The Cardiovascular System: The Heart and Blood Vessels

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The Structure and Function of the Heart

Anatomy of the Heart

The heart is a muscular organ responsible for pumping blood throughout the body via the circulatory system. It consists of four chambers: two atria and two ventricles, separated by valves that ensure unidirectional blood flow. The walls of the heart are composed of three layers: epicardium (outer), myocardium (middle, muscular), and endocardium (inner, endothelial).

Epicardium: The external surface, also known as the visceral pericardium.
Myocardium: Contains cardiac muscle cells, connective tissue, blood vessels, and nerves.
Endocardium: The internal endothelial surface.

Anterior view of a sectioned heart showing internal features and valves

Heart Chambers and Valves

The heart is divided into right and left sides, each with an atrium and a ventricle. The right side receives deoxygenated blood and pumps it to the lungs (pulmonary circulation), while the left side receives oxygenated blood and pumps it to the body (systemic circulation).

Atria: Thin-walled chambers that receive blood.
Ventricles: Thick-walled chambers that pump blood out of the heart.
Valves:
- Atrioventricular (AV) valves: Tricuspid (right) and bicuspid/mitral (left) valves.
- Semilunar valves: Pulmonary (right) and aortic (left) valves.

Diagram of heart valves and chambers

Blood Flow Through the Heart

Pathway of Blood

Blood flows through the heart in a specific sequence, passing through each chamber and valve.

Right atrium
Tricuspid valve
Right ventricle
Pulmonary semilunar valve
Pulmonary trunk and arteries
Lungs (gas exchange)
Pulmonary veins
Left atrium
Bicuspid (mitral) valve
Left ventricle
Aortic semilunar valve
Aorta
Body tissues

Diagram showing blood flow through the heart

The Cardiac Cycle

Phases of the Cardiac Cycle

The cardiac cycle consists of alternating periods of contraction (systole) and relaxation (diastole) of the heart chambers.

Ventricular filling: Blood flows passively into the ventricles during diastole.
Isovolumetric contraction: Ventricles contract with all valves closed, building pressure.
Ventricular ejection: Semilunar valves open, blood is pumped out.
Isovolumetric relaxation: Ventricles relax, all valves closed, pressure drops.

Diagram of cardiac cycle phases

Pressure and Volume Changes

Pressure changes in the chambers and vessels drive the opening and closing of valves, ensuring efficient blood flow.

AV valves open when atrial pressure exceeds ventricular pressure.
Semilunar valves open when ventricular pressure exceeds vessel pressure.
Heart sounds ("lub-dub") are produced by valve closure.

Graph showing pressure, volume, and ECG changes during cardiac cycle

Electrical Activity of the Heart

Intrinsic Conduction System

The heart's electrical activity is generated and propagated by specialized cells.

SA node: Pacemaker cells initiate action potentials.
AV node: Delays impulse, allowing atrial contraction.
Bundle of His and Purkinje fibers: Rapidly conduct impulses to ventricles.

Electrocardiogram (ECG/EKG)

An ECG records the electrical events of the heart.

P wave: Atrial depolarization
QRS complex: Ventricular depolarization
T wave: Ventricular repolarization

ECG tracing with labeled waves and intervals

Cardiac Muscle Action Potentials

Phases of Action Potential in Cardiac Contractile Cells

Cardiac muscle cells exhibit a unique action potential with a plateau phase, preventing tetanus and allowing proper contraction.

Phase 0: Rapid depolarization due to Na+ influx
Phase 1: Partial repolarization due to transient K+ efflux
Phase 2: Plateau phase due to Ca2+ influx
Phase 3: Repolarization due to K+ efflux
Phase 4: Resting membrane potential

Action potential phases and ion permeability changes Action potential of cardiac muscle cells

Excitation-Contraction Coupling

The process by which electrical signals trigger muscle contraction involves Ca2+ influx and release from the sarcoplasmic reticulum, leading to crossbridge cycling.

