The Cardiovascular System: The Heart – Structure, Function, and Physiology

The Heart: Structure, Location, and Function

Overview of Heart Anatomy

The heart is a muscular organ located in the middle mediastinum, between the second rib and fifth intercostal space, resting on the superior surface of the diaphragm. It is roughly cone-shaped, with its apex angled inferiorly and anteriorly. The heart weighs approximately 300g and measures about 9cm in diameter at its widest point.

• Function: Acts as a muscular pump to circulate blood throughout the body.

• Orientation: Two-thirds of the heart lies to the left of the midsternal line.

• Enclosure: Enclosed in the pericardium, a double-walled sac.

Circulatory Pathways

The heart supports two major circulatory circuits: the pulmonary circuit and the systemic circuit. The pulmonary circuit carries blood to and from the lungs, while the systemic circuit delivers blood to and from all body tissues.

• Pulmonary Circuit: Right side of the heart pumps deoxygenated blood to the lungs for oxygenation.

• Systemic Circuit: Left side of the heart pumps oxygenated blood to the rest of the body.

Pericardium and Layers of the Heart Wall

Pericardium

The pericardium is a serous membrane that protects, anchors, and prevents overfilling of the heart. It consists of two main layers:

• Serous Pericardium: Composed of visceral and parietal layers with a pericardial cavity between them.

• Fibrous Pericardium: Dense irregular connective tissue that attaches to the parietal layer and anchors the heart to the diaphragm and great vessels.

Layers of the Heart Wall

The heart wall is composed of three distinct layers:

• Epicardium: The visceral layer of the serous pericardium, consisting of mesothelium and connective tissue.

• Myocardium: The muscular wall of the heart, responsible for contraction.

• Endocardium: The endothelial lining of the inner chambers of the heart.

Chambers and Valves of the Heart

Heart Chambers

The heart contains four chambers: two atria and two ventricles. The atria are separated by the interatrial septum, and the ventricles by the interventricular septum. Auricles are extensions that increase atrial volume.

• Atria: Upper chambers (right and left atrium) receive blood.

• Ventricles: Lower chambers (right and left ventricle) pump blood out of the heart.

Heart Valves

Valves ensure unidirectional blood flow and prevent backflow. They open and close in response to pressure changes from blood volume.

• Atrioventricular (AV) Valves:

  • Tricuspid Valve: Between right atrium and right ventricle.

  • Bicuspid (Mitral) Valve: Between left atrium and left ventricle.

• Semilunar Valves:

  • Pulmonary Valve: Between right ventricle and pulmonary trunk.

  • Aortic Valve: Between left ventricle and aorta.

Valve Malfunction and Replacement

Valve disorders include incompetent valves (backflow) and valvular stenosis (stiff flaps). Defective valves can be replaced with mechanical, animal, or cadaver valves.

Heart Sounds and Phonocardiograms

Heart Sounds

Heart sounds are produced by the closing of heart valves:

• First sound (Lub): AV valves close, beginning of systole.

• Second sound (Dup): Semilunar valves close, beginning of diastole.

• Heart murmurs: Abnormal sounds, often indicative of valve problems.

Pathway of Blood Through the Heart

Blood Flow Sequence

The heart functions as two side-by-side pumps:

• Right side: Receives deoxygenated blood from the body and pumps it to the lungs (pulmonary circuit).

• Left side: Receives oxygenated blood from the lungs and pumps it to the body (systemic circuit).

Blood Volume and Pressure

• Equal volumes of blood are pumped to pulmonary and systemic circuits.

• Pulmonary circuit is short and low-pressure; systemic circuit is longer and encounters more resistance.

Coronary Circulation

Arterial and Venous Supply

Coronary circulation provides functional blood supply to the heart muscle itself. It is the shortest circulation in the body and is delivered when the heart is relaxed. The left ventricle receives most of the coronary blood supply.

Heart Pathologies

Angina Pectoris and Myocardial Infarction

• Angina Pectoris: Thoracic pain caused by fleeting deficiency in blood delivery to the myocardium; cells are weakened.

