Blood - Tissue Functioning and Remodeling in the Circulatory and Ventilatory Systems - page 1

Chapter 1
Blood
“. . . there is a continuous and uninterrupted movement of blood from the heart through the
arteries to the body as a whole, and likewise back from that body as a whole through the
veins to the heart, with such flow and ebb that in such quantity and amount that it must
somehow move in circle.” (A letter from W. Harvey to C. Hofmann [ 1 ])
Blood is a major tissular component of the closed circulatory system, which
supplies nutrients and oxygen using hemoglobin-containing red blood capsules to
the body's cells and removes metabolic wastes. This specialized liquid is composed
of cells suspended in a liquid, the plasma (mostly water). It flows throughout the
body in blood vessels due to a pressure difference set by the heart pump.
Blood cells are involved in tissue adaptation to hypoxia via angiogenesis as well
as treatment of injury via blood coagulation and healing and other types of tissue
damage, such as that caused by infection, by triggering inflammation and immunity.
Carried cells, like vessel wall cells, sense and respond to mechanical stresses exerted
by the flowing blood.
Blood is propelled throughout the circulatory system by the heart. Blood is
ejected from the left and right cardiac ventricles into the systemic and pulmonary
circulation, respectively. In the heart, nodal myocytes of the natural pacemaker —
the sinoatrial node — spontaneously and rhythmically generate an electrochemical
wave, or the so-called action potential, which characterizes heart automatism. This
action potential then propagates down to other atrial and ventricular nodal cells and
cardiomyocytes. The latter also propagate the action potential and contract to propel
the blood. Coupling of (1) generation and propagation of the action potential (heart
electrical activity); (2) cardiac wall contraction and deformation (solid mechanics
aspects of heart activity); and (3) blood filling in and ejection out of both coupled
ventricular pumps associated with closing and opening of auriculoventricular and
ventriculo-arterial valves and myocardium perfusion is a non-trivial task. This
problem has thus been tackled by splitting and separately processing the major
processes. Today, accumulated knowledge, appropriate modelings, and updated
technologies allow their coupling.