In: Anatomy and Physiology
Discuss the mechanisms involved in the formation of the glomerular filtrate, its composition and the regulation of its rate of formation, the GFR.
All blood in the body flows through the kidneys hundreds of times each day. The kidneys push the liquid part of blood through tiny filters (called nephrons), then reabsorb most of the fluid back into the blood. The fluid and waste products that the kidneys don't reabsorb are excreted as urine.
The rate of blood flow through the kidneys is the glomerular filtration rate, or GFR. (The glomeruli are microscopic bundles of blood vessels inside nephrons, and are crucial parts of the filtering system.) The glomerular filtration rate can't be measured directly -- that's where measuring creatinine and creatinine clearance comes in.
Blood entering the glomerulus has filterable and non-filterable components. Filterable blood components include water, nitrogenous waste, and nutrients that will be transferred into the glomerulus to form the glomerular filtrate.
How glomerular filtration works
The first step in making urine is to separate the liquid part of your blood (plasma), which contains all the dissolved solutes, from your blood cells. Each nephron in your kidneys has a microscopic filter, called a glomerulus that is constantly filtering your blood.
Blood that is about to be filtered enters a glomerulus, which is a tuft of blood capillaries (the smallest of blood vessels). The glomerulus is nestled inside a cup-like sac located at the end of each nephron, called a glomerular capsule. Glomerular capillaries have small pores in their walls, just like a very fine mesh sieve. Most capillary beds are sandwiched between arterioles and venules (the small vessels delivering blood to and collecting blood from capillary beds), and the hydrostatic pressure drops as blood travels through the capillary bed into the venules and veins.The glomerulus, on the other hand, is sandwiched between two arterioles - afferent arterioles deliver blood to the glomerulus, while efferent arterioles carry it away. Constriction of efferent arterioles as blood exits the glomerulus provides resistance to blood flow, preventing a pressure drop, which could not be achieved if blood were to flow into venules, which do not really constrict. The two arterioles change in size to increase or decrease blood pressure in the glomerulus. In addition, efferent arterioles are smaller in diameter than afferent arterioles. As a result, pressurized blood enters the glomerulus through a relatively wide tube, but is forced to exit through a narrower tube. Together, these unique features plus the fact that your heart is supplying your kidneys with over a liter of blood per minute (around 20% of its output) maintain a high glomerular capillary pressure and the filtration function of the kidney, regardless of fluctuations in blood flow. For example, the sympathetic nervous system can stimulate the efferent arteriole to constrict during exercise when blood flow to the kidney is reduced.
The physical characteristics of the glomerular capillary wall determine what is filtered and how much is filtered into the glomerular capsule. Working from the inside out, the capillary walls are made up of three layers:
How is the glomerular filtration rate regulated?
It is perfectly normal for your blood pressure to fluctuate throughout the day; however, perhaps surprisingly, this has no effect on your glomerular filtration rate. This is because under normal circumstances, your body can precisely control it:
Intrinsic mechanisms:
Extrinsic mechanisms: