The Cardiovascular System: The Heart and Blood Vessels

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The Structure and Function of the Heart

Heart Anatomy and Internal Features

The human heart is a muscular organ responsible for pumping blood throughout the body. 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. The walls of the heart are composed of three layers: the epicardium (outer layer), myocardium (muscular middle layer), and endocardium (inner endothelial layer).

Epicardium: External surface, consists of connective tissue.
Myocardium: Contains cardiac muscle tissue, connective tissue, blood vessels, and nerves.
Endocardium: Internal, endothelial surface lining the heart chambers.
Intercalated discs: Specialized junctions between cardiac muscle cells for rapid electrical signal transmission.

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

Chambers and Valves of the Heart

The heart's chambers are separated by valves that ensure unidirectional blood flow and prevent backflow. The atria are holding chambers with thin walls, while the ventricles are muscular and responsible for pumping blood out of the heart. The right atrium receives deoxygenated blood from the body, and the left atrium receives oxygenated blood from the lungs.

Right Atrium: Receives blood from the superior vena cava, inferior vena cava, and coronary sinus.
Right Ventricle: Pumps deoxygenated blood to the lungs via the pulmonary trunk and pulmonary semilunar valve.
Left Atrium: Receives oxygenated blood from pulmonary veins.
Left Ventricle: Pumps oxygenated blood to the body via the aorta and aortic semilunar valve. Its wall is thicker to generate higher pressure.
Atrioventricular (AV) Valves: Tricuspid (right) and bicuspid/mitral (left) valves prevent backflow into atria.
Semilunar Valves: Pulmonary (right) and aortic (left) valves prevent backflow into ventricles.

Heart anatomy showing chambers and valves

Blood Flow Through the Heart and Circulatory Pathways

Pulmonary and Systemic Circulation

The heart acts as a dual pump, directing blood through two main circuits: pulmonary (to the lungs) and systemic (to the body). The right side of the heart pumps blood to the lungs for oxygenation, while the left side pumps oxygenated blood to the rest of the body.

Pulmonary Circulation: Right ventricle → pulmonary arteries → lungs → pulmonary veins → left atrium.
Systemic Circulation: Left ventricle → aorta → body tissues → vena cavae → right atrium.
Coronary Circulation: Supplies blood to the heart muscle itself.

Diagram of blood flow through a healthy heart Overview of blood flow through the body and 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. This cycle is responsible for the movement of blood through the heart and into the circulatory system.

Ventricular Filling: Blood flows passively into 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.

Cardiac cycle graph showing electrical, pressure, and volume changes Aorta and arteries stretch and recoil during ventricular contraction and relaxation

Heart Sounds and Valve Function

Heart sounds are produced by the closing of valves. The "lub" sound occurs when AV valves close at the onset of systole, and the "dub" sound occurs when semilunar valves close at the onset of diastole. Valve function is critical for maintaining proper blood flow and preventing regurgitation.

Chordae tendineae: Anchor AV valves to papillary muscles, preventing prolapse.
Valve opening and closing: Governed by pressure differences between chambers and vessels.

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

Electrical Activity of the Heart

Intrinsic Conduction System and ECG/EKG

The heart's electrical activity is coordinated by the intrinsic conduction system, which includes the sinoatrial (SA) node, atrioventricular (AV) node, bundle of His, bundle branches, and Purkinje fibers. This system ensures the orderly depolarization and contraction of the heart chambers. The electrocardiogram (ECG/EKG) records these electrical events.

P wave: Atrial depolarization.
QRS complex: Ventricular depolarization and contraction.
T wave: Ventricular repolarization.
SA node: Pacemaker cells initiate action potentials.

ECG tracing showing P, QRS, and T waves Cardiac cycle and ECG relationship

Action Potentials in Cardiac Muscle

Cardiac muscle cells exhibit unique action potentials characterized by a plateau phase, which prolongs contraction and prevents tetanus. The action potential involves rapid depolarization (Na+ influx), plateau (Ca2+ influx), and repolarization (K+ efflux).

