Nephrons - Histology

What are Nephrons?

Nephrons are the fundamental functional units of the kidneys. Each kidney contains approximately 1 to 1.5 million nephrons, which are responsible for filtering blood, removing waste products, and balancing electrolytes and fluids in the body. Nephrons consist of two main parts: the renal corpuscle and the renal tubule.

What is the Structure of a Nephron?

The renal corpuscle includes the glomerulus and Bowman's capsule. The glomerulus is a network of capillaries where blood filtration begins. Bowman's capsule surrounds the glomerulus and captures the filtered fluid, known as glomerular filtrate. The renal tubule is divided into several segments: the proximal convoluted tubule (PCT), the loop of Henle, the distal convoluted tubule (DCT), and the collecting duct.

How Does Blood Filtration Occur?

Filtration occurs in the glomerulus, where blood pressure forces water and small solutes through the capillary walls into Bowman's capsule. This process is facilitated by the fenestrated endothelium of the glomerular capillaries, the basement membrane, and the podocytes with their foot processes, forming the filtration barrier.

What Happens in the Proximal Convoluted Tubule?

The PCT is responsible for the reabsorption of approximately 65% of the glomerular filtrate, including glucose, amino acids, and electrolytes. The cells lining the PCT have numerous microvilli that increase the surface area for absorption. These cells also contain many mitochondria, providing the necessary energy for active transport mechanisms.

What is the Role of the Loop of Henle?

The loop of Henle creates a concentration gradient in the kidney medulla, crucial for water reabsorption. It consists of a descending limb, which is permeable to water but not solutes, and an ascending limb, which is impermeable to water but actively transports sodium and chloride ions out of the filtrate.

What Happens in the Distal Convoluted Tubule?

The DCT is involved in the selective reabsorption and secretion of ions, contributing to the regulation of blood pH, potassium, and sodium levels. The macula densa, a group of specialized cells in the DCT, senses the sodium concentration and helps regulate the filtration rate through feedback mechanisms.

What is the Function of the Collecting Duct?

The collecting duct receives fluid from multiple nephrons and is the final site for water and electrolyte reabsorption. It plays a critical role in determining the final concentration of urine. The permeability of the collecting duct to water is regulated by antidiuretic hormone (ADH), which increases water reabsorption when the body needs to conserve water.

How Do Nephrons Contribute to Homeostasis?

Nephrons maintain homeostasis by regulating the volume and composition of blood, removing metabolic wastes, and balancing electrolytes and pH. They respond to hormonal signals, such as aldosterone and ADH, to adjust their reabsorptive and secretory activities according to the body's needs.

What are Common Histological Stains Used for Nephrons?

Common histological stains used to study nephrons include hematoxylin and eosin (H&E), which highlight cellular details and tissue architecture. Periodic acid-Schiff (PAS) stain is useful for visualizing basement membranes and brush borders. Silver staining techniques can also be employed to highlight glomerular structures.