• Myocardial Infarction (Heart Attack): Prolonged coronary blockage; areas of cell death are repaired with noncontractile scar tissue.

Microscopic Anatomy of Cardiac Muscle

Cardiac Muscle Cells

Cardiac muscle cells are striated, short, branched, and interconnected. They contain numerous large mitochondria and a single nucleus per cell. Intercalated discs, unique to cardiac muscle, contain desmosomes and gap junctions, allowing the heart to function as a syncytium.

Intrinsic Conduction System

Components and Function

The intrinsic conduction system regulates the rate and strength of heart contractions. About 1% of cardiac cells are autorhythmic, capable of self-excitation. The main components include:

• Sinoatrial (SA) Node: Pacemaker, initiates impulses.

• Atrioventricular (AV) Node: Delays impulses.

• AV Bundle (Bundle of His): Conducts impulses to bundle branches.

• Purkinje Fibers: Complete ventricular contraction.

Electrocardiogram (ECG/EKG)

ECG Features and Interpretation

An ECG is a composite recording of all electrical activity in the heart during the cardiac cycle. It measures voltage and time for action potential propagation.

• P wave: Atrial depolarization

• QRS complex: Ventricular depolarization

• T wave: Ventricular repolarization

• PR interval: Time between start of atrial and ventricular depolarization

• QT interval: Time for ventricular depolarization and repolarization

The Cardiac Cycle

Phases of the Cardiac Cycle

The cardiac cycle includes all events during one complete heartbeat, typically lasting 0.8 seconds at 60-80 bpm. It consists of systole (contraction) and diastole (relaxation).

• Atrial Systole: Atria contract, blood moves to ventricles.

• Isovolumetric Contraction: All valves closed, ventricles begin to contract.

• Ventricular Systole: Ventricles contract, blood ejected.

• Atrial Diastole: Atria relax.

• Isovolumetric Relaxation: All valves closed, ventricles relax.

• Ventricular Diastole: Ventricles relax, cycle repeats.

Cardiac Output and Regulation

Cardiac Output (CO)

Cardiac output is the volume of blood pumped by the heart per minute. It is calculated as:

• CO = HR × SV

• Where HR = heart rate (beats/min), SV = stroke volume (mL/beat)

At rest: $CO = 75 \times 70 = 5250\,\text{mL/min} = 5.25\,\text{L/min}$

Stroke Volume (SV)

Stroke volume is regulated by preload, contractility, and afterload:

• SV = EDV – ESV

• EDV = end diastolic volume; ESV = end systolic volume

At rest, SV should be 50-60% of EDV.

Frank-Starling Principle

The degree of stretch (preload) of cardiac muscle cells before contraction affects force of contraction. Increased venous return increases preload and contraction force.

Regulation of Heart Rate

• Sympathetic stimulation: Increases HR and contractility (norepinephrine).

• Parasympathetic stimulation: Decreases HR (acetylcholine).

• Bainbridge Reflex: Increased venous return stimulates SA node.

• Hormones: Epinephrine and thyroid hormone increase HR.

• Minerals: Ca2+ and K+ levels affect heart function.

Congestive Heart Failure (CHF)

Causes and Effects

• Causes: Coronary atherosclerosis, high blood pressure, multiple infarcts, dilated cardiomyopathy.

• Effects: Left-sided failure causes pulmonary congestion; right-sided failure causes peripheral congestion.

Developmental Aspects and Congenital Defects

Embryonic Heart Formation

• Embryonic chambers: sinus venosus, atrium, ventricle, bulbus cordis.

• Fetal heart structures bypass pulmonary circulation: foramen ovale (connects atria), ductus arteriosus (connects pulmonary trunk and aorta).

Age-Related Changes

• Sclerosis and thickening of valve flaps

• Decreased number of autorhythmic cells

• Decline in cardiac reserve

• Fibrosis of cardiac muscle

• Atherosclerosis