Nephron Function and Urine Formation

The Urinary System

The Nephron: Structure and Function

The nephron is the functional unit of the kidney, responsible for filtering blood and forming urine. Each kidney contains approximately one million nephrons, which work together to maintain the body's fluid and electrolyte balance.

• Filtration: Occurs at the glomerulus, where blood pressure forces water and small solutes out of the blood and into the nephron tubule, forming the filtrate. Large molecules like proteins and blood cells remain in the bloodstream.

• Reabsorption: The process by which water and useful substances (such as glucose, amino acids, and ions) are returned from the filtrate to the blood, primarily in the proximal convoluted tubule.

• Secretion: Additional wastes and excess ions are actively transported from the blood into the nephron tubule, helping to fine-tune the composition of urine.

Key Terms

• Glomerulus: A network of capillaries where filtration of blood takes place.

• Tubule: The structure through which filtrate passes and is modified by reabsorption and secretion.

Summary Table: Nephron Processes

Process	Location	Main Function
Filtration	Glomerulus	Removes water and small solutes from blood
Reabsorption	Proximal tubule, loop of Henle, distal tubule	Returns useful substances to blood
Secretion	Distal tubule, collecting duct	Removes additional wastes and excess ions

Relevant Equation

The overall urine formation can be summarized as:

$\text{Urine} = \text{Filtration} - \text{Reabsorption} + \text{Secretion}$

Example

For example, glucose is filtered at the glomerulus but is almost entirely reabsorbed in the proximal tubule, so it does not normally appear in urine.