Cardiovascular Physiology: Structure, Function, and Clinical Applications

Cardiovascular Physiology

Introduction to Cardiovascular Physiology

The cardiovascular system is essential for transporting materials throughout the body, including nutrients, gases, hormones, and waste products. It consists of the heart (the pump), blood (the fluid), and blood vessels (the tubes). Understanding the structure and function of this system is crucial for comprehending how the body maintains homeostasis and responds to physiological challenges.

Overview of the Cardiovascular System

Components and Functions

• Heart: A muscular organ divided into left and right halves by the septum. Each half contains an atrium (receives blood) and a ventricle (pumps blood out).

• Blood: Composed of cells (erythrocytes, leukocytes, platelets) and plasma.

• Blood Vessels: Arteries (carry blood away from the heart), veins (return blood to the heart), and capillaries (sites of exchange).

• Pulmonary Circulation: Right side of the heart pumps deoxygenated blood to the lungs.

• Systemic Circulation: Left side of the heart pumps oxygenated blood to the body.

Transport Functions

• Materials Entering the Body: Oxygen (lungs to all cells), nutrients and water (intestinal tract to all cells).

• Materials Moved Between Cells: Hormones, immune cells, antibodies, and stored nutrients.

• Materials Leaving the Body: Metabolic wastes (to kidneys), heat (to skin), carbon dioxide (to lungs).

Substance Moved	From	To
Oxygen	Lungs	All cells
Nutrients and water	Intestinal tract	All cells
Wastes	Some cells	Liver for processing
Immune cells, antibodies, clotting proteins	Present in blood continuously	Available to any cell that needs them
Hormones	Endocrine cells	Target cells
Stored nutrients	Liver and adipose tissue	All cells
Metabolic wastes	All cells	Kidneys
Heat	All cells	Skin
Carbon dioxide	All cells	Lungs

Blood Flow, Pressure, and Resistance

Principles of Fluid Flow

Blood flows from regions of higher pressure to regions of lower pressure. The heart creates pressure by contracting, and blood moves through vessels where pressure decreases due to friction and resistance.

• Pressure Gradient (ΔP): The difference in pressure between two points drives flow.

• Hydrostatic Pressure: Pressure exerted by a fluid at rest in all directions.

• Flow Equation: $Flow \propto \Delta P$

Resistance to Flow

• Poiseuille’s Law: Resistance ($R$) is proportional to the length ($L$) of the vessel and the viscosity ($\eta$) of the fluid, and inversely proportional to the fourth power of the radius ($r$):

$R = \frac{8L\eta}{\pi r^4}$

• Vasoconstriction: Decreases vessel radius, increasing resistance and decreasing flow.

• Vasodilation: Increases vessel radius, decreasing resistance and increasing flow.

Velocity of Blood Flow

• Flow Rate (Q): Volume of blood passing a point per unit time.

• Velocity (v): Distance a volume of blood travels per unit time. $v = \frac{Q}{A}$, where $A$ is cross-sectional area.

Structure of the Heart

Anatomy and Chambers

• The heart is located in the center of the thorax, encased in the pericardium, and consists of four chambers: two atria and two ventricles.

• The right side receives deoxygenated blood and pumps it to the lungs; the left side receives oxygenated blood and pumps it to the body.

Heart Valves and Blood Flow

• Atrioventricular (AV) Valves: Tricuspid (right) and bicuspid/mitral (left) valves prevent backflow from ventricles to atria.

• Semilunar Valves: Aortic and pulmonary valves prevent backflow from arteries to ventricles.

• Chordae Tendineae: Prevent valve eversion during ventricular contraction.

Coronary Circulation

• The coronary arteries supply oxygenated blood to the heart muscle itself, while coronary veins return deoxygenated blood to the right atrium.

Cardiac Muscle and Electrical Activity

Cardiac Muscle Structure

• Cardiac muscle cells are striated, branched, and connected by intercalated disks containing gap junctions for electrical coupling.

• Autorhythmic (pacemaker) cells generate spontaneous action potentials, while contractile cells produce force.

Excitation-Contraction Coupling

• Action potentials in pacemaker cells trigger Ca2+-induced Ca2+ release from the sarcoplasmic reticulum, leading to contraction.

• Relaxation occurs as Ca2+ is pumped back into the SR or out of the cell via the Na+-Ca2+ exchanger.

Cardiac Action Potentials

• Contractile Cells: Have a stable resting potential, rapid depolarization (Na+ influx), plateau phase (Ca2+ influx), and repolarization (K+ efflux).

• Autorhythmic Cells: Unstable pacemaker potential, slow depolarization (If channels, Na+ influx), and Ca2+-dependent action potential.

Electrical Conduction and the Cardiac Cycle

Conduction System

• SA node (pacemaker) initiates the heartbeat; signal spreads via internodal pathways to AV node, then through the bundle of His, bundle branches, and Purkinje fibers.

• AV node delay allows atrial contraction before ventricular contraction.

Electrocardiogram (ECG/EKG)

• Records the summed electrical activity of the heart.

• P wave: Atrial depolarization

• QRS complex: Ventricular depolarization (and atrial repolarization)

• T wave: Ventricular repolarization

• U wave: Repolarization of papillary muscles or Purkinje fibers (sometimes seen)

Mechanical Events of the Cardiac Cycle

Phases of the Cardiac Cycle

1. Atrial and Ventricular Diastole: Heart at rest, blood flows from veins to atria and ventricles.

2. Atrial Systole: Atria contract, completing ventricular filling (end-diastolic volume, EDV).

3. Isovolumic Ventricular Contraction: AV valves close (first heart sound, "lub"), pressure rises, no blood ejected.

4. Ventricular Ejection: Semilunar valves open, blood ejected into arteries (end-systolic volume, ESV).

5. Isovolumic Ventricular Relaxation: Semilunar valves close (second heart sound, "dup"), pressure falls, AV valves reopen.

Cardiac Output and Stroke Volume

• Stroke Volume (SV): $SV = EDV - ESV$

• Cardiac Output (CO): $CO = Heart\ Rate \times Stroke\ Volume$

• Average CO at rest is about 5 L/min in adults.

Clinical Applications and Homeostatic Imbalances

Common Disorders

• Broken Heart Syndrome (Takotsubo Cardiomyopathy): Temporary weakening of the heart muscle due to severe emotional or physical stress.

• Pericarditis: Inflammation of the pericardium, causing chest pain and potential cardiac dysfunction.

• Heart Murmurs: Abnormal heart sounds due to valve defects (incompetent or stenotic valves).

• Arrhythmias: Irregular heart rhythms, including atrial and ventricular fibrillation.

• Congestive Heart Failure (CHF): Inadequate cardiac output due to weakened myocardium, often resulting from coronary artery disease, hypertension, or myocardial infarction.

Key Equations and Concepts

• Poiseuille’s Law: $R = \frac{8L\eta}{\pi r^4}$

• Flow Equation: $Flow = \frac{\Delta P}{R}$

• Velocity of Flow: $v = \frac{Q}{A}$

• Cardiac Output: $CO = HR \times SV$

Additional info: This summary integrates foundational concepts in cardiovascular physiology, including structure, function, and clinical relevance, and is suitable for ANP college-level study.