General Organization of CVS and Hemodynamics of Circulation

The cardiovascular system consists of a central pump, the heart, and the interconnected network of blood vessels, which function efficiently to maintain homeostasis. Essential substances such as nutrients and oxygen are continuously picked up from the respective sites of availability and deliv­ered to the cells, and end products of metabolism are contin­uously removed to maintain homeostasis.

6.1.1 Heart as a Pump

The heart is a single organ; anatomically and internally, it comprises four chambers and is divided into right and left halves. The upper chambers, the atria, receive blood returning to the heart and pump it to the lower chambers, the ventricles. The heart serves as an efficient hollow muscu­lar pump that regulates the blood flow in the pulmonary and systemic circulation by imparting pressure to the blood to establish the pressure gradient needed for blood to flow to the organs and tissues.

6.1.1.1 Arteries(ConduitVessels)

Arteries are thick-walled vessels containing smooth muscles, elastin, and collagen fibers. During systole, arteries expand under increased pressure to accept and temporarily store the blood ejected by the heart and then, during diastole, supply this blood to the organs downstream by passive recoil; this is facilitated by the presence of the elastin fibers that can stretch to twice their unloaded length. Arteries further branch into arterioles.

6.1.1.2 Arterioles (ResistanceVessels)

Arterioles are smaller and have much thicker walls with more smooth muscle and less elastic fibers than arteries, and hence, their diameters can be actively changed to regulate the blood flow to the peripheral organs. The diameter of the arterioles can be modulated by the local metabolic process and by the blood-borne substances such as catecholamines. The arterioles are also supplied with vasomotor fibers that are regulated by the centers in the medulla and spinal cord.

Arterioles divide into minute capillaries at the tissue level.

6.1.1.3 Capillaries (ExchangeVessels)

Capillaries are the exchange vessels and are the smallest vessels having a small blood content (5% of blood volume). These vessels are lined by a single layer of endothelial cells that separates the blood from the interstitial fluid by only approximately 1 μm. As they do not have smooth muscle, they lack the ability to change their diameters actively. Capillaries are tiny tubular network composed of only a thin layer of endothelium, which favors selective permeabil­ity of nutrients, water, oxygen, CO₂, and other metabolic waste between blood and tissues, and they are known as exchange vessels. The blood in the capillaries flows as a single layer of RBCs at the center, while WBCs are located in the peripheral side of the capillaries. The highest cross­sectional area of the circulatory system is created by capillaries. Arteriovenous shunts (A-V shunts) are the direct connections between capillaries and veins that result from blood flowing directly from the arteriole into the metarteriole.

6.1.1.4 Venules and Veins (Capacitance Vessels)

After leaving capillaries, blood is collected in venules and veins and returned to the heart. Venous vessels have very thin walls having smooth muscles, and their diameters can actively change. Veins have very thin walls, making them very distensible. Therefore, their diameters change passively in response to small changes in transmural distending pres­sure. The veins continue as vena cava, which drains its content into the right atrium. Vena cava has the largest diameter. Veins have valves in the extremities. These valves stop blood from flowing backward, allowing for one-way blood flow to the heart. The main thoracic and abdominal veins do not have valves. Veins have about 24 times more capacitance than arteries, and about 80% of the total blood volume is located within the venous system.

6.1.2 Hemodynamics of Circulation

Blood flow in the vessels is influenced by many factors as described by the physical principles. However, as blood is not a perfect fluid but consists of both the liquid and cells and the blood vessels are not rigid tubes, the behavior of the circulation deviates, sometimes markedly, from that predicted by these principles.

6.1.2.1 BloodVelocity

It refers to the distance a blood bolus travels per unit of time (mm/time or cm/time). The average velocity of blood move­ment is inversely proportional to the total cross-sectional area at that point, is high in the aorta, reduces steadily in the smaller vessels, and is lowest in the capillaries.

6.1.2.2 Laminar Flow

In the straight blood vessels, the blood flow is normally laminar. Within the blood vessels, the velocity is maximum near the center and decreases near the vessel walls.

6.1.2.3 ResistanceandViscosity

Resistance to the blood flow is directly proportional to vessel length and viscosity of the blood and inversely proportional to radius of the vessel. The major regulating factor for the vascular resistance is the vessel radius to the fourth power. The radius of smaller vessels is controlled by the vascular smooth muscles. In the larger arterial vessels, the radius is controlled mainly by the amount of collagen and elastin. Increase in hematocrit causes appreciable increases in viscos­ity in large vessels, as compared to the smaller vessels (below 100 μm in diameter) (arterioles, capillaries, and venules). In marked anemia, peripheral resistance is decreased, because of the reduction in viscosity, whereas in severe polycythemia, the increase in resistance increases the work of the heart. The rate of blood flow through a vessel is directly proportional to the pressure gradient (as the pressure gradient increases, flow rate increases) and inversely proportional to vascular resis­tance (as resistance increases, flow rate decreases).

