Cardiac Muscle Physiology Overview

Heart Anatomy

• The heart is a muscle that pumps 5.25 L of fluid per minute.
• Located between the lungs in a space called the mediastinum.
• Separated from other structures by the pericardium, a tough membrane.
• The base of the heart is superior and medial to the apex.

Heart Structure

• 4 chambers in the heart: 2 atria and 2 ventricles.
• Atria are superior, collect blood, and push it into lower chambers.
• Ventricles are inferior and pump blood out to the body or lungs.
• 2 circuits: pulmonary and systemic.
• Right ventricle connects to the pulmonary trunk and left ventricle connects to the aorta.

Heart Membranes and Layers

• 2 layers: fibrous and serous pericardium.
• Serous layer has 2 sublayers: parietal and visceral layers separated by pericardial cavity.
• Cavity filled with lubricating fluid.
• Visceral layer is called epicardium and produces fluid.

Heart Features

• 2 auricles: thin-walled structures that can hold blood and empty into atria.
• Sulci: fat-filled grooves that contain coronary blood vessels.

Layers of the Heart

• Epicardium: most superficial layer.
• Myocardium: middle, thickest layer, muscle cells, nerve fibers, and blood vessels.
• Endocardium: innermost layer, lines the chambers filled with blood, covers heart valves.
• Endothelium: cells that line the blood vessels, release endothelins, which are strong vasoconstrictors.

Internal Structures

• Septum: divides the heart into chambers, physical extensions of myocardium, 3 septa in the heart.
• Fossa ovalis: a depression in the interatrial septum, was a foramen ovale (opening) while a fetus.
• Atrioventricular valves: separate atria from ventricles, tricuspid on right and mitral or bicuspid on left.
• Semilunar valves: separate ventricles from blood vessels, pulmonary and aortic valves.

Chambers of the Heart

• Right atrium: receives deoxygenated blood from systemic circulation, contains pectinate muscles.
• Right ventricle: receives deoxygenated blood from the right atrium, has chordae tendineae connected to 3 papillary muscles.
• Left atrium: receives oxygenated blood from the pulmonary veins, no pectinate muscles.
• Left ventricle: thicker muscle, receives oxygenated blood from the left atrium, pushes blood into the aorta.

Valves of the Heart

• Ensure unidirectional blood flow.
• Semilunar valves have no muscular attachments, but make an audible sound as they close.
• Tricuspid and mitral valves close due to papillary muscles.
• Valve disorders are usually caused by inflammation.

Coronary Circulation

• Cyclical and not continuous.
• Arteries branch off the aorta just superior to the semilunar valve.
• Coronary arteries bring blood to the myocardium and other components.
• Anastomosis: blood vessel interconnections to allow blood to flow even if there is a partial blockage in another branch.

Cardiac Muscle and Electrical Activity

• Autorhythmicity: cardiac muscle can initiate an electrical potential at a fixed rate.
• Myocardial conducting cells: initiate and propagate the action potential through the heart.
• Myocardial contractile cells: conduct impulses and contract to pump blood through the body.

Comparative Conduction

• SA node sets normal cardiac rhythm, known as the pacemaker.
• AV node receives impulse from SA node and slows it down.
• Bundle of His receives impulse from AV node and flows down the interventricular septum.
• Purkinje fibers spread the impulse around the ventricles.

Membrane Potentials

• Cardiac conductive cells have Na+ channels that allow a slow influx to create the spontaneous depolarization.
• Cardiac contractile cells depolarize as an impulse reaches them, then they plateau before repolarizing.

Calcium Ions

• Allow for the plateau phase in cardiac contractile cells.
• Bind to troponin to allow the myosin and actin cross-bridge.

Electrocardiogram (ECG)

• Records electrical signals of the heart.
• 3, 5, or 12 leads.
• P wave: depolarization of the atria.
• QRS complex: depolarization of ventricles and it masks the repolarization of the atria.
• T wave: repolarization of ventricles.

Cardiac Cycle

• Begins at atrial contraction and ends with ventricular relaxation.
• Systole: pumping blood into circulation.
• Diastole: chambers are relaxed and filling with blood.

Phases of the Cardiac Cycle

• Atrial systole: atria contract from superior portion following depolarization (P wave).
• Atrial diastole: muscles relax as ventricles begin to contract.
• Ventricular systole: blood in ventricles is the end-diastolic volume (EDV) or preload.
• Ventricular diastole: relaxation, semilunar valves close, and atrioventricular valves are closed too.

Heart Sounds

• Healthy heart has 2 audible sounds: S1 and S2 (lub dub).
• S1: closing of atrioventricular valves.
• S2: closing of semilunar valves.

Murmur

• Caused by turbulent blood flow.
• Inhalation increases blood flow to the right side and may increase the amplitude of a right-sided murmur.
• Expiration partially restricts blood flow to the left side and may increase the amplitude of a left-sided murmur.