4.4 Functions of the Kidney

The kidneys are vital organs that perform two primary roles essential for maintaining homeostasis (a stable internal environment): excretion and osmoregulation.

Excretory Function

The primary excretory role of the kidney is the filtration of blood to remove metabolic wastes. The main waste products filtered are nitrogenous wastes, such as urea, which are toxic if allowed to accumulate. These filtered wastes are then expelled from the body in the form of urine.

The process of urine formation involves:

1. Pressure Filtration: Occurring at the glomerulus where high Blood Pressure→ forces water and solutes into the Bowman's capsule.
2. Selective Reabsorption: Useful substances like glucose and amino acids are taken back into the blood.
3. Tubular Secretion: Additional wastes and hydrogen ions are added to the filtrate to maintain .

Osmoregulatory Function

Osmoregulation is the control of the water and solute balance in the body's fluids. The kidneys adjust the concentration of urine to maintain this balance based on the body's hydration level.

• In Dehydration: When the body is low on water, the kidneys produce a small volume of highly concentrated urine to conserve water.
• In Hydration (Flooding): When the body has excess water, the kidneys produce a large volume of dilute urine to expel the excess water.

Acid-Base Balance

The kidneys also play a crucial role in maintaining the blood pH (around 7.4). They achieve this by secreting hydrogen ions ($H^{+}$) into the tubule and reabsorbing bicarbonate ions ($HC{O}_3^{-}$) back into the blood.

Possible Questions/Answers

Q: What are the two main functions of the kidney? A: The two main functions are excretion (removal of nitrogenous wastes) and osmoregulation (maintenance of water and salt balance).

Q: How does the kidney respond to dehydration? A: During dehydration, the kidney conserves water by producing a small amount of highly concentrated urine.