6.1.2.4 Shear Stress

Shear stress refers to the force exerted by the flowing blood on the endothelium that is parallel to the long axis of the vessel. This shear stress is proportionate to viscosity and changes in the shearing stress produce marked changes in the expression/activation of endothelial cell genes that pro­duce substances like growth factors and integrins.

6.1.2.5 Vascular Compliance

Vascular compliance refers to the ability of a blood vessel wall to respond to the changes in pressure by expanding and contracting passively. The arterial system has less compli­ance as compared to the venous vessels, and hence the venous vessels hold larger volumes of blood. Vessel compli­ance (C) measures a vessel’s capacity to distend and expand in volume when transmural pressure rises and is expressed by the change in volume (ΔV) divided by the change in pressure (ΔP):

Furthermore, the compliance of the vessel is also depen­dent upon the rate by which the change in volume occurs. At higher pressures and volumes, the compliance decreases (i.e., at increased pressures and volumes, vessels become “stiffer”); comparably at lower pressures (venous pressure is usually less than 15 mmHg), the venous compliance is about 10-20 times greater than arterial compliance. Hence, the veins can accommodate large changes in blood volume with only a small change in pressure. Vein compliance has increased in part as a result of venous collapse, which happens at pressures lower than 10 mmHg. Venous compli­ance is comparable to arterial compliance at higher pressures and volumes. This characteristic of the veins makes them suitable for use as arterial bypass grafts.

The compliance status for a blood vessel is dynamic. Even within a given artery or vein, capacitance, distensibility, and compliance are not constants in the cardiovascular system. While smooth muscle contraction improves vascular tone and decreases vascular compliance, vascular smooth muscle relaxation enhances compliance.

Characteristics associated with arteries and veins, such as vascular compliance, blood volume, and vascular resistance, affect the functioning of the heart. Vascular capacitance, compliance, and distensibility are critical determinants of the performance of the heart. During systole, the walls of large elastic arteries (e.g., aorta, common carotid, and pulmo­nary arteries) distend as the blood pressure rises and recoil when the blood pressure falls during diastole. Since the rate of blood entering these elastic arteries exceeds the quantity that leaves these vessels, there is a net storage of blood in the aorta and large arteries during systole, which is discharged during diastole. This feature, known as the windkessel effect, aids to decrease the load on the heart and minimizes the systolic flow and maximizes the diastolic flow in the arterioles. This effect helps in damping the blood pressure fluctuations over the cardiac cycle and assists in maintaining organ perfusion during diastole when cardiac ejection ceases. The windkessel vessels (aorta and large elastic arteries) con­vert the pulsatile inflow to a smooth outflow, and Otto Frank, a German physiologist, developed this concept and named this phenomenon as the windkessel effect.

6.1.2.6 Blood Pressure

This is the pressure exerted by the circulating blood against any unit area of the blood vessel. Stephen Hales, an English clergyman, demonstrated the existence of pressure in the blood vessels in 1730. Blood pressure is highest in aorta (98 mmHg), moderate in capillaries, and lowest in vena cava (3 mmHg). This pressure difference (98 — 3 = 95) moves the blood through systemic vessels. Perfusion pres­sure is the difference in pressure between the aorta and veins. The blood flows more favorably through the blood arteries due to this pressure gradient. The maximum pressure exerted in the arteries when blood is ejected into them during ven­tricular systole is referred to as the systolic pressure that averages 120 mmHg. It indicates the total kinetic energy imparted to the blood by the heart. Diastolic pressure is the minimum pressure within the arteries when blood is draining off into the rest of the vessels during diastole, and it averages 80 mmHg. Blood continues to exit the aorta into the tiny arteries during ventricular diastole, while the volume of blood in the major arteries declines, the arteries become less engorged, and blood exerts less pressure on the arteries.

6.1.2.7 Pulse

It is a wave of expansion and elongation followed by recoiling of the arterial walls and it is due to the forceful entry of the blood from the aorta during each heartbeat. Cardiac systole generates pressure waves that move the pliable and compliant arterial walls. It originates from aorta, spreads throughout arterial system, and disappears at the arterioles. The distension of the arterial walls due to sudden entry of blood and the subsequent increase in the pressure are marked by anacrotic limb of the pulse wave. However, the decreased pressure due to elastic recoiling of the distended arteries is referred to as catacrotic limb or declining slope in the pulse wave. Pulse pressure refers to the difference between systolic and diastolic pressure. The pulse pressure increases as blood flows from aorta to distal arteries and then becomes less and less when the blood moves toward periph­ery. It disappears in the arterioles and capillaries.

