Circulatory System Structure and Blood Flow Dynamics (3B) - MCAT Biological and Biochemical Foundations of Living Systems

Pulmonary semilunar valve. The pulmonary valve allows deoxygenated blood to exit the right ventricle while preventing backflow.

Mitral (bicuspid; left atrioventricular) valve. The mitral valve maintains forward flow of oxygenated blood from left atrium to ventricle during filling.

Tricuspid (right atrioventricular) valve. The tricuspid valve ensures unidirectional flow of deoxygenated blood from right atrium to ventricle.

Left ventricle. The left ventricle pumps oxygenated blood throughout the body via the aorta to meet tissue demands.

$Q = \frac{\Delta P}{R}$. This equation, analogous to Ohm's law, describes blood flow driven by pressure against resistance.

Capillaries. Capillaries' extensive branching results in the largest collective area, optimizing nutrient exchange.

Prevent backflow from arteries into ventricles. Semilunar valves close to sustain arterial pressure and prevent blood from re-entering relaxed ventricles.

Prevent backflow from ventricles into atria. Atrioventricular valves close to maintain pressure and ensure forward propulsion of blood into arteries.

Right ventricle. The right ventricle pumps deoxygenated blood to the lungs for gas exchange via the pulmonary artery.

Pulmonary: gas exchange in lungs; Systemic: delivers $O_2$ to tissues. Pulmonary circulation facilitates oxygenation, while systemic circulation supplies oxygen and nutrients to body tissues.

Venae cavae → RA → RV → pulmonary a. → lungs → pulmonary v. → LA → LV → aorta. This pathway ensures deoxygenated blood is oxygenated in the lungs before being pumped systemically.

Capillaries. Velocity is inversely proportional to cross-sectional area, minimizing speed in capillaries for diffusion.

Veins. Valves in veins counteract gravity and ensure unidirectional blood return to the heart.

Arteries have thicker tunica media; veins have thinner walls and valves. Arteries require robust walls for high pressure, while veins use valves for low-pressure return flow.

$Q = \frac{\pi r^4 \Delta P}{8\eta L}$. Poiseuille's law quantifies how radius, viscosity, and length influence laminar flow in vessels.

Resistance decreases by a factor of $16$. Resistance varies inversely with radius to the fourth power per Poiseuille's law.

Flow decreases by a factor of $2$. Flow is inversely proportional to viscosity in Poiseuille's law, with other factors constant.

$Q = Av$. The continuity equation conserves volume flow by relating it to area and velocity.

Velocity decreases by a factor of $4$. For constant flow, velocity inversely scales with cross-sectional area per continuity equation.

Total peripheral resistance increases. Vasoconstriction reduces vessel radius, elevating resistance via Poiseuille's fourth-power relationship.

$MAP \approx CO \times TPR$. This approximation balances cardiac pumping with vascular resistance to determine arterial pressure.

$MAP$ increases by $25%$. Mean arterial pressure directly proportional to cardiac output when resistance is unchanged.

Exchange of gases, nutrients, and wastes with tissues. Capillaries' thin endothelium and slow flow enable efficient diffusion across vessel walls.

Major resistance vessels controlling tissue perfusion and $BP$. Arterioles regulate blood distribution and pressure via smooth muscle control of vessel diameter.