1 General organization of the cardiovascular system

1.1 Introduction to the cardiovascular system

The primary function of the cardiovascular system is to deliver blood to the tissues, over long distances, and to evacuate waste products from the tissues.

The cardiovascular system has many jobs:

• The cellular distribution of oxygen and nutrients (amino acids, fatty acids, vitamins)

• The elimination of cellular waste (carbon dioxide, lactates)

• The transport of oxygen, carbon dioxide, and hormones

• The regulation of body temperature, blood pH, water volume, mineral salt levels, etc.

The system contributes to homeostasis by helping keep certain physiological values either constant or relatively stable.

The cardiovascular system delivers oxygen and nutrients to the various tissues, according to a hierarchy of regional distribution: first the brain, then the kidneys, the splanchnic territory (digestive system), and finally the limbs.

1.1.1 Circulatory system

The circulatory system refers to the cardiovascular and the lymphatic systems. The cardiovascular system comprises the heart and its vessels: arteries, capillaries, and veins. The heart is a hollow muscle that functions like a pump, propelling the blood through the arteries to circulate though all the tissues of the body.

1.1.2 Vascular network

When looked at from an anatomical rather than a functional point of view, the vascular circuit can be divided in two (Fig. 1.1):

• The systemic or great circulation carries oxygen and nutrients to the tissues. The systemic circuitry feeds the organs of the body through a series of blood vessels arising from the left ventricle via the aorta. The systemic veins collect blood from these organs and convey it back to the right atrium of the heart.

• The pulmonary or small circulation denotes the shuttle between the heart, which generates the flow, and the lungs, which provides the oxygen.

Fig. 1.1 Systemic and pulmonary circulation.

Through this short vascular network, blood moves from the right ventricle, by way of the pulmonary artery, and returns to the left atrium, via the pulmonary veins.

1.1.3 The heart

The heart is a pair of valved muscular pumps whose different functions work in parallel. The ‘left heart’ distributes the systemic circulation, and the ‘right heart’ provides the pulmonary circulation.

The blood flow delivered by the two circulations is more or less the same. However, because the resistance in the pulmonary arterioles is five or six times weaker than the resistance in the systemic arterioles, the pressure is not the same in the two networks.

Both sides of the heart have:

• an atrium (once known as ‘oriellette’) that receives blood

• a ventricle that expels blood:

– from the right ventricle, to the pulmonary artery to enter the lungs

– from the left ventricle to the aorta, in the direction of the other organs.

The main function of the heart is to maintain blood flow.

1.1.4 Vascular sections – resistance and capacitance

In terms of hemodynamics, the right and left heart circuits operate in series. These sectors are nevertheless different in:

• the pressure of their driving blood force

• the compliance of their vessels

• the resistance of their vessels to blood flow.

The mechanical pressure in the vessels depends on the cardiac pump, the refilling pressure of the vessels, and the deformability or compliance of the vascular wall. The more compliant the blood vessel, the more volume can be added without causing a rise in pressure.

High pressure system

The resistant, or high pressure system, displays high resistance and low compliance (the distension capacity of a vessel). It comprises all of the arteries of the systemic circulation from the left ventricle in systole, to as far as the arterioles. It contains only 10–15% of blood volume.

In this system, the progressive ramifications and subdivisions diminish the caliber of the vessels, but increase the total sectional surface. Mean pressure in the aorta is 100 mmHg, six to seven times higher than in the pulmonary artery.

Several subcompartments can be distinguished:

• The reservoir vessels comprise the aorta and its large branches. Their walls are elastic and they convert, without notable modification in average pressure, the intermittent pressure of the cardiac pulsation into a smooth type of flow. These are the main vessels of blood distribution to the periphery.

• The arterioles are the site of regional resistance and are the regulatory vessels of blood flow.

• The precapillary sphincters are smooth muscle bands that lie ‘before’ the capillaries and determine the flow at the entrance to the capillaries in order to control perfusion.

Low pressure system

The capacitance, or low pressure system, has weak resistance and high compliance. It consists of the veins, right heart, pulmonary circulation, left atrium, and left ventricle, in diastole.

The low pressure system retains 75–80% of blood volume. The remainder is found in the peripheral capillaries, which are also included as part of the low pressure system.

This system is named for the relatively low pressure prevailing in it: on average 15 mmHg or 2 kPa. Owing to its large capacity and high compliance, it can act as a blood reservoir. This system’s capacity to hold blood regulates blood volume.

