2 Circulatory physiology

If the laws of fluid dynamics were applied, the cardiovascular system would form an unusually complicated hydraulic system. Our biological vascular plumbing arrangement is a much more difficult study and is harder to conceptualize than the circulation of water through central heating pipes! This system comprises a far greater number of variables than those governing the function of most systems of pumps, pipes, and fluids to be found in the industrial world.

2.1 Circulatory function – generalities

2.1.1 Conceptualization

One can simplify the workings of the circulatory system by looking at functional parts:

• An alternating pump: the heart

• A hydraulic circuit: the vascular system, which can be considered, at least in the short and middle term (independent of renal resorption) a constant volume. This hydraulic circuit (Fig. 2.1) comprises:

• a distribution network:

– the arteries terminating in arterioles, which have variable resistance

– the capillaries, a circuit of tiny blood vessels responsible for the bi-directional exchange of fluid

• a return circuit towards the heart: the veins.

Fig. 2.1 Hydraulic principle schematic.

If at first glance the cardiovascular system appears to be a closed hydraulic system of constant volume, and not an open and linear system, it must be emphasized that:

• the cardiovascular system has some porous tubes (capillaries)

• the cardiovascular system is made up of elastic tubular elements and not rigid structures

• the volume of its contents is subject to changes depending on digestive absorption, as well as renal and digestive elimination.

These characteristics fundamentally complicate any more basic conceptualization.

2.1.2 Pressures

Blood pressure

Blood pressure is the force exerted by the circulating blood on the artery walls. Its pressure by surface unit depends on the volume of blood in the vessels and the compliance of the vessel walls.

Arterial pressure

Arterial pressure results from the pressure exerted by the blood in the large arteries. Blood pressure depends on cardiac output and total peripheral resistance. Arterial pressure fluctuates with each heart beat, according to the pumping of the heart. It:

• increases during the emptying phase (ventricular systole)

• decreases during the filling phase (ventricular diastole).

Systolic pressure

Systolic pressure is the blood pressure measured during the period of cardiac contraction. According to the law of Hagen–Poiseuille, it depends on three factors:

1 cardiac output

2 elasticity of the large arteries

3 viscosity of the blood.

Diastolic pressure

Diastolic pressure corresponds to the arterial pressure during the cardiac relaxation phase. It depends on the speed of blood flow and therefore on the total peripheral resistance.

This attests to the resistance provided by vessels to blood flow. It is a good indicator of arterial wall elasticity.

Average blood pressure

Blood pressure values oscillate between a maximum (P_s) and a minimum (P_d) value. The average arterial pressure (P_ave) is obtained by integrating the pressure curve in the course of a cardiac cycle. In practice, for the peripheral arteries, take one-third of the difference between maximum and minimum pressures, and add to the minimum pressure.

This can be expressed by the formula:

For example, in a person whose arterial pressure is 140/80 mmHg, the average pressure is:

Values

Normal arterial blood pressure in a healthy 40-year-old man is 140 mmHg during systole at the maximum and 80 mmHg during diastole at the minimum.

According to the World Health Organization, pressure is considered pathological (arterial hypertension) if systole pressure is greater than 160 mmHg and/or diastolic pressure is higher than 95 mmHg.

Arterial pressure varies with:

• Sex: up to the age of 40 years males have higher arterial values than women, becoming lower than women after age 50 (D’Alché 2008).

• Age: on average blood pressure rises with age.

• Measurement circumstances.

• Cardiac output: a rise in output tends to increase blood pressure. For example, during physical exertion, arterial pressure increases. By contrast, blood pressure falls during sleep. Systolic pressure comes down as much as 10 to 30 mmHg, and diastolic pressure lowers 5 to 10 mmHg. (D’Alché 2008).

• Blood volume: increased blood volume increases blood pressure. An injection of 250 mL water causes blood pressure to rise by 10 mmHg within 60 min.

• Blood flow to individual organs depends on the degree of vasoconstriction of the arteries supplying the particular organ. Digestion modifies arterial pressure.

