The glomerulus is a microscopic filtering unit within the kidneys, maintaining overall body health. It functions like a tiny sieve, cleansing the blood. This filtering action removes waste products and excess fluids, supporting the body’s internal balance.
The Glomerulus in the Kidney
The glomerulus is an integral part of the nephron, the kidney’s functional unit. Each kidney contains approximately one million nephrons per kidney.
The glomerulus appears as a small tuft of specialized blood vessels, specifically capillaries. These capillaries are enveloped by a cup-shaped structure known as Bowman’s capsule. This arrangement allows for the collection of filtered substances.
The Filtration Process
The glomerulus filters blood, initiating urine formation. Blood flows into the glomerular capillaries under pressure, allowing small molecules to pass through a three-layer filtration membrane. This membrane consists of the fenestrated capillary wall, a basement membrane, and an outer layer of podocytes.
The capillary wall’s endothelial cells have fenestrations that permit fluid, solutes, and small proteins to pass, but prevent blood cells from exiting the bloodstream. The basement membrane acts as a secondary barrier, restricting larger plasma proteins due to both size and negative charge. Podocytes, with their interdigitating foot processes, form filtration slits that act as a final selective barrier, ensuring large molecules are retained in the blood.
Through this multi-layered barrier, substances like water, glucose, salts, and urea are filtered from the blood to become “filtrate.” Conversely, larger components such as red blood cells, white blood cells, and large proteins like albumin are blocked and remain in the bloodstream. This filtration process is driven by the hydrostatic pressure of the blood within the glomerular capillaries, which is higher than in most other capillary beds.
Factors Influencing Filtration
Glomerular Filtration Rate (GFR) quantifies glomerular function, measuring the volume of filtrate formed per minute. The body maintains a stable GFR, even when blood pressure fluctuates. This autoregulation involves adjusting the diameter of the arterioles that supply and drain the glomerulus.
When blood pressure rises, the afferent arteriole, which brings blood to the glomerulus, constricts. This reduces blood flow into the capillaries, preventing excessive filtration. If blood pressure drops, this arteriole dilates to increase blood flow and maintain filtration. Hormonal systems also contribute to this regulation, such as the renin-angiotensin system, influencing arteriole diameter to adjust GFR in response to changes in blood volume and pressure.
When Filtration Fails
Damage to the glomerulus can compromise its filtering ability, leading to health issues. If the filtration barrier becomes overly permeable, substances like proteins and blood cells can leak into the urine. This leakage can result in proteinuria, where excess protein, particularly albumin, is found in the urine, often causing it to appear foamy.
The presence of blood in the urine, known as hematuria, is another sign of impaired glomerular function. Protein loss can also lead to reduced protein levels in the blood, causing fluid to accumulate in tissues. This results in swelling, particularly in the face, hands, ankles, or feet, a condition called edema. Glomerular damage can stem from various causes, including chronic conditions like diabetes and high blood pressure, or autoimmune diseases where the immune system mistakenly attacks the glomeruli.