Capillaries are the vessels of exchange between blood and tissues. Veins collect the blood and return it to the heart. The distribution of blood volumes is depicted in Fig. 1.2.

Fig. 1.2 Blood volume distribution (Silbernagl & Despopoulos 1985).

1.1.5 Blood distribution

The cardiovascular system is designed to deliver oxygen and nutrients, according to the prevailing need: brain, then kidneys, splanchnic (digestive) territories, and limbs.

Tissue blood flow or irrigation is adjusted precisely to the functional need of each tissue and organ – neither more nor less. In general, the most active tissues receive the greatest perfusion. The selection of one territory at the expense of another occurs by vasoconstriction (reduction in vessel caliber) in the neglected territories.

Hyperthermia refers to an increase in blood supply to a tissue. By contrast, ischemia means the opposite: interrupted circulation within a tissue. The term active hyperthermia corresponds to an increased flow rate linked to increased tissue or organ activity. This is what happens in skeletal muscles during strenuous activity.

At rest

At rest, cardiac output is around 5–6 L/min. The blood arrives in the tissues with the same average arterial pressure and approximately the same force. The blood flow within each organ depends solely on the extent of the vascularization and local arterial resistance.

At rest, the digestive system and the kidneys attract about 50% of the blood distributed, whereas the heart brings in only 3–4% (Table 1.1 & Fig. 1.3).

Table 1.1 Resting blood flow

Organs	% total
Digestive	25
Kidneys	20
Brain	13
Skin	9
Heart	4

Fig. 1.3 Blood output at rest and with exertion.

Adapted from Marieb (2005).

With exertion

During heavy muscle work, the cardiac output increases to 17–25 L/min. Distribution to various tissues is modified, with the exception of the brain, whose supply remains remarkably constant (Table 1.2 & Fig. 1.3).

Table 1.2 Blood flow on exertion

Organs	% total
Heart	4
Brain	14
Kidney	20
Skeletal muscle	22
Skin	8
Other organs	12

1.1.6 Types of circulation

Depending on the purpose of the blood running through an organ, three types of circulation occur:

• Nourishing circulation: cerebral arteries, muscular arteries, coronary arteries, bronchial arteries, hepatic arteries

• Functional circulation: pulmonary arteries, portal vein

• Mixed circulation: renal arteries, mesenteric arteries, cutaneous arteries.

Blood flow distribution calculation does not clearly reflect the true needs of the organ, but accounts for circulatory draw based on extra work or environmental changes. For this reason the oxygen extraction coefficient provides a clearer picture of the intrinsic requirement of an organ than the isolated notion of blood flow.

1.2 The heart

Formed in the third week in embryo, the heart remains rhythmic throughout life: 100 000 beats per day, 2 billion times in an average lifetime. One is hard pressed to imagine a machine capable of the same work. It has its own rhythm, independent of our will, adjustable according to events taking place outside or inside the body. The heart can race following an emotion, an exertion, or a high temperature. Its weight varies according to a person’s size and sex, averaging 275 g.

1.2.1 Anatomy review

Form and orientation

On the outside, the heart is a somewhat rounded conical form, lying on its side.

• The apex of the heart is directed caudad, anteriorly and to the left.

• The base of the heart, opposite the apex, is directed cephalad, and faces mainly posteriorly and to the right.

• The main axis of the heart is oblique from posterior to anterior, from right to left. This obliqueness varies with the shape of the thorax; the heart lying more vertically in a narrower thorax.

Chambers

The hollow muscle of the heart is divided into right and left parts by the interatrial septum. Each side of the heart has two chambers, an atrium (superior chamber) and a ventricle (lower chamber) connected by atrioventricular valves (Fig. 1.4).

Fig. 1.4 Cardiac chambers

Atria

Functionally, the atria are receiving chambers, into which blood flows from the circulation. As they do not have to contract strongly for the blood to pass into the ventricles just beneath them, the atria are small in size, and their walls are relatively thin. They contribute only modestly to the filling of the ventricles and the pumping action of the heart.

Three veins enter the right atrium:

• the superior vena cava

• the inferior vena cava

• the coronary sinus.

Four pulmonary veins, two right and two left, enter the left atrium.

