Principles of heart movements and circulation
The blood circulation takes care of the transport of oxygen and nutrients to the cells and the removal of CO2 and other waste products from these cells. The circulation plays an essential role in the thermoregulation of our body and in the distribution of hormones and other substances necessary to regulate the various chemical processes.
Blood (5 litres for an adult of approx. 70 kg) circulates in a closed circuit consisting of large and small vessels and the heart (Figure ~\ref{Fig:menscir}). Roughly, the circulation can be divided into two systems connected in series:
the systemic circulation or large or peripheral circulation
the pulmonary circulation or small circulation
The heart takes care of the propulsion of blood through the circulatory system. It consists of two parts: the left heart and the right heart. Each of these two parts is composed of two spaces: an atrium and a ventricle. The ventricles or chambers supply the main pumping force, made possible by the contraction of the heart muscle together with two essential valves, the inlet and the outlet valve. The atria mainly serve as blood reservoirs, making it possible for the ventricles to fill up quickly during relaxation.
figuur 2.1 Schematic of the human circulation. Note that the pressures in the left circulation are higher than the right circulation at similar locations. The muscle of the left heart is also thicker than the right heart muscle.
The left heart propels blood into the systemic circulation. When the left ventricle contracts, blood is pushed via the open aortic valve (see Chapter~\ref{ch:pulserend}) into the aorta. This blood flows to all parts of the body, with the exception of the lungs, via a network of arteries with ever smaller diameters until it reaches the capillaries where an exchange of substances will take place. Subsequently, the blood flows back to the right atrium via a network of venules and venes with ever increasing diameters. The right heart conveys blood to the lungs via the arteria pulmonalis (lung artery). Via a branching arterial system, blood is transported to the lung capillaries where O2 is absorbed and CO2 is released (Hoofdstuk~\ref{ch:massa}). Then the oxygenated blood enters the left atrium and the cycle can start all over again.
As indicated in Figure~\ref{Fig:menscir}, the pressures in the systemic circulation are higher than in the pulmonary circulation. A more precise scheme of the circulation is given in Figure 2.2. Note that the course of some of the flows differs from those described above.
The circulation of blood through the heart muscle itself, the so called coronary circulation, flows out into the right heart (see Chapt.~\ref{ch:corcir}).
Blood from the gastrointestinal organs having absorbed nutrients, as well as blood from the spleen, pancreas and gallbladder, does not enter the venous circulation directly. This blood flows via the vena porta to the liver, where it is made suitable for the circulation. Waste products and toxic material (e.g. alcohol) are absorbed by the liver. Moreover, nutrients are absorbed and stored in the liver as well. This direct flow to the liver forms a kind of security, but is also economical, because the nutrients and waste products in this flow occur in a much higher concentration than would be the case if it had been mixed with the circulating blood.
To be able to perform its functions, the liver needs arterial blood from the systemic circulation. This is supplied by the liver artery.
For more on basic principles of human circulation, see http://www.phys.mcw.edu/medphy/chout/Lectures/Lecture%2017.pdf
Anatomy of heart and circulation
A longitudinal section as well as a cross section of the heart are given in Figure ~\ref{Fig:doorsnede}. The left ventricle is the most important part of the heart. Because of the high pressure during systole, the muscle of the left ventricle is thicker (approx. 1 cm in diastole) than the right ventricular wall (approx. 3 mm in diastole). In fact, the right ventricle is built against the left ventricle.
Left) Frontal section of the heart, Right) cross section of the heart, RV = richt ventricle, LV = left ventricle, RA = right atrium,
LA = left atrium, TV = tricuspidal valve, MV = mitra valve, PV = pulmonary valve, AV = aortic valve, PT = pulmonary trunk, AO = aorta, PPM = papillary muscle for closing the valves. (Uit Arts 1978, p11)}
The locations of the different arteries and veins are given in Figure~\ref{Fig:mensvaten}a and b, respectively. Blood leaves the ventricle through the aorta. From the ascending aorta, blood flows through the aortic arch into the descending aorta located along the spine. From the aortic arch the aorta branches out into arteries delivering blood to the head and arms. At the level of the umbilicus, close to the spine, the aorta branches out into arteries delivering blood to the legs.
The configuration of the venous system is comparable to that of the arterial system. All through the body, arteries, veins and nerves are always found together, enveloped by a thin membrane. Like a sort of pipeline system. The analogy becomes even more clear when considering the names in Figures~\ref{Fig:mensvaten}a and b.
