i need Understand the role of capillaries in fluidnutrient e

i need Understand the role of capillaries in fluid/nutrient exchange. How is blood directed to or away from particular capillary beds?

Solution

Capillaries are the tiniest blood vessels that are present in the body of an organism and facilitate the transition between arteries and veins. The Arteries bring the blood from the heart to the various organs of the body and the size of these vessels decreases to form arterioles and the further decreases till the time it becomes a capillary. These capillaries are small in size but more in number and thus form a network over any specific organ. these capillaries then as the blood flows further turn into broader blood vessels and eventually the veins which carry the blood back to the heart. These capillaries can be called as microscopic blood vessels as many a times they may have walls that are made of only a single celled endothelial layer. Their microscopic structure makes material exchange possible as capillaries are the only blood vessels that can carry out the fluid and nutrient exchange. Due to the presence of a large network the surface area for exchange by these capillaries is huge. The capillaries at any time hold only about 5% of the total blood volume and the human body has approximately 40 billion capillaries. the size of the capillaries is only 5–10 nm in diameter. At any given time only a around 25 percent of capillaries are fully filled with blood, especially in tissues at rest, as blood flow in microvessels is dependent on the metabolic activity of the tissue and is regulated at the sites of their origin by sphincter muscles.

There are three types of capillaries based on their structures – Continuous capillaries, fenestrated capillaries and sinusoids. Continuous capillaries are found in the skin and muscles which have an uninterrupted lining of endothelial cells held together by tight junctions. Fenestrated capillaries are found wherever active capillary absorption or filtrate formation occurs like small intestine etc., having pores in the endothelium to facilitate diffusion. The Sinusoids have a slow blood flow and are modified by leaky pored capillaries with lumens for exchange of large molecules. The structurally different capillaries are present to facilitate the transport according to the functional requirement of the body.

The main function of capillaries is exchange of material – Gases, nutrients and fluids. The exchange of gases is primarily of oxygen and carbon di oxide. This exchange occurs by basic diffusion through the semipermeable layers. The gases move from higher to lower concentration. In the lungs the oxygen enters the blood though the blood capillaries and the carbon di oxide is taken up at regions where oxygen is given out of the blood, in organs that have accumulated the increased carbon di oxide levels after respiration process is done. Gases can easily pass through the semi permeable layers of the capillary walls. The lipid soluble substances can also easily pass through the wall as its made of a lipid bilayer. Any non lipid soluble substance is transported across the wall by Transcytosis by forming pinocytotic vessels at one side and then releasing it at other end. The capillaries also help carry out exchange of water and solutes by osmosis across the pressure gradient. There exists two different kinds of pressure i.e. Hydrostatic pressure within the capillary vessels and osmotic pressure. Hydrostatic pressure is influenced by the overall blood pressure. These two forces interact in a network of capillaries in between arteries and veins called the capillary bed. These beds are found in major muscles, tissues, and anywhere that needs nutrients from the blood. As blood begins to move into capillaries from the arteries, the small size of the blood vessel increases blood pressure. Blood pressure is higher than osmotic pressure so the fluids containing oxygen and other nutrients are diffused out of the capillary and into the surrounding body tissue. Capillary beds are basically used for diffusion between the interstitial fluid and blood regulated by a fluid balance. Fluids cross the capillary walls by ultrafiltration and reabsorption in a capillary bed. At the arteriole section the pressure difference between blood and interstitial fluid forces out movement of plasma. This bulk movement of plasma out if the capillary is called ultrafiltration. In reabsorption the interstitial fluid moves across into the capillary by osmotic pressure.