Anatomy and Physiology of the Heart

Lecture Objectives

• Describe the human heart's anatomical location.
• Describe the heart's structure, including pericardium, heart wall, surfaces, apex, and base.
• Explain how the heart is divided into chambers and describe their structure.
• Compare heart chamber thicknesses and relate thickness to function.
• Describe the structure and function of heart valves.
• Trace blood flow through the heart, pulmonary, and systemic circulation.
• Trace blood flow through coronary circulation and describe its function.
• Describe the microscopic structure and function of cardiac muscle tissue.
• Explain the basis for autorhythmicity of cardiac muscle tissue.
• Describe the structure and function of the cardiac conduction system.
• Compare cardiac and skeletal muscle action potentials.
• Define refractory periods.
• Explain how heart structure prevents tetanus.
• Define electrocardiogram (ECG).
• Explain the P, QRS, and T waves of ECGs.
• Describe cardiac cycle events, especially blood pressure and volume.
• Define heart sounds and explain their physiological basis.
• Define cardiac output and factors regulating it/stroke volume and heart rate.
• Explain how exercise affects heart structure and function.

Anatomy of the Human Heart

• The heart is roughly the size of a clenched fist.
• It's located in the mediastinum of the thoracic cavity.
• Mass: ~250g in females, ~300g in males.
• The apex (pointed tip) rests on the diaphragm, slightly to the left.
• The base (opposite the apex) is angled slightly posterior.
• The heart is positioned with its right surface facing the right lung, left surface facing the left lung, and anterior surface deep to the sternum.
• The pericardium is a double-layered sac surrounding the heart:
  • A fibrous pericardium, strong and inelastic, anchoring the heart in the mediastinum
  • A serous pericardium, thinner and more fragile, divided into parietal (fused with fibrous) and visceral layers (also known as the epicardium). The pericardial fluid in between the layers reduces friction.

Heart Wall Structure

• The heart wall is composed of three layers:
  • Epicardium (visceral layer of the serous pericardium), with connective tissue and fat.
  • Myocardium, cardiac muscle tissue.
  • Endocardium, lining the chambers and valves, continuous with blood vessels and reducing friction.

Chambers and Valves

• The heart has four chambers: two atria and two ventricles.
• Atria receive blood from veins and are thin-walled.
• Ventricles pump blood into arteries and are thicker-walled.
  • The right ventricle is thinner than the left.
• Heart valves ensure unidirectional blood flow.
  • Tricuspid and mitral (bicuspid) are atrioventricular valves.
  • Pulmonary and aortic are semilunar valves.
• Valves open and close based on pressure differences.

Cardiac Conduction System

• The cardiac conducting system coordinates and regulates heart contractions.
• The sinoatrial (SA) node initiates the heartbeat (pacemaker).
• The atrioventricular (AV) node delays the signal to allow the atria to contract before the ventricles.
• Signals travel through the AV bundle (bundle of His), bundle branches, and Purkinje fibers to stimulate ventricle contractions.

Cardiac Action Potentials

• Cardiac action potentials are unique compared to skeletal muscle potentials due to a plateau phase.
• VGNCs initiate depolarization.
• VGCCs (voltage-gated calcium channels) maintain the plateau; it is important for the strong and sustained contraction of cardiac cells.
• Repolarization of cardiac cells brings the membrane back to its resting potential, allowing for the refractory period and preventing tetanus.

Structure and Function of Cardiac Muscle

• Cardiac muscle cells are branched, striated, and have a single nucleus.
• Intercalated discs contain desmosomes and gap junctions, allowing for coordinated contractions.
• Cardiac muscle tissue has a high density of mitochondria enabling high oxidative respiration.
• Calcium ions play a critical role in contraction.

Coronary Circulation

• Coronary vessels supply the heart with oxygenated blood.
• Coronary arteries branch from the aorta.
• Blood flows from high pressure in the coronary arteries to low pressure in coronary capillaries.
• Deoxygenated blood is returned to the heart by the coronary veins, emptying into the coronary sinus, which empties into the right atrium.

ECG (Electrocardiogram)

• An ECG records electrical activity in the heart.
• P wave: atrial depolarization (contraction).
• QRS complex: ventricular depolarization (contraction).
• T wave: ventricular repolarization (relaxation).
• The intervals and segments between waves provide additional information.

Cardiac Cycle

• The cardiac cycle is a complete cycle of contraction and relaxation of the heart.
• Atrial systole and diastole: The atria contract to fill the ventricles. Ventricles fill with blood as atria relax.
• Ventricular systole and diastole: The ventricles contract to pump blood into the arteries. The ventricles relax and refill with blood from the atria.
• Pressure changes drive valve openings and closings.
• Heart sounds are produced by blood turbulence related to valve closure.
• S1 (lub): AV valves closing.
• S2 (dup): Semilunar valves closing.

Cardiac Output

• Cardiac output (CO) is the amount of blood pumped per minute.
• Stroke volume (SV): Volume of blood pumped per contraction.
• Heart rate (HR): Number of contractions per minute.
• CO = SV × HR.
• Cardiac reserve is the difference between maximum and resting CO.

Regulation of Cardiac Activity

• The autonomic nervous system (ANS) regulates heart activity, influencing heart rate and stroke volume.
• Sympathetic stimulation increases heart rate and contractility via norepinephrine.
• Parasympathetic stimulation (vagus nerve) decreases heart rate via acetylcholine.
• Other factors affecting heart rate and contractility include hormones and body temperature.
• Exercise, for example, increases cardiac output through increased heart rate and stroke output.