Two Modes of Transport Are Used in the Cardiovascular System: Bulk Flow and Diffusion
Blood moves through blood vessels by bulk flow. The most important feature of bulk flow is that it is rapid over long distances. Blood that is pumped out of the heart travels quickly through the aorta and its various branches, and it reaches distant parts of the body, including the head and limbs, within 10 seconds.
Transport requires energy, and the source of energy for bulk flow is a hydrostatic pressure difference; unless the pressure at one end of a blood vessel is higher than the pressure at the other end, flow will not occur. The difference in pressure between two points in a blood vessel is called the perfusion pressure difference or, more often, simply perfusion pressure. Perfusion literally means “flow through,” and the perfusion pressure is the pressure difference that causes blood to flow through blood vessels. 'Γhe muscular pumping action of the heart creates the perfusion pressures that constitute the driving force for bulk flow of blood through the circulation.It is important to distinguish between perfusion pressure difference and transmural pressure difference (usually shortened to transmural pressure). Transmural means “across the wall,” and transmural pressure is the difference between the blood pressure inside a blood vessel and the fluid pressure in the tissue immediately outside the vessel (transmural pressure equals inside pressure minus outside pressure). Transmural pressure is the pressure difference that would cause blood to flow out of a vessel if a hole were poked in the vessel wall. Transmural pressure is also called distending pressure, because it corresponds to the net outward “push” on the wall of a blood vessel. Figure 18-1 emphasizes the distinction between perfusion pressure and transmural pressure.
FIGURE 18-1 ■ Fluid pressures associated with a blood vessel.
Pin∣βt, P0ul∣βt, and Pinside refer to blood pressure within the vessel. Poutside refers to the pressure in the tissue fluid (interstitial fluid) immediately outside the blood vessel. Perfusion pressure is the pressure difference along the length of a blood VesseLTransmuraI pressure (distending pressure) is the pressure difference across the wall of the vessel, indicated here at the midpoint of the vessel. Perfusion pressure is the driving force for blood flow through the vessel, whereas transmural pressure is the driving force that would cause blood to spill out of the vessel if there were a hole in it.Diffusion is the second mode of transport in the cardiovascular system. Diffusion is the primary mechanism by which dissolved substances move across the walls of blood vessels, from the bloodstream into the interstitial fluid, or vice versa. Interstitial fluid is the extracellular fluid outside capillaries. It is the fluid that bathes each cell of a tissue. Most of the movement of substances between the blood and the interstitial fluid takes place across the walls of the capillaries, the smallest blood vessels. For a substance (e.g., oxygen) to move from the bloodstream to a tissue cell, it first diffuses across the wall of a capillary and into the tissue interstitial fluid, then diffuses from the interstitial fluid into the tissue cell.
The source of energy for diffusion is a concentration difference. A substance diffuses from the bloodstream, across the wall of a capillary, and into the interstitial fluid only if the concentration of the substance is higher in the blood than in the interstitial fluid (and if the capillary wall is permeable to the substance). If the concentration of a substance is higher in the interstitial fluid than in the blood, the substance will diffuse from the interstitial fluid into the capillary blood. It is important to distinguish diffusion, in which a substance moves passively from an area of high concentration toward an area of low concentration, from active transport, in which substances are forced to move in a direction opposite to their concentration gradient. In general, substances are not transported actively across the walls of capillaries. The movement of substances between the bloodstream and the interstitial fluid occurs by passive diffusion.