Chapter 14: Cardiovascular Physiology: Structure, Function, and Electrical Activity of the Heart

Cardiovascular System

Overview and Functions

The cardiovascular system is responsible for the transport of substances throughout the body, including oxygen, nutrients, hormones, and waste products. It consists of the heart, blood vessels, and blood.

• Heart: Muscular organ that pumps blood.

• Blood vessels: Network of arteries, veins, and capillaries that transport blood.

• Blood: Fluid that carries essential substances.

• Transport of substances: Includes oxygen, nutrients, hormones, and waste removal.

Substance Moved	From	To
Oxygen	Lungs	All cells
Nutrients and water	Intestinal tract	All cells
Waste	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
Blood leaving body	All cells	Kidneys, lungs
Carbon dioxide	All cells	Lungs

Blood Vessels

Types and Pathways

Blood flows through a series of vessels in a closed circuit:

• Arteries: Carry blood away from the heart.

• Arterioles: Small branches of arteries leading to capillaries.

• Capillaries: Sites of exchange between blood and tissues.

• Venules: Collect blood from capillaries.

• Veins: Return blood to the heart.

Pathway: heart → arteries → arterioles → capillaries → venules → veins → heart

Series Flow Through the Cardiovascular System

Pulmonary and Systemic Circuits

The cardiovascular system is a closed system with two main circuits:

• Pulmonary circuit: Carries blood between the heart and lungs for gas exchange.

• Systemic circuit: Delivers oxygenated blood from the heart to the rest of the body and returns deoxygenated blood.

Structure of the Heart

Pericardium and Chambers

The heart is encased within a membranous, fluid-filled sac called the pericardium, which lubricates the heart and decreases friction during contraction. The heart sits above the diaphragm and consists of four chambers: two atria and two ventricles.

Valves and Blood Flow

Types and Functions of Heart Valves

Heart valves ensure unidirectional blood flow and prevent backflow. They open and close passively in response to pressure gradients.

• Atrioventricular (AV) valves:

  • Right AV valve: tricuspid valve

  • Left AV valve: bicuspid (mitral) valve

  • Papillary muscles and chordae tendineae prevent AV valves from inverting into the atria.

• Semilunar valves:

  • Aortic valve

  • Pulmonary valve

Valves open when pressure in the preceding chamber exceeds that in the next chamber.

Fluid Flow and Pressure Gradients

Principles of Blood Flow

Blood flow through vessels is driven by pressure gradients:

• Pressure (P): Force exerted by blood.

• Pressure gradient (ΔP): Drives flow from high to low pressure.

• Bulk flow: Movement of blood due to pressure differences.

Equation:

$\text{Flow} \propto \Delta P$

Flow only occurs if there is a positive pressure gradient ($\Delta P > 0$).

Resistance in the Cardiovascular System

Systemic vs. Pulmonary Circuits

The pressure gradient in the systemic circuit is much greater than in the pulmonary circuit, but flow through both is equal due to differences in resistance.

Equation:

$\text{Flow} = \frac{\Delta P}{R}$

Lower resistance in the pulmonary circuit results in lower pressure.

Resistance Opposes Flow

Relationship Between Resistance and Flow

Flow through a tube is inversely proportional to resistance:

$\text{Flow} \propto \frac{1}{R}$

• If resistance increases, flow decreases.

• If resistance decreases, flow increases.

Poiseuille’s Law

Determinants of Resistance

Poiseuille’s Law describes how resistance depends on vessel length, fluid viscosity, and vessel radius:

$R \propto \frac{L \eta}{r^4}$

• Length (L): Resistance increases as length increases.

• Viscosity (η): Resistance increases as viscosity increases.

• Radius (r): Resistance decreases as radius increases (small changes in radius have large effects).

Vasoconstriction: Decrease in vessel diameter increases resistance and decreases flow. Vasodilation: Increase in vessel diameter decreases resistance and increases flow.

Flow Rate and Cross-Sectional Area

Velocity of Blood Flow

Flow rate is the volume of blood passing a point per unit time. Velocity is the distance a fixed volume travels in a given period.

