Module 8 Intro

Module 8—Circulation, Immunity, and Excretion

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Circulation and the Action of Capilliaries

 

Capilliaries 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. 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 unidirectional blood flow.

 

Plasma Interstitial Fluid Exchange

 

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.

 

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 capilliary. At the arteriol 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 arteriol 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.

 

In the diagram to the left, the force exerted by water inside the capillary will eventually equal the force of diffusion from the interstitial fluid, creating equilibrium. When equilibrium is reached, water continues to flow, but it flows both ways in equal amounts as well as forces. This is why osmotic pressure remains constant over the length of a capillary.

 

Hydrostatic pressure tends to cause fluid to leave the plasma, and osmotic pressure pulls it back. These two forces balance each other under normal conditions.

 

Hydrostatic pressure gradually decreases over the length of a capillary, while osmotic pressure remains constant.  If these pressures were graphed, they would look approximately like the following figure.

 

Biology 20 © 2008 Alberta Education