Exchange of Matter

Capillaries are the smallest blood vessels in the body. They are the only vessels thin enough for the exchange of matter by diffusion. The cells of the body are constantly bathed in interstitial fluid, or extracellular fluid. Any material exchanged must pass through the interstitial fluid. Nutrients, gas, and other materials are transported into and out of the capillaries, depending on if they are necessary reactants or waste products of metabolism. The exchange of matter from capillary to interstitial fluid or vice versa is dependent on concentration gradients.

Substances move from high concentrations to low concentrations. This is referred to as a concentration gradient. Blood flow is also dependent on pressure differences. Fluid moves from high pressure to low pressure. Therefore, the pressure in the capillaries is lower than it is in the arteries but higher than in the veins. This ensures uni-directional blood flow.

 Read

Read "Circulation and the Action of Capillaries" on pages 287 to 288 of your textbook.

 Hydrostatic and Osmotic Pressures

The movement of substances in and out of the capillaries is very complex. You are required to understand the basic movement as explained above and in the textbook. The following is a more detailed explanation of why these movements occur. Although this is not a required concept  in this unit, it will be helpful in this section and in the excretory section.

The aqueous solutions that compose the plasma and the interstitial fluid readily exchange through the thin walls of most of your body's capillaries. The forces that govern this exchange are hydrostatic pressure (the blood pressure within the capillaries) and osmotic pressure (the pressure created by osmosis).

Hydrostatic Pressure

The capillary wall acts as a filtration barrier. Most of the fluid within the capillaries is retained, but some fluid filters through pores between the cells. This fluid is pushed by the pressure difference between the capillary blood and the interstitial fluid. Water and small solutes can pass freely through these pores.

Hydrostatic pressure is based on the loss of water and solutes from the capillary. At the arterial end of a capillary, the hydrostatic pressure is higher because not much water or solutes (nutrients) have diffused out of the capillary. At the venous end of the capillary, the hydrostatic pressure is lower because substances have diffused out of the capillary and into the interstitial fluid and other cells. The capillary walls (both cells and pores) are impermeable to the plasma proteins and lipids. Under normal pressure conditions, the proteins and lipids stay within the plasma. However, if there is an imbalance, proteins and lipids may leave the plasma.

With hydrostatic pressure, water is more highly concentrated in the capillary. This influx of water from osmosis increases the hydrostatic pressure in the capillary. This tends to force the water back to the interstitial fluid. Hydrostatic pressure will be greater at the arterial end. This means there will be more water in the capillary in relation to other solutes. As solutes and water leave the blood, the hydrostatic pressure decreases toward the venous end of the capillary.

Osmotic Pressure

Because the capillary wall is permeable to water but essentially impermeable to plasma proteins, these molecules generate an osmotic pressure. This results in a pressure that draws water from the interstitial fluid into the blood plasma. Osmotic pressure remains constant over the length of a capillary.