Plateau phase: Maintains depolarization, allowing sustained contraction.
Ion channels: Voltage-gated Na+, Ca2+, and K+ channels regulate the phases.
Excitation-contraction coupling: Ca2+ influx triggers further Ca2+ release from the sarcoplasmic reticulum, enabling contraction.

Action potential and membrane permeability changes in cardiac muscle Action potential of cardiac muscles Excitation-contraction coupling in cardiac contractile cells

Action Potentials in Autorhythmic Cells (Pacemaker Cells)

Autorhythmic cells, such as those in the SA node, generate spontaneous action potentials due to a slow depolarization called the pacemaker potential. This is regulated by changes in Na+, Ca2+, and K+ permeability.

Pacemaker potential: Slow depolarization until threshold is reached.
Depolarization: Ca2+ influx.
Repolarization: K+ efflux.

Self-induced action potential in pacemaker cells Comparison of action potentials in pacemaker and contractile cells

Regulation of Heart Rate and Cardiac Output

Autonomic Nervous System Control

The autonomic nervous system (ANS) modulates heart rate and contractility. Parasympathetic stimulation (via the vagus nerve) decreases heart rate, while sympathetic stimulation increases heart rate and contractility. Cardiac output is the product of heart rate and stroke volume.

Cardiac Output (CO): $CO = HR \times SV$
Parasympathetic effects: Decrease depolarization rate, lower heart rate.
Sympathetic effects: Increase depolarization rate, raise heart rate and contractility.

Cardiac output calculation and factors affecting stroke volume Table of ANS effects on heart activity Parasympathetic and sympathetic stimulation effects on heart and blood pressure

Blood Pressure Regulation

Vascular Tone and Local Factors

Arteriolar smooth muscle maintains a baseline tone, which can be modulated by intrinsic (myogenic) and extrinsic (ANS) factors. Vasoconstriction increases resistance and decreases flow, while vasodilation decreases resistance and increases flow. Local metabolic changes, hormones, and neural inputs regulate arteriolar radius.

Vasoconstriction: Increased contraction, higher resistance, lower flow.
Vasodilation: Decreased contraction, lower resistance, higher flow.
Key chemicals: Norepinephrine, epinephrine, vasopressin, angiotensin II, histamine.

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

Baroreceptor Reflex and Blood Pressure Homeostasis

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, which adjusts heart rate, stroke volume, and vessel tone via the ANS.

Increased blood pressure: Decreases sympathetic activity, increases parasympathetic activity, lowers heart rate and vessel tone.
Decreased blood pressure: Increases sympathetic activity, decreases parasympathetic activity, raises heart rate and vessel tone.
Mean Arterial Pressure (MAP): $MAP = CO \times TPR$

Baroreceptor reflex pathway Baroreceptor reflex response to blood pressure changes Homeostatic control correcting a decrease in blood pressure

Summary Table: Effects of the Autonomic Nervous System on Heart Activity

Area Affected	Parasympathetic Stimulation	Sympathetic Stimulation
SA node	Decreases depolarization rate, decreases heart rate	Increases depolarization rate, increases heart rate
AV node	Decreases excitability, increases AV nodal delay	Increases excitability, decreases AV nodal delay
Ventricular conduction pathway	No effect	Increases conduction, increases contractility
Atrial muscle	Weakens contraction	Increases contractility
Ventricular muscle	No effect	Increases contractility
Adrenal medulla	No effect	Promotes secretion of epinephrine and norepinephrine
Veins	No effect	Increases venous return

Additional info:

Cardiac muscle blends features of both skeletal and smooth muscle, being striated and involuntary.
Cardiac output can increase significantly during exercise due to increased heart rate and stroke volume.
Hormones such as aldosterone, vasopressin, and angiotensin II play important roles in blood pressure regulation.
Mean arterial pressure is the main driving force for blood flow and is regulated by the body to maintain homeostasis.