Essential Concepts in Cardiovascular Physiology

Pulmonary Circuit

The pulmonary circuit involves the right side of the heart, which pumps blood that is partially depleted of oxygen content and contains elevated CO2 as a result of gas exchange in various tissues. This blood is delivered from the right side of the heart into the lungs. In the lungs, oxygen is loaded into the blood, and CO2 is released. This oxygenated blood then travels to the left side of the heart and is pumped to body tissues via the systemic circuit. The systemic circuit pumps an equal amount of blood from both ventricles. Generally, veins pump blood back to the heart, while arteries pump blood away from the heart.

Cardiac Drift

Cardiac drift refers to the increase in heart rate and decrease in stroke volume observed during prolonged exercise. This phenomenon is primarily due to the influence of rising body temperature on dehydration and a reduction in plasma volume.

Cardiac Muscle Fibers

Cardiac muscle fibers are shorter than skeletal muscle fibers, branched, and their contraction is involuntary. They are connected via intercalated discs. The myocardium contains one primary fiber type that is similar to Type I, slow-twitch fibers found in skeletal muscle. These fibers are highly aerobic and contain a larger number of mitochondria compared to Type I skeletal muscle fibers.

SA Node (Sinoatrial Node)

The SA node is specialized tissue located in the right atrium that generates electrical impulses to initiate the heartbeat. It is often referred to as the heart's natural pacemaker.

AV Node (Atrioventricular Node)

The AV node is muscle tissue located in the interventricular septum. It functions in the transmission of cardiac impulses from the atria to the ventricles. Purkinje fibers then complete ventricular contraction.

AVO2 Difference (Arteriovenous Oxygen Difference)

The AVO2 difference represents the amount of oxygen (O2) that is taken up from 100 mL of blood by the tissues during one trip around the systemic circuit. An increase in AVO2 difference during exercise is due to an increased amount of O2 taken up and used for the oxidative production of ATP. Typical values are approximately 20 mL O2/100 mL blood. At rest, the value is typically 5 to 6 mL, increasing to 15 to 16 mL at VO2max. VO2 is equal to the product of cardiac output and the AVO2 difference; an increase in either cardiac output or AVO2 difference will elevate VO2.

Factors Affecting Blood Flow

Blood flow is determined by the relationship: Blood Flow = Pressure / Resistance. The most important factor determining resistance to blood flow is the radius of the blood vessel. Other key factors include:

• Vessel radius
• Vessel length
• Blood viscosity

The formula for resistance (R) is: R = (length × viscosity) / radius. The greatest resistance in the circulatory system is found in the arterioles.

Hematocrit

Hematocrit is the percentage of blood composed of cells. Blood viscosity, which influences the difficulty with which blood flows, is significantly affected by increases or decreases in red blood cells.