Common Misconceptions in Biology

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Common Misconceptions in Biology

Circulatory system
(a) Formation of tissue fluid and exchange of materials in the capillary network
In a capillary network, the movement of fluid between the blood and tissue fluid is mainly determined
by two opposing forces: (1) the hydrostatic pressure difference and (2) the osmotic potential
difference between the blood and the tissue fluid. In the part of the capillary network near the arterial
end, blood pressure is much higher than that of the tissue fluid so that the difference in hydrostatic
pressure exceeds the osmotic difference between the two fluids. As a result, some plasma is filtered
out of the capillaries under pressure into the tissue space to form tissue fluid. As blood moves along
the narrow capillaries, the blood pressure drops continuously so that the difference in hydrostatic
pressure between the blood and tissue fluid decreases steadily or may even be reversed. At the
venous end of the capillary network, most the tissue fluid formed at the arterial end is reabsorbed
back into the capillaries by osmosis
This mechanism results in a continuous formation of tissue fluid from the plasma by filtration and the
return of tissue fluid back to the blood by osmosis . This circulation of tissue fluid is
essential for the regulation of blood pressure and blood volume. For instance, when there is a drop
in blood pressure such as due to haemorrhage, less tissue fluid would be formed at the arterial end
and more tissue fluid will be absorbed back into the blood at the venous end of the capillary
network. The net flow of tissue fluid into the capillary network results in a rise in blood volume and
blood pressure to normal .The opposite occurs when the blood pressure or blood
volume increases.
When the osmotic potential of the blood is raised, e.g. due to a low plasma protein level, the
osmotic difference between the blood and tissue fluid decreases. This results in a net formation of
tissue fluid from the plasma as the volume of tissue fluid formed exceeds that returned to the plasma
As the tissue fluid volume (about 10 litres) is three to four times larger than the volume of plasma
(about 3 litres), the tissue fluid serves as a reservoir which can supply additional fluid to the
circulatory system or draw off excess. This mechanism of tissue fluid formation and withdrawal is
important in maintaining a constant plasma volume.
This process, however, plays a relatively minor role in the exchange of nutrients and metabolic
wastes between the blood and the tissue. Although some nutrients and wastes are carried by mass
flow during this fluid movement, most nutrients and metabolic wastes are transported between blood
and tissue cells by diffusion according to the concentration gradient of these substances. Thus
glucose diffuses from the blood to the tissue cells across the capillary wall along the whole length of
the capillary network, not just restricted to the arterial end. Similarly, waste substances produced by
the body cells diffuse into the blood along the capillaries in the opposite direction This
process of diffusion is facilitated by the very thin capillary wall and the numerous branches of the
capillary network which present a very large surface area for diffusion to occur. The low rate of
blood flow along the capillaries also allows enough time for diffusion to take place.
Many local textbooks elaborate on the formation of tissue fluid but give little description on the
diffusion of substances between the blood and the tissue. Thus many students develop the mistaken
belief that the formation and absorption of tissue fluid in the capillary network is essential for the
transport of materials between blood and body cells, and fail to appreciate the importance of
diffusion in the exchange of materials in the capillary network (HKEA, 1992, p. 50).
(b) Adaptive features of the capillary network
It is a well known fact that blood flows very slowly along the capillaries and this low flow rate
facilitates the exchange of materials by diffusion between the blood and tissue fluid. In accounting for
the low blood flow rate in the capillaries, many students wrongly think that it is due to the narrow
diameter of the capillaries which presents a high