Hemodynamics and Circulatory System

Hemodynamics and Venous Return

• Homeostasis is maintained by the body's regulating mechanisms to create an optimal internal environment for cell survival.
• Systemic Circulation carries oxygen-rich blood from the heart to body tissues and returns deoxygenated blood to the heart. About 84% of blood volume circulates in this system.
• Pulmonary Circulation carries deoxygenated blood to the lungs for oxygenation and returns oxygenated blood to the heart. About 16% of blood volume is in this circuit.
• The circulatory system transports oxygen, nutrients, waste products, and hormones throughout the body.

Circulatory System and Blood Vessels

• Blood vessels distribute and exchange substances (nutrients/waste); have three layers: tunica intima, tunica media, and tunica externa/adventitia.
• Arteries have thick walls with elastin for stretching and recoil, and smooth muscle for constriction/vasodilation.
  • Elastic Arteries (e.g., aorta) have high elastin content near the heart, conducting large volumes of blood.
  • Muscular Arteries (distributing arteries) play a role in regulating vasoconstriction/vasodilation, farther from the heart where blood pressure has decreased, and contain less elastin.
  • Arterioles are small, have significant resistance to blood flow, and their diameter regulates blood flow into capillaries via precapillary sphincters.
• Capillaries are the sites of gas and nutrient exchange with tissues; walls contain a single layer of cells for efficient diffusion.
• Venules collect blood from capillaries and merge into veins.
• Veins are capacitance vessels; have thinner walls, larger lumens, and venous valves preventing backflow.

Hemodynamics

• Velocity is the distance a particle of fluid travels per unit time; flow rate is volume of fluid per unit time.
• Velocity is inversely proportional to cross-sectional area; blood velocity is slowest in capillaries due to their large total cross-sectional area.
• Bernoulli's Principle: the sum of potential, kinetic, and pressure energy per unit mass of an incompressible fluid remains constant. Increased velocity results in a decrease in lateral pressure.
• Pressure is exerted laterally against the vessel walls; higher velocity is related to pressure difference.
• Poiseuille's Law: fluid flow through a vessel is directly proportional to the fourth power of the radius and pressure difference, and inversely proportional to viscosity and length.

Blood Pressure

• Blood pressure (BP) is the force of blood against vessel walls, measured as systolic/diastolic pressure.
• Mean arterial pressure (MAP) is the average pressure during a cardiac cycle; a better indicator of tissue perfusion than systolic pressure.
• Pulse Pressure is the difference between systolic and diastolic pressure.

Regulation of Arterial Blood Pressure

• Intrinsic Control: Local factors like myogenic mechanisms, endothelial factors (nitric oxide, endothelin), and tissue metabolites regulate local blood flow.
• Extrinsic Control (Humoral): The renin-angiotensin-aldosterone system (RAAS) and other hormones regulate systemic arterial blood pressure and volume long-term.
• Extrinsic Control (Neural): The autonomic nervous system regulates short-term blood pressure through the sympathetic and parasympathetic nervous systems.
  • Baroreceptors: pressure sensors; changes in blood pressure trigger reflex changes in heart rate and vascular resistance.
  • Chemoreceptors: detect changes in blood pH, oxygen, and carbon dioxide levels; trigger compensatory mechanisms to maintain homeostasis.

Venous Return

• Venous system: has high capacitance; large reservoir for circulating blood (70%); adjust volume of blood returning to the heart; distensible and has low resistance.
• Venous Compliance: veins' ability to stretch and hold blood volumes; inversely related to venous pressure.
• Venous Valves: prevent backflow and facilitate one-way blood flow toward the heart.
• Factors affecting venous return: skeletal muscle pump, respiratory pump, sympathetic vasoconstriction, cardiac suction effect, and gravity.
• Central Venous Pressure (CVP): pressure in the large veins near the heart; it reflects volume load on the right side of the heart.
• Pulmonary Venous Pressure (PVP): pressure within the pulmonary veins; it affects left-sided filling pressures and is a part of the pulmonary circulation.