Cardiovascular Physiology: Structure, Function, and Mechanisms

Chapter 14: Cardiovascular Physiology

14.1 Overview of the Cardiovascular System

The cardiovascular system is essential for the transport of materials throughout the body, supporting cellular function and homeostasis.

• Components:

  • Heart: Acts as a pump to circulate blood.

  • Blood: The fluid medium carrying cells and plasma.

  • Blood vessels: Tubular structures (arteries, veins, capillaries) that transport blood.

• Transport Functions:

  • Delivers nutrients, water, and gases from the external environment.

  • Transfers hormones, immune cells, and antibodies between cells.

  • Removes waste products such as CO2, heat, and metabolic byproducts.

Table 14.1: Transport in the Cardiovascular System

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	Any cell that needs them
Hormones	Endocrine cells	Target cells
Stored nutrients	Liver and adipose tissue	All cells
Metabolic wastes	All cells	Kidneys
Carbon dioxide	All cells	Lungs

14.2 Pressure, Volume, Flow, and Resistance

Blood flow in the cardiovascular system is governed by physical principles of pressure, volume, and resistance.

• Blood Flow:

  • Liquids move from regions of high pressure to low pressure.

  • Pressure gradient (ΔP): The difference in pressure between two regions drives flow.

• Fluid Motion Components:

  • Dynamic (kinetic energy)

  • Lateral (hydrostatic pressure): Pressure exerted by a fluid at rest, in all directions.

• Pressure Changes:

  • Contraction of the heart creates pressure without changing blood volume (driving pressure).

  • Vessel dilation decreases pressure; vessel constriction increases pressure.

  • Volume changes affect blood pressure.

• Flow Relationships:

  • Flow through a tube is directly proportional to the pressure gradient: $\text{Flow} \propto \Delta P$

  • The higher the pressure gradient, the greater the fluid flow.

• Resistance:

  • Flow is inversely proportional to resistance: $\text{Flow} \propto \frac{1}{R}$

  • Poiseuille’s Law: $R = \frac{8L\eta}{\pi r^4}$ Where $L$ = length, $\eta$ = viscosity, $r$ = radius.

  • Resistance increases with length and viscosity, decreases with the fourth power of radius.

  • Vasoconstriction increases resistance; vasodilation decreases resistance.

• Velocity of Flow:

  • Velocity ($v$) depends on flow rate ($Q$) and cross-sectional area ($A$): $v = \frac{Q}{A}$

• Mean Arterial Pressure (MAP):

  • MAP is the primary driving force for blood flow.

  • MAP is maintained during heart relaxation.

  • MAP is proportional to cardiac output (CO) and peripheral resistance (PR): $\text{MAP} \propto \text{CO} \times \text{PR}$

14.3 Cardiac Muscle and The Heart

The heart is a muscular organ composed of specialized cardiac muscle cells, organized into four chambers and surrounded by connective tissue.

• Chambers:

  • Two thin-walled atria (upper chambers)

  • Two thick-walled ventricles (lower chambers)

• Blood Flow Pathways:

  • Deoxygenated blood: Vena cava → right atrium → right ventricle → pulmonary trunk

  • Oxygenated blood: Pulmonary veins → left atrium → left ventricle → aorta

• Connective Tissue Rings: Serve as origin and insertion for cardiac muscle fibers.

Table 14.2: The Heart and Major Blood Vessels

Structure	Receives Blood From	Sends Blood To
Right atrium	Venae cavae	Right ventricle
Right ventricle	Right atrium	Lungs
Left atrium	Pulmonary veins	Left ventricle
Left ventricle	Left atrium	Body except for lungs
Pulmonary artery	Right ventricle	Lungs
Pulmonary vein	Lungs	Left atrium
Aorta	Left ventricle	Systemic arteries

Heart Valves Ensure One-Way Flow

• Atrioventricular (AV) valves:

  • Located between atria and ventricles.

  • Chordae tendineae prevent eversion during ventricular contraction.

  • Tricuspid valve (right side), bicuspid/mitral valve (left side).

• Semilunar valves:

  • Located between ventricles and arteries.

  • Aortic valve and pulmonary valve.

• Coronary Circulation:

  • Supplies blood to the heart muscle via coronary arteries and veins.

Cardiac Muscle Cells

• Autorhythmic cells (pacemakers):

  • Initiate and coordinate contraction.

  • Smaller, fewer contractile fibers, lack organized sarcomeres.

• Contractile cells:

  • Striated fibers organized into sarcomeres.

  • Single nucleus per fiber, joined by intercalated disks and gap junctions.

  • T-tubules are larger and branch; sarcoplasmic reticulum is smaller.

  • Mitochondria occupy one-third of cell volume.

Cardiac Excitation-Contraction (EC) Coupling

• Action potential starts with pacemaker cells.

• Ca2+-induced Ca2+ release:

  • Voltage-gated L-type Ca2+ channels open in cell membrane.

  • Ryanodine receptors (RyR) in SR open in response to Ca2+ influx (Ca2+ spark).

  • Summed sparks create a Ca2+ signal.

  • Calcium binds to troponin, initiating crossbridge cycling as in skeletal muscle.

  • Relaxation: Ca2+ removed via Ca2+-ATPase (into SR) and Na+-Ca2+ exchanger (out of cell).

Myocardial Action Potentials

Contractile Cells

• Phase 4: Resting membrane potential

• Phase 0: Depolarization (Na+ inflow)

• Phase 1: Initial repolarization (Na+ channels close)

• Phase 2: Plateau (Ca2+ inflow)

• Phase 3: Rapid repolarization (K+ outflow)

Autorhythmic Cells

• Unstable membrane potential (pacemaker potential)

• Depolarization: Open If channel (Na+ inflow), then voltage-gated Ca2+ channels (Ca2+ inflow)

• At threshold: Second type of voltage-gated Ca2+ channels open (steep depolarization)

• At peak: Ca2+ channels close, slow K+ channels open (repolarization)

Table 14.3: Comparison of Action Potentials in Cardiac and Skeletal Muscle

	Skeletal Muscle	Contractile Myocardium	Autorhythmic Myocardium
Membrane Potential	Stable at -70 to -80 mV	Stable at -90 mV	Unstable, starts at -60 mV
Rising Phase	Na+ entry	Na+ entry	Ca2+ entry
Repolarization	Rapid, K+ efflux	Extended, K+ efflux	K+ efflux
Duration	Short (1-2 ms)	Extended (200+ ms)	Variable (generally 150+ ms)
Refractory Period	Brief	Long	Not significant in normal function

14.4 The Heart as a Pump

Electrical signals coordinate the contraction of the heart, ensuring efficient blood flow.

• Conduction Pathways:

  • Sinoatrial (SA) node → Atrioventricular (AV) node → AV bundle (bundle of His) → left and right bundle branches → Purkinje fibers

  • SA node sets the pace (70 bpm); AV node (50 bpm) and Purkinje fibers (25–40 bpm) can act as pacemakers if needed.

  • AV node delay allows atria to contract before ventricles.

• Pacemakers:

  • Specialized cells that set the heart rate.

Summary of Key Concepts

• The cardiovascular system consists of the heart, blood vessels, and blood, and is responsible for the transport of essential substances and waste.

• Blood flow is driven by pressure gradients and opposed by resistance, which is determined by vessel length, viscosity, and radius.

• The heart is composed of specialized muscle cells that contract rhythmically and are regulated by electrical signals.

• Valves ensure one-way flow, and the conduction system coordinates the timing of contractions.

Additional info: These notes expand on the provided slides with definitions, equations, and context for college-level Anatomy & Physiology students.