General Principles of Hemodynamics

Organization of Cardiovascular System

The human heart has 4 chambers which are the two atria and the two ventricles. These 4 chambers are divided into 2 functional units referred to as the left heart and the right heart. These atria and ventricle are single functional unit, separated by the atrioventricular valves. These AV valves are one way valves and allow blood flow in the forward direction only.

Right heart is formed by the right atrium and the right ventricle, and it forms one functional unit. The right atrium receives the venous deoxygenated blood from three following sources

1. Superior vena cava: brings deoxygenated blood from the head, neck and upper limb region.
2.  Inferior vena cava: brings deoxygenated blood from the lower extremities, the abdominal region and the rest of the body except the heart itself.
3.  Coronary sinus: brings deoxygenated blood from the veins of the heart itself.

During diastole when the atria contract, this deoxygenated blood is pumped into the right ventricle. During systole, the right ventricle pumps this deoxygenated blood out of the heart and into the pulmonary circuit via the pulmonary artery.

The right heart plus the pulmonary arteries, capillaries and veins together form the pulmonary circulation.

The right side of the heart deals with deoxygenated blood only and it functions to send this deoxygenated blood to the pulmonary circulation to get oxygenated.

Left heart forms another functional unit and consists of the left atrium and left ventricle. The left atrium receives oxygenated blood from the pulmonary circuit via the  four pulmonary veins, two from each lung. When the atria contract, the left atrium pumps this oxygenated blood into the left ventricle. During systole, this oxygenated blood is pumped out of the heart via aorta, when the left ventricle contracts. The aorta then carries this oxygenated blood into the systemic circulation.

The left heart plus the systemic arteries (starting at the aorta), capillaries and veins together form the systemic circulation.

The left heart deals with the oxygenated blood and sends it to the systemic circulation via the aorta.

NOTE

Arteries conduct blood away from the heart towards the tissues. Arteries normally carry oxygenated blood away from the heart, but an exception to this rule are the pulmonary arteries and the umbilical arteries(during fetal life only) which carry deoxygenated blood away from the heart and toward the lungs & placenta respectively. Veins normally carry deoxygenated blood, except the pulmonary veins in adults and the umbilical vein (during fetal life only) which bring back oxygenated blood to the heart from the lungs and the placenta respectively.

human heart

As part of the systemic perfusion, the oxygenated blood in the aorta is eventually transported to the following six major systemic tissues. These systemic tissues receive blood via a parallel system of arteries which originate at various levels from the aorta itself.

1. Cerebral: The CNS plus the head & neck region. 15% of the cardiac output enters the cerebral arteries.
2. Coronary: The myocardium itself which receives oxygenated blood during diastole in contrast to the rest of the body which receives oxygenated blood as part of systole. 5% of the total cardiac output is designated for the myocardial perfusion via the kidneys.
3.  Splanchnic: The gastrointestinal system and its accessory organs such as the liver, spleen, pancreas and the biliary system. 25% of the total cardiac output reaches the GIT system via the splanchnic arteries.
4. Renal: The kidneys and the genitourinary system. Kidneys, as part of the renal system, receive 25% of the total cardiac output.
5. Skeletal: Roughly 25% of the total cardiac output is reaches the skeletal system. Exercise increases the percentage of cardiac output which is made available for the skeletal system. Bones and the musculature of the body form part of this system.
6.  Cutaneous: The skin and its associated structures (sebaceous glands, hair follicles). Around 5% of the total cardiac output reaches the cutaneous circulation.

Direction of Blood Flow and chemical composition of Blood

There are four pulmonary veins which bring back oxygenated blood from the lungs to left atrium. This blood is rich in oxygen (PaO2=100 mm Hg) and low in carbon dioxide (PaCO2=40 mm Hg). The mitral valve which forms the left atrioventricular valve, allows passage of blood from the left atrium into the left ventricle during the diastole phase. When the left ventricle begins to contract and its pressure rises more than the left atrial pressure, the mitral valve closes to prevent backflow of the blood. This ensures anterograde flow of blood in to the aorta i.e., the forward direction of blood flow. Backflow from the aorta back into the left atrium is prevented by the semilunar aortic valve. It’s important to remember that all the valves of the heart are tricuspid i.e., having three cusps, except the mitral valve which is bicuspid i.e., having two cusps. However, only the right atrioventricular valve is referred to as the tricuspid valve.

