Glomerulus

Recommended video: Kidney histology [33:48]

Have a thorough look at the microscopic structure of the kidney and its cell types.

The glomerulus is the main filtering unit of the kidney. It is formed by a network of small blood vessels (capillaries) enclosed within a sac called the Bowman's capsule. The space inside the capsule that surrounds the glomeruli is known as the Bowman's space. Each glomerulus is located at the beginning of the nephron.  

The blood supply to the glomerulus is provided via the afferent arteriole. The blood then flows through the capillary network, where it gets filtered, and then leaves the glomerulus via the efferent arteriole. The ultrafiltrate is collected in the Bowman's space and drains directly into the proximal tubule of the nephron. 

The glomerulus is composed mainly of three cell types:

• Endothelial cells
• Smooth muscle cells and specialized muscle cells
• Epithelial cells (parietal cells, podocytes)

The endothelial cells that surround the capillaries have numerous fenestrations that allow rapid filtration of the blood to occur. The smooth muscle cells surround the afferent and efferent arterioles. These muscle cells regulate the vascular resistance in these vessels and thus influence the glomerular filtration rate. The specialized muscle cells include the intraglomerular mesangial cells, extraglomerular mesangial cells, and granular cells. Each of these cells has a distinct way of regulating blood flow and glomerular filtration. The cells that form the capsule that forms the Bowman space are called the parietal cells. Podocytes are specialized epithelial cells located in the Bowman's space. They form extensions (foot processes) that completely surround the glomerular capillaries and play a role in restricting the passage of plasma proteins into the urine.

Clinically, the glomerulus is considered to be the most important part of the kidney mainly because most of the disorders that affect the kidney involve the glomerulus.