A glomerulus, plural glomeruli, is a knot of extremely tiny blood vessels involved in filtering the blood to form urine. There are many glomeruli inside the kidney, each contained within a capsule known as Bowman’s capsule. Blood at high pressure passes into the glomerulus through a tiny artery. Water and soluble waste products filter out of the blood through the leaky walls of the glomerulus and enter a space inside Bowman’s capsule before draining away through a tube, called a renal tubule, into the bladder. The remaining blood leaves the glomerulus through another small artery to rejoin the general circulation.
Each glomerulus and its associated renal tubule make up a unit known as a nephron, and in humans there are more than a million nephrons in each kidney. Normally, the glomerular filtration rate is so efficient that the whole of the blood volume passes through the kidneys within about five minutes. The tiny artery supplying the glomerulus is called an afferent arteriole, and it divides into numerous tiny vessels, or capillaries, to form the capillary tuft that is the glomerulus. Blood pressure inside the glomerulus is relatively high, helping to force blood out of the capillaries. The capillary walls are full of minute pores through which filtration occurs.
Bowman’s capsule has two walls with a space in between. One wall fits tightly around the glomerular capillaries and serves as an extra layer through which blood is filtered. Special cells called podocytes make up this wall, with finger-like projections which interlock to form filtration slits. Blood must pass through these slits as well as filtering through the pores in the glomerular capillaries; this helps prevent large molecules from passing into the space in Bowman’s capsule. Podocytes also help hold the glomerular structure together, stopping it from expanding in response to the high pressure inside it.
Glomerular disease, where the glomeruli become inflamed or scarred, usually causes problems with filtration. A wide range of illnesses can involve the glomeruli, including HIV and diabetes. Typically, the glomerular filter becomes more permeable, resulting in loss of protein and blood into the urine. Removal of waste products may not be as effective, leading to them building up in the blood. Symptoms may include foamy and pink urine and swollen ankles or hands.
Treatment of glomerular disease depends on the specific illness, but if the damage to glomerular tissue is extensive enough that the kidneys fail, patients will need dialysis. Dialysis may involve filtering blood through a machine, known as hemodialysis, or through a membrane in the abdomen, known as peritoneal dialysis. There may also be the option of having a kidney transplant.