The kidneys are two bean-shaped organs that serve as the body’s natural blood filters. Within each kidney, microscopic structures work continuously to clean the blood and maintain overall health. An important component in this complex system is the glomerulus, which performs the initial step in filtering waste and excess substances from the bloodstream.
Glomerulus Structure and Location
The glomerulus is a network of capillaries twisted into a ball-like shape. It is located at the beginning of a nephron, the kidney’s functional filtering unit. Each kidney contains approximately one million nephrons, and each nephron houses one glomerulus.
This capillary tuft is enclosed within a cup-shaped sac called Bowman’s capsule. The space within the capsule surrounding the glomeruli is known as Bowman’s space. The glomerulus, along with its surrounding Bowman’s capsule, forms a structure called the renal corpuscle, the basic filtration unit of the kidney. Blood enters the glomerulus through the afferent arteriole and exits through the efferent arteriole.
How the Glomerulus Filters Blood
Glomerular filtration is the initial step in urine formation, where the kidneys filter fluid and waste products from the blood. This process is driven by hydrostatic pressure, the force exerted by the blood against the capillary walls, pushing water and small dissolved substances out of the blood vessels and into Bowman’s capsule. Unlike most capillary beds, the glomerulus has both an afferent and efferent arteriole, which helps maintain the high pressure necessary for filtration.
The filtration barrier, located between the blood in the glomerular capillaries and the fluid in Bowman’s capsule, is a three-layered structure. The first layer consists of the fenestrated endothelial cells of the glomerular capillaries, which have small pores, about 70 nanometers, that allow rapid fluid passage while preventing blood cells from filtering through. Surrounding these cells is a negatively charged glycocalyx that helps repel similarly charged molecules.
The second layer is the glomerular basement membrane, a layer composed of type IV collagen, heparan sulfate proteoglycans, and laminin. This membrane is 250 to 400 nanometers thick and acts as a barrier to large blood proteins like albumin and globulin. The third layer is formed by epithelial cells called podocytes, part of Bowman’s capsule, with foot-like processes that create tiny filtration slits. These slits are bridged by a thin diaphragm with small pores, further preventing large molecules, such as proteins, from passing into the filtrate.
This selective filtration process allows water, salts, glucose, amino acids, and waste products like urea and creatinine to pass from the blood into Bowman’s capsule, forming a fluid called glomerular filtrate. Blood cells and large proteins are retained in the bloodstream. The glomerular filtrate then moves into the renal tubule for further processing.
The Glomerulus’s Role in Overall Health
The functioning of the glomerulus is important for maintaining the body’s internal balance, a state known as homeostasis. Its main function of filtering blood is important for removing metabolic waste products, such as urea and creatinine, byproducts of the body’s normal processes. Without this constant removal, these substances would accumulate to toxic levels.
Beyond waste elimination, the glomerulus plays a role in fluid and electrolyte balance. By filtering excess water and ions like sodium and potassium, it helps regulate the volume and composition of bodily fluids. This regulation is also interconnected with blood pressure control, as the kidneys, through processes initiated by glomerular filtration, influence the fluid volume in the circulatory system. The rate at which the glomeruli filter blood, known as the glomerular filtration rate (GFR), is a measure of overall kidney function and is adjusted by the kidneys to maintain a stable internal environment.
What Happens When the Glomerulus is Damaged?
When the glomeruli are damaged, their ability to filter blood effectively is compromised, leading to health consequences. Instead of removing waste and excess fluids, damaged glomeruli may allow substances, such as proteins and red blood cells, to leak into the urine. The presence of excessive protein in the urine, known as proteinuria, can cause the urine to appear foamy.
The loss of proteins like albumin from the blood can decrease their levels in the bloodstream. Since albumin helps draw fluid from the body into the blood, its reduction can result in fluid accumulation in tissues, causing swelling, also known as edema, often noticeable as swelling in the body. Additionally, impaired filtration can lead to a buildup of waste products in the blood, a condition known as uremia or azotemia, causing symptoms like fatigue and nausea. If not managed, this decline in glomerular function can progress to chronic kidney disease and eventually kidney failure.