Microcirculation Through Capillary Beds
Capillary beds are found in nearly every tissue and organ of the body. Capillaries are minuscule exchange sites where oxygen, nutrients, and hormones pass from the bloodstream into the interstitial fluid (space between capillary walls and cells), and carbon dioxide and metabolic wastes pass from the interstitial fluid back to the bloodstream.
A capillary bed is an interconnected network of capillaries. Capillary beds contain different types of capillaries (read about continuous, fenestrated, and sinusoidal capillaries in my previous blog) depending on the organ in which they are located. Blood flow through a capillary bed is called microcirculation. A capillary bed consists of true capillaries, which are the hundreds of tiny capillaries in which gas and nutrient exchange occurs between the capillary wall and interstitial fluid, and a vascular shunt, which bypasses the true capillaries.
The main arteriole that leads into the capillary bed is called the terminal arteriole. The terminal arteriole branches into the metarteriole, which diverges into the true capillaries. True capillaries eventually converge into the thoroughfare channel, which merges into the postcapillary venule. When cells do not need nutrients, the smooth muscle cells of the precapillary sphincter contract, preventing blood flow into the true capillaries. As a result, blood flows through the vascular shunt, directly from the metarteriole to the thoroughfare channel. However, when cells are in need of nutrients, the smooth muscle cells of the precapillary sphincter are relaxed, allowing blood to flow from the metarteriole into the true capillaries.
Figure 2: Blood flows through true capillaries, allowing diffusion of gases and nutrients to occur between capillaries and interstitial fluid
If the precapillary sphincters are relaxed and blood flows into the true capillaries, how do substances cross the capillary wall to reach cells in the interstitial fluid? Substances move across the capillary wall in a process called diffusion. Diffusion is the movement of substances from an area of high concentration to an area of low concentration. Thus, oxygen and nutrients diffuse from the capillary (where they are abundant) to the interstitial fluid (where they are found in a lower concentration). Carbon dioxide and wastes diffuse by the same mechanism.
Depending on their chemical properties, molecules diffuse across the capillary wall in different ways. Oxygen, carbon dioxide, and other hydrophobic molecules pass the capillary wall through the phospholipid bilayer of an endothelial cell's membrane. A phospholipid bilayer consists of two layers of phospholipids arranged in opposite directions. Because the central portion of a phospholipid bilayer is hydrophobic, hydrophilic substances such as glucose and amino acids are unable to diffuse across the endothelial cell's membrane.
Figure 4: Phospholipid bilayer in an endothelial cell's plasma membrane
Instead, hydrophilic substances pass the capillary wall through its fenestrations and intercellular clefts. Not all proteins, however, cross the capillary wall through diffusion. Some proteins and molecules that are too large to diffuse through fenestrations and intercellular clefts are transported by pinocytotic vesicles and caveolae. If, for example, if a protein has to be transported from the capillary to the interstitial fluid, it will attach to the endothelial cell's membrane. Then, the membrane will fold itself inward, forming a vesicle. After the vesicle has reached the other end of the membrane, it will empty its contents, allowing the protein to enter the interstitial fluid. The membrane that originally surrounded the vesicle will become part of the outer membrane of the endothelial cell. Most large molecules are able to move across a sinusoidal capillary's wall in the absence of pinocytotic vesicles and caveolae since it has larger fenestrations and wider intercellular clefts than those of continuous and fenestrated capillaries.
Figure 5: Movement of different types of substances across the capillary wall
Capillary Pressure
As gases, nutrients, and hormones are diffusing across the capillary wall, a lot of fluid circulating in the blood ends up moving between the capillaries and the interstitial space. In fact, throughout the day, an estimated 20 liters of fluid is lost to the interstitial space at the arterial end before it reenters the capillaries at the venous end (approximately seven times the plasma volume)! The movement of fluid in and out of a capillary has a big impact on the pressure exerted on its walls.
