Medc0002 - 6 Capillary Exchange PDF

Preview

Summary

cardiovascular systems in health and disease notes 6...

Description

CAPILLARY EXCHANGE ➔ Overview ◆ The exchange of molecules from the bloodstream to tissues of the body, carrying waste products in need of disposal, and delivering needed nutrients, is key for the body to function at its optimum. This process occurs in blood vessels called capillaries ➔ Structure of capillaries ◆ The structure of capillaries facilitates the exchange of molecules ◆ FICK’S LAW: rate of diffusion is proportional to the concentration difference and area available for diffusion, and inversely proportional to the diffusion distance ● To maximize the area for diffusion, many capillaries supply the same tissue. A constant blood flow through these capillaries maintains a large concentration gradient to allow for the exchange of molecules with the tissue. ● Singular capillaries have a large resistance, however many capillaries in parallel have their resistance reduced, allowing for more efficient blood supply to the tissues ● The diffusion distance is very little because the endothelium of capillaries is just 1 cell thick, so capillaries have a very small diameter

➔ Gas exchange ◆ Gas exchange is an example of capillary exchange: it occurs between the end parts of the lungs and the pulmonary capillaries ◆ Pulmonary capillaries optimize diffusion rate by receiving a constant blood supply, having a very thin membrane, and forming a network of capillaries over the alveoli. ◆ Furthermore, the alveoli have a huge surface area which also increases the area available for diffusion ◆ Problems: many diseases can interfere with this diffusion rate optimization. ● e.g. some pulmonary diseases cause fibrosis or oedema, which increases the diffusion distance that the molecule has to travel. Other

diseases, such as emphysema, cause the alveoli to lose their shape cluster with one another, forming a single alveoli which has a smaller surface area than the previous two.

➔ Starling’s forces ◆ The movement of fluids between capillaries and tissues is controlled by 4 forces: 1) BLOOD HYDROSTATIC PRESSURE: pressure exerted from the blood into the capillaries against the capillary wall. This P forces fluids out of capillaries 2) BLOOD COLLOID OSMOTIC PRESSURE: pressure exerted by proteins in the blood (mainly albumin in capillaries). Normally, these proteins are unable to diffuse into the interstitium, only when there's disease, they can. This P tries to pull fluid into the blood. 3) INTERSTITIAL HYDROSTATIC PRESSURE: pressure of the fluid in the interstitium. This P forces fluid back into the capillaries 4) INTERSTITIAL COLLOID OSMOTIC PRESSURE: pressure of proteins in the interstitium. This P pulls fluid out of the capillaries.

➔ Clinical relevance: Kwashiorkor ◆ Disease caused by severe malnutrition: limited amount of protein intake, causes the liver to produce less plasma proteins such as albumin. This decreases the blood’s colloid osmotic pressure, meaning less fluid will be returned to the capillaries, and will stay in the tissues. ◆ Consequence: build up of fluid → oedema. In Kwashiorkor this may mask the typical muscle wastage commonly seen in other malnutrition diseases, as patients with Kwashiorkor have distended abdomens but thin extremities. ➔ Clinical relevance: inflammation ◆ Inflammation occurs in response to cell injury

◆ It stimulates the dilation of arterioles, thereby increasing the hydrostatic pressure in teh capillaries ◆ Chemicals present in inflammation such as histamines, causes capillaries to become more permeable,which allows protein to flow into the interstitium, thereby increasing the interstitial colloid osmotic pressure ● These two factors both cause an increase of fluid to move out into the interstitium and explains the typical swelling seen in inflamed areas...