Mean arterial pressure (MAP) can be roughly estimated as follows:

This method is useful to find out mean pressure in major arteries distal to aorta but not in aorta because the pattern of arterial pressure pulsation changes as the pulse moves away from the heart.

Arterial BP is always expressed in mmHg, whereas capil­lary and venous pressure can be expressed as mm H₂O.

6.1.2.8 Factors Influencing Blood Pressure

1. Heart rate: During systole, the arterial pressure increases. The rate at which blood enters the arterial system exceeds the rate at which it drains through arterioles and capillaries, increasing the pressure. During diastole, the BP decreases since blood passes out of arteries into capillaries. When other factors remain constant, a decrease in the heart rate causes a fall in blood pressure and an increase raises blood pressure.

2. Peripheral resistance: Total peripheral resistance (TPR) is the resistance to the blood flow in systemic vessels. It is caused by internal friction produced by the viscosity of blood and is mostly present in arterioles and capillaries. Blood pressure is directly proportional to the peripheral resistance. When most capillaries are open, peripheral resistance decreases and blood pressure is reduced. Resis­tance is inversely proportional to the fourth power of the vessel radius and varies directly with the viscosity and length of the vessel. Total peripheral resistance (TPR) can be calculated as follows:

3. Elasticity of arteries: The elastic recoil of large arteries is responsible for the continuous flow of blood in the arterioles and capillaries. During systole when more blood enters aorta and large arteries, they expand to store the blood, i.e., potential energy is stored, and during diastole, the stored blood is released due to elastic recoil of the walls of arteries causing blood to flow into arterioles during diastole. Arterial stiffening increases systolic pres­sure (due to reduced expansion of artery) and decreases diastolic pressure (since less blood is stored by reduced expansion).

4. Volume flow: An increase and decrease in the blood volume of circulatory system cause increase and decrease of blood pressure. When hemorrhage occurs in a large artery, the blood pressure (BP) falls immediately. On moderate hemorrhage and when small arteries are cut, the fall in BP will be negligible because of the compensa­tory vasomotor mechanism and splenic constriction.

5. Diameter of the blood vessels: Arterial blood pressure varies indirectly with the diameter of blood vessel. If the diameter increases, the peripheral resistance decreases, leading to decrease in the blood pressure.

6.1.2.9 Determination of Arterial Blood Pressure

It helps in diagnosing the defects of heart and circulatory system. It can be measured by two means, the direct and indirect (clinical) methods.

The first direct measurement of mean arterial blood pres­sure was carried out in horse by Stephen Hales in 1730.

The usual physical examination in veterinary clinical prac­tice does not include measurement of ABP. Pulse pressure is often indirectly assessed during physical examination by digital palpation of a peripheral artery. In veterinary practice, blood pressure cuffs are less frequently used, but the pulse is usually palpated by placing the fingertips over a major artery, such as the femoral artery. Palpation of an artery allows the clinician to sense the pulse pressure on the basis of the magnitude of pulsations that are felt in the artery. However, the character of peripheral arterial pulsations primarily reflects the pulse pressure and not the absolute level of ABP. Thus, a horse with a systolic/diastolic ABP of 100/60 mmHg may have a facial artery pulsation that is very similar to that of another horse with ABP of 180/140 mmHg. Thus, assessment of the level of ABP (i.e., diagnosis of systemic hypertension) requires measurement of ABP.

The ABP may be measured in animals by direct or indirect techniques.

Direct techniques rely on the penetration of a peripheral artery with a catheter or a needle and connection via a fluid- filled tube to a pressure transduction system. This technique generally provides an accurate and precise measure of ABP and is useful in anesthetized animals, particularly large animals. However, because anesthetics and sedatives alter ABP and physical or emotional distress can dramatically elevate blood pressure, they are not used for screening of animals for the presence of systemic hypertension and indi­rect techniques are usually employed in veterinary practice for this purpose. Indirect techniques rely on the placement of a cuff over an extremity (limb or tail). The cuff is inflated to occlude a peripheral artery. As pressure is automatically or manually reduced in the cuff, the restoration of arterial flow can be detected distally by a variety of methods. These devices generally utilize ultrasonic Doppler, oscillometric, or photoplethysmographic principles to detect the restoration of flow distal to the cuff. Systolic ABP may then be estimated from cuff pressure at the time of restoration of flow. Some indirect devices (e.g., ultrasonic Doppler flowmeters) are generally used only to estimate systolic arterial blood pres­sure (ABP), while other devices often provide an estimate of systolic, mean, and diastolic ABP.

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