Figure 6: Pressures created by fluid exchange between capillary and interstitial fluid
Capillary hydrostatic pressure is the pressure that fluids inside the capillary exert on the capillary wall; it is relatively higher at the arterial end of a capillary bed than it is at the venous end. If fluids exert a high pressure on a capillary wall, they tend to leave the capillary and enter the interstitial space. Thus, as blood flows through a capillary bed, blood pressure decreases from about 35 mm Hg to 17 mm Hg due to the loss of fluid at the arterial end. Capillary hydrostatic pressure is opposed by interstitial fluid hydrostatic pressure, which is the pressure that the interstitial fluid exerts on the capillary wall. This pressure acts outside of capillaries, and tends to force fluids inside capillaries. However, it is usually low (close to zero) because lymphatic vessels are constantly draining the interstitial fluid. Therefore, the net hydrostatic pressure (the difference between the capillary and interstitial fluid hydrostatic pressures) is very close to the capillary hydrostatic pressure.
Some plasma proteins, such as albumin, are too large to diffuse across the capillary wall. As a result, they stay in the capillary and tend to attract fluids because the fluid concentration in their vicinity is less than that in the interstitial space. Capillary oncotic pressure, also known as colloid osmotic pressure, is the pressure that these nondiffusible proteins (or colloids) exert on the inside of capillary walls. It is about 26 mm Hg. Interstitial fluid oncotic pressure is the pressure that proteins in the interstitial fluid exert on the capillary wall. Because few proteins are present in the interstitial fluid, interstitial fluid oncotic pressure is low, ranging from 0.1 mm Hg to 5 mm Hg. While hydrostatic pressure changes drastically from the arterial end to the venous end of a capillary bed, oncotic pressure stays constant. Thus, if the capillary oncotic pressure were 26 mm Hg and the interstitial fluid oncotic pressure were 3 mm Hg, the net oncotic pressure would be 23 mm Hg.
The net filtration pressure (NFP) reflects whether there is a net gain or loss of fluid as blood moves through the capillary bed. If there is more fluid in the capillaries than the interstitial space (which means there is a high capillary hydrostatic pressure), fluid will move into the interstitial space. This is called filtration and usually occurs at the arterial end of a capillary bed. On the other hand, if there is more albumin inside the capillaries, (which means there is a high capillary oncotic pressure), fluid will enter the capillaries, which is called reabsorption and occurs at the venous end. NFP is calculated (at the arterial end) by subtracting the net oncotic pressure from the net hydrostatic pressure because the hydrostatic pressure is higher at the arterial end.
NFP= (Capillary hydrostatic pressure - interstitial fluid hydrostatic pressure) - (capillary oncotic pressure - interstitial fluid oncotic pressure)
If the capillary hydrostatic pressure = 34 mm Hg, interstitial fluid hydrostatic pressure = 1 mm Hg, capillary oncotic pressure = 26 mm Hg, and interstitial fluid oncotic pressure = 3 mm Hg, the NFP will be:
(34 - 1) - (26 - 3) = 10 mm Hg
This signifies that fluid is leaving the capillary at a pressure of 10 mm Hg. To calculate the NFP at the venous end, the net hydrostatic pressure has to be subtracted from the net oncotic pressure because oncotic pressure is higher.
If the capillary hydrostatic pressure = 17 mm Hg, interstitial fluid hydrostatic pressure = 1 mm Hg, capillary oncotic pressure = 26 mm Hg, and interstitial fluid oncotic pressure = 3 mm Hg, the NFP will be:
(26 - 3) - (17 - 1 ) = 7 mm Hg
This signifies that a pressure of 7 mm Hg is forcing fluids to enter the capillary from the interstitial space.
Hydrostatic and oncotic pressures are forces that balance each other out; "hydrostatic pressure pushes" while "osmotic pressure sucks."
Image References:
"Blood Vessels." Antranik, antranik.org/blood-vessels/
"Blood Vessels." DK Find Out, dkfindout.com.
"Cancer Discoveries." Cheryl Millett, cherylmillett.com/cancer-discoveries/
"Difference Between Hydrostatic and Oncotic Pressure." 21 Nov. 2017, Pediaa, pediaa.com/difference-between-hydrostatic-and-oncotic-pressure/
slideserve.com/noelle/ch-19
"Structure and Function of Blood Vessels." Lumen Learning, courses.lumenlearning.com/ap2/chapter/structure-and-function-of-blood-vessels/
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