## 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 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. | Condition | Body's Water Level | Kidney's Response | Urine Produced | | :--- | :--- | :--- | :--- | | **Dehydration** | Low | Conserve water | Small volume, concentrated | | **Hydration** | High (Flooding) | Expel excess water | Large volume, dilute | ### 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 ($HCO_3^-$) 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.