area to volume ratio for selective reabsorption of glucose and amino acids.

2. Each epithelial cell has many mitochondria to release energy for active transport of glucose and amino acids during selective reabsorption.