13.2 - The Urinary System

Structure and Function of the Kidneys

The primary organs of the urinary system are the kidneys. The human kidney has 2 distinct regions – cortex and medulla. The two main functions that the kidneys perform are excretion and osmoregulation. The functions of the urinary system include:

• Regulating water and salt balance in the body
• Regulating osmotic pressure and ionic level
• Excreting waste products
• Regulating blood pH

Figure 7 - The human urinary system (left) and the longitudinal section of a kidney (right).

Formation of Urine

The functional unit of a kidney is the nephron. A nephron consists of 3 major parts:

• Glomerulus
• Bowman's capsule
• Renal tubules which are made up of the proximal convoluted tubule, loop of Henle and distal convoluted tubule.

Figure 8 - The structure of a nephron.

1. The process of urine formation begins with ultrafiltration in the Bowman's capsule. Bood that reaches the nephron is under high hydrostatic pressure due to the larger diameter of the afferent arteriole than the efferent arteriole. Ultrafiltration happens when the high hydrostatic pressure forces the fluid into the Bowman's capsule. The fluid that enters the Bowman's capsule is called the glomerular filtrate. The glomerular filtrate has the same composition as blood plasma, but does not contain red blood cells, platelets and plasma proteins as they are too large to pass through the afferent arterioles.
2. Reabsorption takes place when substances move across the walls of the renal tubule into the capillary network. In the proximal convoluted tubule , sodium (Na+) ions are actively pumped into the capillary network while the chloride (Cl-) ions flow passively. The reabsorption of glucose and amino acids occur through active transport. The movement of solutes into the capillary network decreases the solute concentration of the filtrate, but increases the solute concentration in the capillary network. Therefore, water moves into the blood capillary by osmosis.
3. In the loop of Henle, water and Na+ ions are reabsorbed through osmosis and active transport respectively. In the distal convoluted tubule, more water, Na+ and Cl- ions are reabsorbed.
4. Secretion takes place in the renal tubules, but is especially active at the distal convoluted tubule. It is the process of secreting waste and excess substances initially not filtered into the renal tubules. Secretion occurs by simple diffusion and active transport. Secreted substances include hydrogen (H+) ions, potassium (K+) ions, ammonia (NH3), urea, creatinine, toxins and drugs.
5. When the renal fluid reaches the collecting duct, most of the water has been reabsorbed into the bloodstream with little amount of salt left. The remaining fluid is now called urine , flows down the collecting duct. Some urea which are small enough diffuse into the blood capillaries. Urine contains water, urea, NaCl salt, uric acid and creatinine. From the collecting duct, urine flows down the ureter and is collected in the bladder before excreted out through the urethra.

Mechanism of Homeostasis and Osmoregulation

The body gains and loses water every day. Despite so, the body is able to maintain its water content constantly. This process of maintenance is known as osmoregulation. Osmoregulation is achieved mainly by regulating the volume of urine production and excretion from the kidneys.