Bowman's capsule, also known as a glomerular capsule or capsula glomeruli, is a double-walled, cup-shaped structure inside the kidney. The kidney contains up to two million units called nephrons. Each consists of a tube, one closed end of which is swollen to form Bowman's capsule while the other opens into the renal pelvis, the space from which urine leaves the kidney. A knot of tiny blood vessels called a glomerulus sits inside the capsule. Blood entering the kidney passes through the glomerulus and is filtered through blood vessel walls and the outer wall of Bowman's capsule, before the resulting filtrate collects inside the capsule.
The function of the kidney involves removing waste and harmful products from the blood, while conserving useful substances and water. In order to achieve this, different parts of the kidney act together to filter out and then reabsorb different factors from the blood. Eventually, the correct composition is achieved to maintain the body's fluid and electrolyte balance. Urine is formed at the end of the process. It contains waste material and fluid which is expelled from the body during urination.
Bowman's capsule and the glomerulus form a unit called the renal corpuscle, in which the initial process of filtration occurs. A small artery carries blood into the glomerulus, where it is filtered at high pressure through two layers of cells before the remaining blood leaves through another artery. The first layer of cells, the lining of the glomerular blood vessels, is full of holes that are too small to allow blood cells and proteins to pass out, but which allow smaller molecules through. Substances leaving the blood then pass through a second cell layer, formed by the outer wall of Bowman's capsule.
The outer layer of Bowman's capsule consists of specialized cells called podocytes, which have long thin projections. These are arranged to interlink with the projections from other podocytes like interlocked fingers, leaving narrow slits between them. Small particles which have filtered through the holes in glomerular blood vessels must pass through these slits before entering the space inside Bowman's capsule. The composition of the filtrate at this point is the same as blood, apart from the lack of blood cells and proteins.
Next, the filtrate passes through a series of tubes in which some factors are absorbed back into the blood and others are secreted out. The body actively controls many of these processes so that the composition of body fluids can be regulated. Finally, the remaining urine drains through a tube called the collecting duct into the renal pelvis, ureter and bladder.