Clinical note

With arterial hypertension the heart must work harder and consume more energy to supply the same output. Arterial hypertension therefore represents a form of energy squandering for both the heart and the organism.

2.1.3 Arterial tension

In France, the term ‘arterial tension’ refers to arterial pressure measurement.

Normally, the term fluid pressure denotes fluid pressure on the walls of a deformable tube. Blood vessels are elastic tubes and as such are deformable. Laplace’s law expresses the relationship between the tension, T, exerted on the wall of such a tube and the prevailing pressure at the interior of the tube (Fig. 2.2).

Fig. 2.2 Pressure tension relationship in an elastic tube.

This relationship between tension and pressure allows us to understand the role that vessels play in blood pressure.

2.1.4 Arterial pulse

The arterial pulse is the wave of rhythmic arterial pressure perceived by palpating an artery. The normal number of pulse beats per minute varies from 60 to 80.

The pulse is caused by the sudden increase in blood pressure, ejected by the left ventricle into the aorta and large arteries. It propagates along the entire length of the arterial tree.

The regular recurring expansion and contraction of an artery (the pulse) is produced by waves of pressure and is not the same as the sensation of fluid pulsation linked to a rhythmic blood flow, passing under the fingers (pulse taking). Moreover, the pulse wave velocity is much higher (from 5 to 15 m/s) than the speed of blood!

2.1.5 Compliance and elasticity

In a deformable hollow organ the ratio between changes in volume and pressure is defined as compliance (or capacitance or volume dispensability). The inverse relationship defines elasticity.

In cardiovascular physiology, the term compliance can be understood to mean the opposite of rigidity. It describes the ability of a vessel, or of the heart, to return to form. Veins have greater compliance than arteries and arterioles. Inversely, arteries and arterioles possess greater elasticity than veins. These differences are due to the structure of their walls and their shape: circular for the arteries and elliptical for the veins. The large compliance of the venous network allows them to serve as a blood reservoir.

2.2 Cardiac physiology

As described above, the heart’s valves permit a one-way blood flow from the atria to the ventricles, and from the ventricles to the aorta or pulmonary artery. Each ventricle ejects about 5 L/min, which amounts to 2.5 million liters an hour at rest.

2.2.1 Cardiac mass

In mammals whose weight varies somewhere between that of a mouse and a horse, the cardiac mass represents about 0.6 of living weight. Thus a 500-kg horse has a cardiac mass of 3000 g, whereas a 2.5-kg cat’s heart weighs just 15 g. A blue whale’s heart weighs in at 450 kg!

Heart rate

The functioning of the cardiac pump is discontinuous. Heart rate is intermittent and pulsatile. The heart beats normally about 70 times per minute (bpm).

In mammals, cardiac frequency is much more rapid if the heart is small. For example, the heart rate of a mouse is on average 500 bpm, whereas that of an elephant is only about 25 bpm. In mammals, the relationship between the resting heart rate and life expectancy is indubitable. The lower the heart beat, the longer the lifespan.

In humans, many studies show a relationship between a raised resting heart rate and an increased mortality rate in subjects at cardiovascular risk. A low resting heart rate seems to be associated with longevity.

Bradycardia is a slower than normal heart rate, whereas tachycardia describes a faster than normal heart rate. Many factors can alter heart rate such as age, sex, physical exertion, and stress.

Cardiac cycle

The contraction of the heart, or systole, is followed by a period of rest, called diastole. Together they constitute the cardiac cycle (Fig. 2.3).

Fig. 2.3 Cardiac cycle.

When the heart rate changes, diastole is shortened or extended, whereas the duration of the systole remains relatively unchanged, at least on a broad scale frequency.

At 75 bpm, the duration of systole is 0.27 s, whereas that of diastole is 0.53 s, giving a systole to diastole relationship of about 1 to 2.

The resting time between heart beats is about twice that of a contraction.

Terminology

Cardiovascular literature can be daunting in its use of prefixes. Notions such as ‘telediastolic’ or even ‘protosystolic’ are encountered.

