Excretion in humans (kidneys and skin) — KCSE Biology

Urine formation occurs in the nephron, the functional unit of the kidney. This process involves three main stages: filtration, reabsorption, and secretion.

1. Filtration: Blood enters the nephron through the glomerulus, where it is filtered. Water, salts, glucose, and urea pass into the Bowman’s capsule, while larger molecules like proteins remain in the blood.

2. Reabsorption: As the filtrate moves through the proximal convoluted tubule, essential substances such as glucose and certain ions are reabsorbed back into the bloodstream. This process is vital for maintaining homeostasis and conserving nutrients.

3. Secretion: In the distal convoluted tubule, additional wastes and excess ions are secreted into the filtrate from the blood. This helps regulate pH and electrolyte balance.

Finally, the processed filtrate, now called urine, collects in the collecting duct and is transported to the bladder for excretion. Understanding these stages is crucial for appreciating how the kidneys maintain fluid and electrolyte balance in the body.

The skin plays a vital role in excretion, primarily through the process of sweating. Sweat glands, located in the dermis layer of the skin, produce sweat, which is composed mainly of water, salts, and urea. This process helps to regulate body temperature and remove waste products from the body.

When the body temperature rises, sweat is secreted onto the skin's surface. As it evaporates, it cools the body down. Additionally, during sweating, excess salts and urea are expelled, contributing to the body's excretory functions.

In summary, the skin's excretory function can be described as:

• Sweat production: Helps remove waste.
• Temperature regulation: Maintains homeostasis.
• Composition of sweat: Contains water, salts, and urea.

Excretion in humans is crucial for maintaining homeostasis. The primary organs involved are the kidneys and skin.

Kidneys:

• The kidneys filter blood to remove waste products and excess substances.
• They consist of nephrons, the functional units responsible for urine formation.
• Each nephron has a glomerulus for filtration and a renal tubule for reabsorption and secretion.

Skin:

• The skin excretes waste through sweat, which contains water, salts, and urea.
• Sweat glands help regulate body temperature and eliminate toxins.

Process of Excretion:

1. Filtration: Blood enters the glomerulus, where water and solutes are filtered out.
2. Reabsorption: Essential substances like glucose and ions are reabsorbed in the renal tubule.
3. Secretion: Additional wastes are secreted into the tubule, forming urine.
4. Excretion: Urine is stored in the bladder and expelled from the body.

Understanding these processes is vital for grasping how the body maintains its internal environment and removes harmful substances.