Question

Describe how substances are exchanged at the blood capillaries.

Answer 1

Capillaries are the smallest blood vessels in the body which facilitates the exchange of carbon dioxide,oxygen, hormones, nutrients and waste products.

Capillary exchange is a process by which exchange of material occurs from the blood in the caillary into the tissues of our body.

It is mainly carried out by means of 3 mechanisms: Diffusion,transcytosis and bulk flow

Diffusion is the process which allows flow of small molecules across the capillaries.Eg:Glucose and Oxygen from the blood into the tissues and CO2from the tissues into the blood. It is based on the difference of gradients between blood and interstitium(molecules move from high-concentrated to low-concentrated areas)

Transcytosis is the process by which large molecules move across the capillary membrane.Mainly lipid soluble molecules are transorted by ths process. At first, molecules from the blood are endocytosed by the capillary endothelial cell to form a lipid vesicle and then it is exocytosed into the tissue side.

Bulk flow is a process by which small, lipid insoluble molecules are being transported across the capillary membrane.It is dependent on net filtration pressure which is derived from 4 starling forces.

This capillary dynamics is driven by 2 opposing factors mainly,which are oncotic pressure and hydrostatic pressure.

oncotic pressure is a pulling force exerted by the proteins in te blood plasma or interstitial fluid.

Hydrostatic pressure is a pushing force generated by the pressure of fluid on the capillary wall.

Net filtration pressure is derived from the 4 starling forces(oncotic pressure in capillary, oncotic pressure in interstitium, hydrostatic pressure in capilllary and hydrostatic pressure in interstitium)

SO, blood hydrostatic pressure and interstitial fluid oncotic pressure favors the fluid flow from blood into interstitium, and blood oncotic pressure and interstitial fluid hydrostatic pressure favors fluid flow from interstitium into blood.