From the aorta, the blood is transported to the systemic tissues which are mentioned above. The aorta divides into large and medium sized arteries, which eventually give arise the arterioles. The arterioles continue to form capillaries, and these capillaries merge together to form venules at their venous ends. The venules eventually end up forming the veins. The veins are low pressure vessels which return the deoxygenated blood back to the right heart via the three above mentioned sources of venous return to the heart. This deoxygenated blood is low in oxygen (40 mm Hg) and rich in carbon dioxide (47 mm Hg). The right atrioventricular valve, which is also referred to as the tricuspid valve, allows this deoxygenated blood to flow from the right atrium into the right ventricle. During ventricular systole when the left ventricle contracts, this deoxygenated blood is pumped out of the right side of the heart via the pulmonary artery. The backflow of this deoxygenated blood into the right side of the heart during ventricular diastole is prevented by the semilunar pulmonary valve. This deoxygenated blood reaches the lungs and enters the pulmonary circuit to get deoxygenated. At this point the blood completes its route both around the pulmonary and systemic circuits.

Series and parallel Circuits

The right and left sides of the heart are connected in a series circuit to both the pulmonary and systemic tissues respectively. By series circuit, what’s meant here is that quantitatively, the blood flow through the lungs is equal to the blood flow through the rest of the body. For simplicity in understanding, it should be considered that the lungs are connected to the rest of the body in a series circuit. During a single cardiac cycle, the right and left ventricular outputs are the same.

However, the blood supply of the systemic tissues is connected in a parallel circuit. This means that each organ system is supplied by an artery which originates as a branch of the aorta. This ensures that the blood which reaches a particular organ system is perfused at the same partial pressure of oxygen, as that of the site at which the branch originated from the aorta. If the organ systems were connected and perfused via a series circuit, by the time the blood reached the last organ system, it would have been completely depleted of oxygen and nutrients. The sum of blood flow to these individual systems adds up to the total left ventricular output (the cardiac output).

Pressures in cardiovascular system

SYSTOLIC BLOOD PRESSURE (SBP)

Systole is time interval when the ventricles are contracting. Systolic BP therefore is the pressure in the systemic arteries when the left ventricle is contracting. Therefore, SBP is the highest blood pressure in the systemic arteries during a cardiac cycle.

Average SBP in healthy adults is 120 mm of Hg.

 DIASTOLIC BLOOD PRESSURE (DBP)

Diastole is the time interval when the ventricles are relaxing and therefore receiving blood from the atria. Diastolic BP is the pressure in the systemic arteries when the left ventricle is relaxing. DBP therefore is the lowest pressure in systemic arteries during a cardiac cycle. Average DBP in healthy adults is 80 mm of Hg.

 PULSE PRESSURE (PP)

This is the difference between the systolic blood pressure and the diastolic blood pressure in the systemic arteries at any given time. Pulse pressure can therefore be calculated as following:

Pulse pressure (PP) = Difference between the systolic & diastolic blood pressures.

PP= SBP- DBP

PP = 120-80= 40 mm of Hg

Variations in pulse pressure can be in anemia, fever, blood loss etc. Narrow pulse pressure can be seen when there is blood loss, aortic regurgitation. Pulse pressure widens during exercise.

MEAN ARTERIAL PRESSURE

MAP is the average arterial pressure of the systemic arteries. However, quantitatively it’s not an arithmetic mean of the SBP & DBP. Since the ventricular muscle spends 2/3 of the time of a cardiac cycle in diastole, the MAP is closer to the DBP, than it’s to the SBP.

MAP signifies the perfusion pressure of the tissues. If the MAP of the patient decreases below 60 mm of Hg, then it should be a cause of concern for the doctor. What this signifies is that a perfusion pressure below 60 mm of Hg would not be able to meet the nutritional needs of the systemic tissues. So, the MAP, which is easier to calculate quantitatively, can be used in lieu of systemic perfusion pressure.

MAP = (CO x SVR) + CVP {CVP is negligible, so it can be ignored}

MAP = (CO x SVR)

CVP= central venous pressure

SVR (systemic vascular resistance) is the sum of resistance of all the vessels in the systemic circuit. However, the major component of systemic vascular resistance is the arteriolar resistance.

Also, MAP can be calculated by using the following formula if the SBP and DBP are known:

MAP = DBP + 1/3 (SBP – DBP)

For a normal healthy adult, SBP = 120 & DBP = 80

So, MAP = 80 + 1/3 (120 – 80)

MAP = 80 + 1/3 (40)

MAP = 80 + 13.33

MAP = 93 mm of Hg (After rounding off)

Alternatively MAP can also be calculated by the following formula:

MAP= 2/3 DBP + 1/3 SBP

MAP= (2/3 x 80) + (1/3 x 120)

MAP= 53.33 + 40 = 93 mm of Hg

The Mean Arterial Pressure is necessary pressure to bring blood supply to the systemic tissues to ensure their nutritional needs.