Urinary system physiology

Physiology of the urinary system

The urinary system has several important roles that help regulate homeostasis in the body:

• Maintains fluid volume. The urinary system increases fluid loss in urine when fluid volume is high. It decreases the fluid loss when the fluid volume is low (e.g., with dehydration).
• Maintains the normal composition of body fluids. The urinary system can increase or decrease the loss of certain electrolytes in the urine as it regulates the normal makeup of the body fluids. This helps to keep the pH of the blood within normal limits.
• Maintains an adequate blood pressure. Renin, an enzyme, is secreted in the urinary system and is involved in increasing the blood pressure.
• Controls red blood cell production. Erythropoietin is secreted by the urinary system, which triggers red blood cell production.
• Activates vitamin D. The kidneys are involved in the final step of vitamin D activation. Vitamin D is important in the absorption of calcium and phosphorus.

Many of these roles are performed during the formation of urine. The body rids itself of unneeded substances produced during the metabolic process. Urine formation happens in three steps: filtration (covered in detail below) forms the initial filtrate at the glomerulus; reabsorption then returns needed substances, such as water, electrolytes, glucose, and amino acids, from that filtrate back into the blood, so they’re normally absent from final urine; and secretion moves additional wastes from the blood into the filtrate before it leaves the kidney as urine.

Filtration

Filtration is the first step in urine formation. It is a continual process that involves the renal corpuscle. If you recall, the renal corpuscle consists of a cup-shaped structure (Bowman capsule) that surrounds the network of capillaries (glomerulus).

The blood is brought to the kidneys by the renal arteries. The arteries branch into smaller vessels that transport the blood throughout the kidney. As the blood gets to the nephron, an afferent arteriole brings the blood into the glomerulus, and an efferent arteriole takes the blood away. Eventually, the blood moves into the renal vein and out of the kidney.

Let us focus on what occurs in the glomerulus. The diameter of the afferent arteriole is larger than the efferent arteriole’s diameter. This size difference creates a bottleneck: blood enters the glomerulus faster than it can leave, keeping the pressure inside the glomerulus high. This high pressure forces water and dissolved substances through the one-celled wall of the capillary and into the Bowman’s capsule. The substance in the Bowman capsule is known as filtrate. (Think of a garden hose that is wide at one part and narrow at the other. The pressure builds up behind the narrow section. Water can leak from any weakened areas in the wall of the hose. This is similar to what occurs in the glomerulus.)

Several dissolved substances can move through the capillary wall:

• Electrolytes (i.e., sodium, chloride, and potassium)
• Waste products (i.e., urea, metabolites)
• Other substances (i.e., amino acids and glucose)

White blood cells, red blood cells, and plasma proteins are too large to pass through the capillary wall. With normal kidney function, they remain in the capillary.

If the blood pressure falls (i.e., with hemorrhaging), the pressure in the glomerulus is not high enough to cause movement of the fluids, so little to no filtrate is created, and the person will have little to no urine output.