Ca2+ binds to troponin, moving tropomyosin and allowing actin-myosin interaction.
Extent of contraction depends on cytosolic Ca2+ concentration.

Excitation-contraction coupling in cardiac contractile cells

Action Potential of Autorhythmic Cells

Pacemaker Potential

Autorhythmic cells (e.g., SA node) generate spontaneous action potentials due to slow Na+ and Ca2+ influx and reduced K+ efflux.

Threshold reached at -40 mV, Ca2+ channels open for rapid depolarization.
Repolarization occurs as Ca2+ channels close and K+ channels open.

Pacemaker potential in autorhythmic cells Comparison of action potentials in pacemaker and contractile cells

Regulation of Heart Rate and Cardiac Output

Autonomic Nervous System Control

The heart rate is regulated by the autonomic nervous system (ANS):

Parasympathetic (vagus nerve): Releases ACh, opens K+ channels, slows depolarization, decreases heart rate.
Sympathetic: Releases norepinephrine, closes K+ channels, increases Ca2+ influx, speeds depolarization, increases heart rate.

Cardiac Output

Cardiac output (CO) is the volume of blood pumped by each ventricle per minute.

Formula:
Heart Rate (HR): Beats per minute
Stroke Volume (SV): Volume of blood pumped per beat

Blood Pressure Regulation

Factors Affecting Blood Pressure

Blood pressure is determined by cardiac output and peripheral resistance.

Increase in output or resistance raises blood pressure.
Decrease in output or resistance lowers blood pressure.
Blood volume, vessel diameter, and viscosity also play roles.

Diagram of vasoconstriction and vasodilation in arterioles

Local and Extrinsic Control of Blood Flow

Local factors: Metabolic changes, histamine, myogenic activity, shear stress, temperature.
Extrinsic factors: Sympathetic stimulation, hormones (norepinephrine, epinephrine, vasopressin, angiotensin II).

Flowchart of factors affecting arteriolar radius Flowchart of local metabolic and myogenic responses

Baroreceptor Reflex and Blood Pressure Homeostasis

Baroreceptor Reflex

Baroreceptors in the aorta and carotid sinus monitor blood pressure and send signals to the cardiovascular center in the medulla.

Increased pressure decreases sympathetic activity, causing vasodilation and reduced heart rate.
Decreased pressure increases sympathetic activity, causing vasoconstriction and increased heart rate.

Diagram of baroreceptor reflex pathways Flowchart of baroreceptor reflex responses

Effects of the Autonomic Nervous System on Heart Activity

Comparison Table

Area Affected	Parasympathetic Effect	Sympathetic Effect
SA node	Decreases rate of depolarization, decreases heart rate	Increases rate of depolarization, increases heart rate
AV node	Decreases excitability	Increases excitability
Ventricular conduction pathway	No effect	Increases conduction velocity
Atrial muscle	Weakens contraction	Strengthens contraction
Adrenal medulla	No effect	Promotes secretion of epinephrine
Veins	No effect	Increases vasoconstriction

Table of ANS effects on heart activity

Summary of ANS Effects on Blood Pressure

Flowchart of ANS effects on heart and blood pressure

Elastic Properties of Arteries

Role of Elastic Arteries

The aorta and elastic arteries stretch during ventricular contraction and recoil during relaxation, helping maintain continuous blood flow. Diagram of elastic artery function during systole and diastole

Pressure Changes in the Circulatory System

Blood Pressure Gradient

Blood pressure is highest in the arteries and decreases as blood moves through arterioles, capillaries, venules, and veins. Graph of blood pressure changes across vessels

Additional info:

Cardiac muscle cells are striated, branched, and possess intercalated discs for rapid electrical transmission.
Cardiac output and blood pressure are tightly regulated by neural and hormonal mechanisms.
Vasovagal response is a rapid decrease in blood pressure due to increased parasympathetic stimulation, leading to fainting.