What are the transport mechanisms used by capillaries?

Capillaries, the smallest blood vessels in the circulatory system, facilitate the exchange of substances between the blood and the surrounding tissues. They use several transport mechanisms to accomplish this vital function:

1. Diffusion: Diffusion is the primary mechanism by which substances move across the capillary walls. It involves the movement of molecules from an area of higher concentration to an area of lower concentration. Oxygen, carbon dioxide, glucose, and other small molecules diffuse across the thin capillary walls, driven by concentration gradients.

2. Filtration: Filtration occurs when fluid and solutes are forced out of the capillary into the interstitial fluid (the fluid surrounding the cells) due to hydrostatic pressure. The hydrostatic pressure within the capillary is higher than the osmotic pressure (the pressure exerted by the dissolved particles in the fluid), causing fluid and small solutes to filter out.

3. Reabsorption: Reabsorption is the opposite of filtration and occurs when fluid and solutes move back into the capillary from the interstitial fluid. It happens when the osmotic pressure within the capillary is higher than the hydrostatic pressure. Proteins, such as albumin, exert osmotic pressure and help retain fluid within the blood vessels.

4. Vesicular Transport: Transcytosis, also known as vesicular transport, is a mechanism used to transport larger molecules or particles across the capillary endothelium. It involves the formation of vesicles that engulf the substances and carry them across the capillary wall. This process is particularly important for transporting certain proteins, lipoproteins, and macromolecules.

5. Channel-Mediated Transport: Some capillaries contain specialized channels or pores that allow specific molecules to pass through. These channels are formed by proteins embedded in the capillary endothelial cell membranes. For example, water-soluble molecules like glucose can utilize specific channels called aquaporins to facilitate their transport.

The interplay of these transport mechanisms ensures that essential nutrients, oxygen, and other substances are delivered to the tissues, while waste products, such as carbon dioxide and urea, are removed. This continuous exchange of substances is crucial for maintaining homeostasis and supporting cellular functions throughout the body.