Blood - Tissue Functioning and Remodeling in the Circulatory and Ventilatory Systems - page 2

The cardiac pump sets up a pressure wave that impinges strongly upon walls of
adjacent elastic arteries, which are then capable of ensuring a blood flow during
the ventricular diastole. The blood vessel wall is a living tissue that quickly reacts
to loads applied on it by the flowing blood. In any segment of the vasculature,
endothelial and smooth muscle cells sense space and time variations in small-
magnitude wall shear stress and large-magnitude wall stretch generated by the
flowing blood. These cells respond with a short time scale (from seconds to hours)
to adapt the vessel caliber according to the loading, especially when changes exceed
the limits of the usual stress range. The mechanotransduction pathways determine
the local vasomotor tone and subsequently the lumen bore of the reacting blood
vessel. This regulatory mechanism is much faster than the nervous and hormonal
control. It sets up the level of local resistance to blood flow: (1) in large arteries,
which ensure the blood distribution in the body, to limit the cardiac postload; (2) in
small resistive arteries, which irrigate the body's tissues, to maintain the flow rate;
and (3) in arterioles, which perfuse a cell population, to adapt the blood supply to
metabolic needs of active cells.

1.1 Blood Functions

The blood performs 3 major functions: (1) transport through the body; (2) regulation
of bulk equilibria; and (3) body immune defense against foreign bodies.
Blood supplies oxygen, hence energy, and conveys nutrients (vitamins, mineral
ions, glucose, amino acids, fatty acids, among other glucids, protids, and lipids)
to the tissues and removes carbon dioxide and waste products of cell metabolism
toward lungs and purification organs. Kidneys filter blood. Toxins are not only
removed in urine, but also by sweating. Blood transmits metabolic factors and
messengers such as hormones to target organs.
Blood volume and electrolyte concentration are regulated. Blood maintains the
body temperature (36.4-37.1 ◦ C) and acid-base equilibrium, controlling blood pH
that remains in the range 7.35 to 7.45. 1 Blood circulation transports heat throughout
the body, thereby contributing to the thermoregulation. Increased blood flow at
the body's surface during warm weather or exercise permits higher heat loss.
Conversely, when the ambient temperature decays, blood flow near the skin lowers
to prevent heat loss.
Blood participates in the body's defense against infection, as it transports
immunocytes and antibodies, as well as in repair after injury. Innate immunity yields

1 The major blood buffer is composed of bicarbonate ions, from which carbonic acid and water can
be obtained. Carbonic acid is unstable and decomposes into carbon dioxide and water.