24.5:
Capillary Beds
A capillary bed is a network of 10 to 100 capillaries that run between an arteriole and a venule, facilitating microcirculation.
The oxygenated blood first enters the terminal arteriole and flows into the capillary bed.
While passing through the capillary bed, oxygen, nutrients, and waste products from the blood are exchanged with the interstitial fluid.
The oxygen-poor blood later drains into the postcapillary venule.
The amount of blood passing through the capillary bed varies based on the tissue requirements.
For instance, after a meal, blood flows through the capillary bed, exchanging the nutrients released from digestion.
However, once digestion is complete, the capillary pathways close, and blood bypasses the capillary bed to drain directly into the postcapillary venule.
This redirection is achieved through the contraction of the precapillary sphincters, which are composed of smooth muscle fibers. They are located at the junctions of metarterioles — the short vessels connecting arterioles and capillaries.
Once the precapillary sphincters are closed, blood flows through the metarteriole and the thoroughfare channel, directly reaching the postcapillary venule.
24.5:
Capillary Beds
Capillary beds are networks of tiny blood vessels that play a crucial role in the circulatory system. These beds are where the exchange of gases, nutrients, and waste products occurs between the blood and surrounding tissues. Each capillary bed consists of numerous capillaries, which are the smallest blood vessels in the body, typically only one cell-thick. This thinness allows for the efficient diffusion of substances.
Capillaries connect arterioles, small branches of arteries, to venules, small branches of veins. Blood flow through capillary beds is regulated by pre-capillary sphincters, which can constrict or dilate to control the amount of blood entering the capillaries. This regulation is crucial for maintaining tissue homeostasis and responding to varying metabolic demands.
The structure of capillaries varies depending on their location and function. Continuous capillaries, found in most tissues, have tightly packed endothelial cells, limiting the passage of large molecules. Fenestrated capillaries, found in organs requiring extensive exchange, like the kidneys and intestines, have small pores that increase permeability. Sinusoidal capillaries, found in the liver, spleen, and bone marrow, have larger gaps, allowing the passage of larger molecules and cells.
The exchange process in capillary beds involves diffusion, filtration, and osmosis. Oxygen and nutrients diffuse from the blood into tissues, while carbon dioxide and metabolic wastes diffuse from tissues into the blood. Filtration occurs due to hydrostatic pressure, pushing fluid out of capillaries, while osmosis draws fluid back in due to osmotic pressure.
Capillary beds are vital for maintaining tissue health, supporting cellular activities, and ensuring efficient systemic circulation. Their dynamic regulation ensures tissues receive adequate oxygen and nutrients while removing waste products.