One difference with the arterial system is that most veins are located close to the surface of the body. Arteries are always located deeper. Hence, there are veins that are not located along an artery, e.g. the vena jugularis externa. It is clear that damage of a vein has less severe consequences in terms of blood loss than damage of an artery. However, damage of the jugular veins is very dangerous because of the suction of air. Venous blood from the legs and abdomen is collected in the vena cava inferior and from head and arms in the vena cava superior. Finally, both veins drain in the right atrium.
Distribution, flow and pressure of blood in the circulation
The vessels of the systemic circulation are distinguished by their cross section and their relative position in the vascular system. As the different types of arteries and veins diminish in diameter, their number increases.
In lying position the arterial pressure close to the heart is approximately 13.3 kPa (100 mmHg) and 12.7 kPa in the brain and feet. In standing position these presures are 26.0 kPa (195 mmHg) at the feet and 7 kPa (52.6 mmHg) in the head due to hydrostatic pressure differences. Naturally, these influences also occur in the venous system. The venous blood pressure in the head is subatmospherical, namely in the order of -.5 kPa. These influences are illustrated in Figure~\ref{Fig:houding}. The hydrostatic pressure variations have little or no influence on the total blood volume of the arterial system, because the walls of the arteries are reasonably stiff. (see also http://www.phys.mcw.edu/medphy/chout/Lectures/Lecture%2018.pdf about the distensibility and compliance of the arterial and venous system; the distensibility is the increase in volume necessary to induce a unit pressure change). The walls of the veins, however, are flaccid and are influenced to a great extent by the tone of the smooth muscle tissue in the wall (also present in the arterial wall). When someone is suddenly standing up, he may become unwell, due to the sudden increase in volume of the venes in the lower part of the body as a result of the hydrostatic pressure. Because of this, the blood flow to the heart and thus to the brain is temporarily reduced. In this context it should be pointed out that the valves in the venous system prevent backflow to the capillaries. These valves are important for keeping the venous pressure low in the extremities. Muscle activity in legs and arms make sure that blood is pumped in the direction of the venae cavae. The valves are closed to prevent backflow of blood.
n particular, backflow may occur in the cavities behind the aortic valve
gives the tube-Reynolds numbers under peak flow conditions in the different arteries of a human and the mean velocities in the capillaries and veins. Note that, due to damping in the circulation, the blood flow velocity and blood pressure in the capillaries and veins have lost their pulsatile character. From Table~\ref{tab:piek} we can see that the circulatory flow will be laminar, in general. Only in the aorta, immediately behind the valve, the Reynold's number can reach such high values that turbulence may occur. Under normal circumstances and with a well-functioning aortic valve turbulence does not occur. During heavy labour, the cardiac output (the amount of blood pumped around per time unit) and therefore also the Reynold's number can increase with a factor of 5, which will lead to turbulence. Also in case of a stenotic aortic valve (hardened valve membrane) turbulence will occur.
In the systemic circulation, the mean blood pressure decreases from 13.3 kPa (100 mmHg) in the aorta to 3 kPa (20 mmHg) in the capillairies and to 0.27 kPa (2 mmHg) in the venae cavae. These pressure drops are caused by friction losses of the flowing blood. The largest pressure drop occurs in the arterioles. This is also the place where the blood flow control is effectuated. The resistance at this level can vary such that the blood flow through an organ can increase by a factor of 5 while the perfusion pressure remains constant. The pressure drop (in mean pressure) in the large vessels is small. Between the aorta and the large arterial branches this pressure drop is in the order of 0.7 Kpa.
Under normal circumstances the kinetic energy of a volume-unit of flowing blood can be neglected in relation to its potential energy (or its pressure). This density of kinetic energy is in the order of 1/2rv2 (r= mass density, v = the mean blood flow velocity) and for the aorta in the order of 3 %. During heavy labour, the cardiac output can increase by a factor of 5 and therefore, 1/2rv2 with a factor 25
The kinetic energy term is also of importance when one is dealing with stenotic arteries, because this will cause the blood flow velocity to increase substantially. In case of stenoses (e.g. coronary stenoses) one should realise that the conversion of potential energy into kinetic energy at the entrance of a stenosis is only partly reversable at the outlet of the stenosis Apart from the increased viscous resistance in a stenosis, there will be an additional pressure drop because of this process.
Influence of posture on arterial and venous pressures. The numbers are estimated pressure levels with the right atrium as reference.
For more on basic principles of human circulation, see http://www.phys.mcw.edu/medphy/chout/Lectures/Lecture%2017.pdf
http://www.phys.mcw.edu/medphy/chout/Lectures/Lecture%2018.pdf about the distensibility and compliance of the arterial and venous system
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