Equation:

$v = \frac{Q}{A}$

• Velocity is faster in narrow sections.

• Velocity is slower in wider sections.

Example: If flow rate is 12 cm3/min and cross-sectional area is 3 cm2, then $v = 4$ cm/min.

Cardiac Muscle

Types and Structure

Cardiac muscle consists of contractile cells and autorhythmic (pacemaker) cells.

• Contractile cells: Striated fibers organized into sarcomeres; responsible for contraction.

• Autorhythmic cells: Generate and coordinate electrical signals; do not have organized sarcomeres.

Cardiac muscle cells are branched, have a single nucleus, and are connected by intercalated disks containing desmosomes (for force transfer) and gap junctions (for electrical signal transmission).

The Conduction System of the Heart

Pacemaker and Conduction Fibers

The heart’s electrical system coordinates contraction:

• Pacemaker cells: Spontaneously depolarize to generate action potentials (e.g., SA node).

• Conduction fibers: Rapidly conduct action potentials to the myocardium.

Main components:

• Sinoatrial (SA) node: Pacemaker of the heart.

• Atrioventricular (AV) node

• Internodal pathways

• Bundle of His

• Purkinje fibers

Electrical Conduction and Excitation

Pathways and Timing

Electrical signals spread through the heart in a coordinated manner:

• Interatrial pathway: SA node → right atrium → left atrium (simultaneous contraction).

• Internodal pathway: SA node → AV node.

• AV node transmission: Only pathway from atria to ventricles; slow conduction causes AV nodal delay (0.1 sec), allowing atria to contract before ventricles.

SA node sets the pace at ~70 bpm; AV node and Purkinje fibers have slower intrinsic rates.

Ionic Basis of Electrical Activity in the Heart

Pacemaker Potentials and Action Potentials

Autorhythmic cells have pacemaker potentials due to spontaneous depolarizations:

• Closing K+ channels and opening Na+ (funny) and Ca2+ (T-type) channels cause depolarization to threshold.

• Opening fast Ca2+ (L-type) channels triggers the action potential.

• Repolarization occurs via opening K+ channels.

Phase	Ion Channel Gating	Ion Movement
Pacemaker potential	Funny channels open, T-type Ca2+ channels open	Na+ in, Ca2+ in
Action potential	L-type Ca2+ channels open	Ca2+ in
Repolarization	K+ channels open	K+ out

Myocardial Action Potentials

Contractile Cell Electrical Activity

Myocardial contractile cells have distinct action potentials:

• Depolarization: Due to Na+ entry.

• Plateau phase: Due to Ca2+ entry, prolonging the action potential and preventing tetanus.

• Repolarization: Due to K+ exit.

Phase	Ion Channel Gating	Ion Movement
Rapid depolarization	Na+ channels open	Na+ in
Small repolarization	Na+ channels inactivated, K+ channels open	K+ out
Plateau	Ca2+ channels open	Ca2+ in
Repolarization	K+ channels open	K+ out
Resting potential	Na+ and Ca2+ channels closed	Little movement

Cardiac Muscle vs. Skeletal Muscle

Excitation-Contraction Coupling

Cardiac muscle shares properties with both skeletal and smooth muscle:

• Skeletal muscle similarities: T-tubules, sarcoplasmic reticulum, troponin-tropomyosin regulation.

• Smooth muscle similarities: Gap junctions, reliance on extracellular Ca2+.

Cardiac muscle has a long refractory period, preventing tetanus and ensuring rhythmic contractions.

Summary Table: Key Equations and Relationships

Concept	Equation
Flow and Pressure Gradient	$\text{Flow} \propto \Delta P$
Flow and Resistance	$\text{Flow} = \frac{\Delta P}{R}$
Poiseuille’s Law	$R \propto \frac{L \eta}{r^4}$
Velocity of Flow	$v = \frac{Q}{A}$

Additional info: These notes expand on the provided slides with definitions, examples, and equations for a comprehensive review of cardiovascular physiology, suitable for Anatomy & Physiology college students.