Although they are brief events in the cardiac cycle, diastole and systole are of a particular duration nonetheless. Sometimes it is necessary to be more specific about a particular moment during these occurrences or, in contrast, it can be helpful to refer to the overall picture. Table 2.1 summarizes the meaning of these various prefixes.

Table 2.1 Meaning of prefixes with regard to the cardiovascular system

Cardiac cycle

The heart can be likened to a hollow muscle that propels blood by alternately contracting and relaxing. This sequence affects the size of various cavities, especially the ventricles. During ventricular contraction (systole) the walls thicken and the cavity size diminishes, whereas during ventricular relaxation the walls thin and the cavity enlarges (Fig. 2.4).

Fig. 2.4 Ventricular cross-section in diastole and systole.

The cardiac cycle is a repetitive sequence of systole (period of myocardial contraction) and diastole (period of myocardial relaxation). Within this sequence the cardiac valves open and close at key moments, to manage the flow of blood.

The adult human heart ‘beats’ about 70 times per minute, which means the four action phases of the heart are accomplished in less than 1 s. The four phases (Fig. 2.5) are:

• The ventricular filling phase, during which the atrioventricular valve is open. Blood flows from the atrium towards the ventricle, while the aortic valve (for the left ventricle) and the pulmonary valve (for the right ventricle) remain closed. During this phase, blood flow first flows passively from the atrium to the ventricle, and then more actively during auricular diastole.

• In the isovolumetric contraction phase, myocardial ventricular contraction increases the pressure in the ventricle, and all valves are closed.

• Systolic ejection begins when pressure in the ventricle (ventricular pressure) overtakes the pressure in the aorta (aortic pressure) or the pulmonary artery, during the course of which the blood is expelled from the ventricle. The semilunar valves are opened.

• Isovolumetric relaxation begins at the end of the ventricular contraction. Ventricular pressure falls below that of the aorta and the pulmonary artery causing closure of the semilunar valves. The atrioventricular valve remains closed.

Fig. 2.5 The cardiac cycle.

2.2.2 Cardiac output

Cardiac output is the volume of blood ejected by each ventricle per unit of time. It can be calculated from the heart rate × stroke volume.

With a heart rate of 70 bpm and a volume of systolic ejection of 80 mL, this represents a cardiac output of 5.6 L. A man weighing 70 kg possesses about 6 L of blood, which means that his total blood mass is pumped in about 1 min.

Regulation of output

The right and left heart can be likened to two pumps arranged in series. This implies that the two ventricles have the same output. Suppose that the right ventricular output is 5.1 L/min and that of the left ventricle only 5 L/min. After 10 min, 1 L of blood will have accumulated in the lungs, causing pulmonary edema.

As the two pumps operate at the same rate, output regulation can occur only through a change in the volume ejected from the two ventricles, dependent on the force of contraction of the ventricle itself.

Frank–Starling law

Each ventricle must be able to change its force of contraction and therefore stroke volume in response to changes in venous return. The Frank–Starling law describes the mechanism by which changes in pressure alter stroke volume. It states that the force of ventricular contraction is increased when the ventricle is stretched prior to contraction. The myocardial fibers experience an increase in load due to the extra blood entering the heart. And the force of contraction of the cardiac muscle is proportional to its initial length. The capacity of the stretched heart to contract is a quality shared by all striated muscles.

The Frank–Starling law allows the heart to be synchronized with the venous return without depending on external regulation to make alterations. It is vital that stroke volume of both ventricles match so exactly that neither stagnation nor total emptying of the pulmonary circulation can occur, either of which would be fatal.

Volume of systolic ejection

Systolic ejection volume (V_s) is the quantity of blood ejected from the left ventricle during systole and depends on three factors:

1 The pre-charge corresponds to the volume of blood present in the left ventricle just before ejection. Volume depends on the pressure of venous return to the heart, called central venous pressure (CVP).

2 The contractility of the myocardium, regulated by the autonomic nervous system.

3 The post-charge represents those factors that oppose the work of the heart:

– Parietal resistance of the ventricle

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