Ventricles

The ventricles, with their thick walls, form the bulk of the heart. From a functional viewpoint, they insure that the heart pumps properly. The right ventricle ejects the blood into the pulmonary trunk, which dispatches it into the lungs where gaseous exchange occurs. The left ventricle pumps blood into the aorta whose successive branches feed all the organs.

Valves

Valves, which separate the chambers, keep the blood flowing in one direction inside the heart.

Arterioventricular valves

Each arterioventricular valve is formed by a fold of endocardium, reinforced by fibrous tissue. The right arterioventricular valve comprises of three valvules or cuspids (tricuspid valve), whereas the left arterioventricular valve has just two (bicuspid or mitral valve). They prevent the blood from flowing back during contraction of the ventricles.

Arterial valves

The valves of the aorta and the pulmonary trunk are located at the base of the aorta and the pulmonary trunk. Each of these valves, also called sigmoid, have there semilunar valvules in the form of a crescent or a pocket. They open when the ventricles contract to pump the blood into the arterial trunk. They close and prevent blood from returning to the ventricles, during ventricular relaxation when the ventricles are refilling.

Structure of the heart

The heart is formed by a thick muscle called the myocardium, and lined internally by a smooth membrane layer called the endocardium. This arrangement is continuous in the pericardium, which is a double-walled sac composed of the serous pericardium layer and the fibrous pericardium layer.

Myocardium

The myocardium is made up of:

• striated muscle fibers anchored to a fibrous framework

• differentiated fibers forming the nodal tissue or conducting system of the heart.

Fibrous skeleton

The fibrous framework or fibrous skeleton of the heart denotes the four rings of dense collagen that surround the orifices of the auriculoventricular and arterial valves. The two auriculoventricular rings and the aortic ring are located in the same fibrous body. The pulmonary ring is situated anterior and superior to the others.

Muscular fibers

The heart is composed of ventricular and auricular fibers (Fig. 1.5).

Fig. 1.5 Tendinomuscular structure of the heart.

The ventricular fibers consist of:

• fibers specific to each ventricle whose oblique moorings attach to the fibrous rings of the skeleton

• Common fibers that envelop and join the two sacs formed by the individual fibers.

‘The ventricular heart is composed of two muscular sacs contained in a third muscular sac’ (Winslow, cited by Bouchet & Cuilleret). The ventricular fibers fasten solely to the periphery of the auriculoventricular rings.

Conduction system of the heart

The heart has an intrinsic system by which it is able to beat without the participation of external nerves. This property is termed autorhythmicity. This autonomous command system is the tissue nodal or impulse conducting system (Fig. 1.6) made up of three parts:

• The sinoatrial (SA) node (Keith and Flack) is located in the wall of the right atrium, at the junction of the superior vena cava and the external atrial orifice. This is the pacemaker of the heart.

• The atrioventricular (AV) node (Aschoff–Tawara) is a smaller collection of nodal tissue situated in the interauricular septum, near the atrioventricular valves. It functions as a secondary pacemaker.

• The atrioventricular (AV) bundle (of His) is a collection of specialized muscle fibers beginning at the AV node and dividing into right and left bundles at the junction of the membranous and muscular parts of the interventricular septum.

• The AV bundles proceed on either side of the septum, deep to the endocardium, and then ramify into the subendocardial branches (Purkinje network), which extend into the walls of the respective ventricles.

Fig. 1.6 Conduction system of the heart.

Endocardium

The endocardium is a thin internal layer lining all cardiac cavities and continuous with the endothelium of the vessels. This fine smooth glistening membrane permits the blood to circulate easily inside the heart. A fold of endocardium forms the valvules of the orifices anchored on the fibrous skeleton, arising from the periorificial ring.

Pericardium

The pericardium is a fibroserous sac surrounding the heart and the roots of the great vessels at the base of the heart. Extending 12–14 cm in height with a width of 13–14 cm, it comprises two layers:

• The serosal pericardium: this glistening serous membrane facilitates all manner of movements, glidings, and deformations of the heart, in relation to its neighboring organs.

• The fibrous pericardium clothes the serous pericardium. The heart is protected and somewhat tethered in place inside this fibrous sac.

Generally in the body, mobile organs or those capable of significant movement are often surrounded by connective tissue that attenuates mechanical forces at play, thus harmonizing connections with neighboring organs. The lungs are a good example of this: they deploy considerable mechanical forces within the thorax.

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