Glomerular filtration is the first step in the kidneys' production of urine for the body to excrete. The glomerulus is part of a nephron; each healthy human kidney contains 1 million nephrons and all of them function to filter the blood to remove waste products. A glomerulus is made up of an afferent ("arriving") arteriole, an efferent ("exiting") arteriole, and a capillary bed within a structure called Bowman's capsule. The blood from the body arrives at the nephron via the afferent arteriole and is filtered at the capillary bed through four forces before leaving the capsule via the efferent arteriole. Those four forces determine the rate of filtration at the glomerulus, two of which are "oncotic" and two "hydrostatic."
The glomerulus contains two distinct regions: the blood vessels and the tubes of the nephron where blood is not normally found. Blood flows into Bowman's capsule, where the capillaries have some unique anatomical features to allow for filtration of only certain substances. First, the capillary walls are "fenestrated," which means there are tiny holes that can allow molecules such as water, ions and waste products to pass through. These holes are too small, however, for larger substances such as plasma proteins or red blood cells to cross. Second, the capillaries are covered with specialized filtration cells called "podocytes" which have many finger-like protrusions on their surface. These allow the cells to "interdigitate," like interlacing fingers, which further restrict the size of molecules able to cross from the blood to the tubes. The contents of the tubes are therefore made up of waste products, water, ions and other small molecules, which together are termed "filtrate."
Oncotic Versus Hydrostatic Pressure
The two kinds of pressures result in filtration at the glomerulus, and can change the rate at which blood is filtered. Hydrostatic pressure is a "pushing pressure" exerted by water. Water will always try to "push" itself across a membrane to ensure an equal concentration of water on each side of the membrane. For example, in a container with a water-permeable divider down the middle and very salty water on one side and pure water on the other, the pure water would flow across the divider to dilute the salty side. The concentration would become closer to equal because of the force exerted by the hydrostatic pressure from the pure water side.
Oncotic pressure is a "pulling pressure" exerted by dissolved substances in water. In the previous example, the salt in the water would have exerted an oncotic pressure to "pull" pure water across the divider.
Hydrostatic pressure inside the glomerular capillaries is the primary determining factor in the rate at which blood is filtered. It is based on the blood pressure of the body. This is the force exerted to push water (and its ions and small molecules) across the capillary membrane from the blood to the tubular capsule. The rate of this filtering process is directly related to the blood pressure as it enters the capillary bed.
The hydrostatic pressure is opposed by the pressure exerted by the substances present in the blood, such as proteins and large molecules. These create an oncotic "pulling" pressure that serves to pull some of the water back into the blood away from Bowman's capsule. However, the blood pressure (hydrostatic force) is typically not affected significantly by the opposing oncotic force.
Other Filtration Forces
The liquid, or filtrate, in Bowman's capsule contains both water and substances which exert their respective pulling and pushing forces at the glomerular membrane. However, both of these forces are minimal in comparison to the capillary blood pressure. Capsule/tubular hydrostatic pressure is the pressure that pushes the water back across the membrane toward the blood in the capillary bed. Capillary oncotic pressure is the pulling pressure of the substances in the filtrate, promoting filtration and increased flow across the membrane toward the tubules. However, because the substances here are so much smaller than the plasma proteins and other cells in the capillary blood, the force they exert is insignificant.
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