The Cardiovascular System: The Heart and Blood Vessels

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The Heart: Structure and Function

Anatomy of the Heart

The human heart is a muscular organ responsible for pumping blood throughout the body via the circulatory system. It consists of four chambers: two atria (upper chambers) and two ventricles (lower chambers). The heart is divided into right and left sides, each serving distinct circulatory functions.

Right Atrium: Receives deoxygenated blood from the body via the superior and inferior vena cava.
Right Ventricle: Pumps deoxygenated blood to the lungs through the pulmonary trunk.
Left Atrium: Receives oxygenated blood from the lungs via the pulmonary veins.
Left Ventricle: Pumps oxygenated blood to the body through the aorta.
Valves: Ensure unidirectional blood flow and prevent backflow. Includes atrioventricular (AV) valves (tricuspid and bicuspid/mitral) and semilunar valves (pulmonary and aortic).

Anterior view of a frontally sectioned heart showing internal features and valves

Heart Valves and Blood Flow

Valves play a crucial role in directing blood flow through the heart. The AV valves (tricuspid and bicuspid) separate the atria from the ventricles, while the semilunar valves (pulmonary and aortic) separate the ventricles from the major arteries.

Tricuspid Valve: Between right atrium and right ventricle.
Bicuspid (Mitral) Valve: Between left atrium and left ventricle.
Pulmonary Semilunar Valve: Between right ventricle and pulmonary trunk.
Aortic Semilunar Valve: Between left ventricle and aorta.

Bicuspid valve open and closed, showing chordae tendineae and papillary muscle Superior view of heart valves with atria removed

Circulatory Pathways

Pulmonary and Systemic Circulation

The heart functions as a dual pump, supporting two main circulatory pathways:

Pulmonary Circulation: Carries deoxygenated blood from the right ventricle to the lungs for oxygenation, then returns oxygenated blood to the left atrium.
Systemic Circulation: Distributes oxygenated blood from the left ventricle to the body, returning deoxygenated blood to the right atrium.

Diagram showing blood flow through a healthy heart Overview of blood flow through the heart and body

Cardiac Cycle

Phases of the Cardiac Cycle

The cardiac cycle consists of alternating periods of contraction (systole) and relaxation (diastole) of the heart chambers. This cycle ensures efficient blood flow and is associated with characteristic heart sounds.

Atrial Systole: Atria contract, pushing blood into ventricles.
Ventricular Systole: Ventricles contract, ejecting blood into pulmonary and systemic circuits.
Diastole: Chambers relax and fill with blood.
Heart Sounds: "Lub" (AV valves closing) and "Dub" (semilunar valves closing).

Cardiac cycle: electrical events, pressure and volume changes, heart sounds Propagation of action potential through the heart

Electrical Activity of the Heart

Intrinsic Conduction System

The heart's electrical activity is coordinated by the intrinsic conduction system, which includes specialized autorhythmic cells. The sequence of depolarization ensures synchronized contraction.

SA Node: Pacemaker cells initiate the action potential.
AV Node: Delays the impulse, allowing atrial contraction.
Bundle of His and Purkinje Fibers: Rapidly distribute the impulse to ventricles.

EKG/ECG tracing showing electrical events in the heart Cardiac cycle and ECG correlation

Action Potentials in Cardiac Muscle

Cardiac muscle cells exhibit unique action potentials characterized by a plateau phase, which prevents tetanus and ensures proper contraction duration.

Depolarization: Rapid influx of Na+.
Plateau Phase: Slow influx of Ca2+ maintains depolarization.
Repolarization: Efflux of K+ restores resting potential.

Equation:

Action potential and membrane permeability changes in cardiac muscle Action potential of cardiac muscles

Excitation-Contraction Coupling

Excitation-contraction coupling in cardiac cells involves Ca2+ influx, which triggers further Ca2+ release from the sarcoplasmic reticulum, enabling contraction.

CICR: Calcium-induced calcium release amplifies intracellular Ca2+.
Relaxation: Ca2+ is removed via pumps, allowing muscle relaxation.

Excitation-contraction coupling in cardiac contractile cells

Regulation of Heart Rate and Cardiac Output

Autonomic Nervous System Control

The autonomic nervous system (ANS) modulates heart rate and contractility through sympathetic and parasympathetic pathways.

Parasympathetic (Vagus Nerve): Releases ACh, slows heart rate.
Sympathetic: Releases norepinephrine, increases heart rate and contractility.

Regulation of heart rate via pacemaker cells Effects of the autonomic nervous system on heart activity

Area Affected	Parasympathetic Stimulation	Sympathetic Stimulation
SA node	Decreases rate of depolarization to threshold; decreases heart rate	Increases rate of depolarization to threshold; increases heart rate
AV node	Decreases excitability; increases AV nodal delay	Increases excitability; decreases AV nodal delay
Ventricular conduction pathway	No effect	Increases excitability; hastens conduction through His and Purkinje cells
Atrial muscle	Weakens contractility	Increases contractility
Ventricular muscle	No effect	Increases contractility
Adrenal medulla	No effect	Promotes secretion of epinephrine
Veins	No effect	Increases venous return

ANS effects on heart activity

Cardiac Output

Cardiac output is the volume of blood pumped by each ventricle per minute. It is determined by heart rate and stroke volume.

Intrinsic Factors: Stretching of arteries, increased Ca2+, mechanoreceptors.
Sympathetic Activation: Increases stroke volume and heart rate.

Equation:

Cardiac output and factors affecting stroke volume Cardiac output and stroke volume regulation

Blood Pressure and Vascular Regulation

Blood Pressure Dynamics

Blood pressure is the force exerted by blood against vessel walls. It is highest in the arteries and decreases through the circulatory system.

Systolic Pressure: Pressure during ventricular contraction.
Diastolic Pressure: Pressure during ventricular relaxation.
Mean Arterial Pressure (MAP): Average pressure driving blood flow.

Equation:

Graph of blood pressure across the circulatory system

Vascular Tone and Resistance

Arteriolar tone is maintained by smooth muscle contraction and is influenced by intrinsic and extrinsic factors. Vasoconstriction increases resistance and decreases flow, while vasodilation decreases resistance and increases flow.

Intrinsic Factors: Myogenic activity, metabolic changes.
Extrinsic Factors: Sympathetic stimulation, hormones.

Normal arteriolar tone, vasoconstriction, and vasodilation Factors affecting arteriolar radius and blood flow

Baroreceptor Reflex and Blood Pressure Homeostasis

Baroreceptor Reflex

The baroreceptor reflex is a rapid mechanism for maintaining blood pressure homeostasis. Baroreceptors in the aorta and carotid sinus detect changes in blood pressure and send signals to the cardiovascular center in the medulla oblongata.

Increased BP: Decreases sympathetic activity, increases parasympathetic activity.
Decreased BP: Increases sympathetic activity, decreases parasympathetic activity.

Baroreceptor reflex pathway Other influences on blood pressure Monitoring blood pressure and homeostatic control Homeostatic control: correcting a decrease in blood pressure

Summary Table: Circulatory Pathways

Pathway	Origin	Destination	Function
Pulmonary	Right ventricle	Lungs	Oxygenates blood
Systemic	Left ventricle	Body tissues	Delivers oxygen and nutrients
Coronary	Left ventricle	Heart tissue	Supplies heart muscle

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