Heart Anatomy, Cardiac Muscle, and Cardiac Physiology Study Guide

Heart Anatomy

Overview of Heart Structure

The heart is a muscular organ responsible for pumping blood throughout the body. It is divided into chambers and surrounded by protective layers.

• Chambers: The heart has four chambers: two atria (upper) and two ventricles (lower).

• Layers: The heart wall consists of three layers: epicardium (outer), myocardium (middle, muscular), and endocardium (inner).

• Pericardium: The heart is enclosed in a double-walled sac called the pericardium, which provides protection and reduces friction.

Heart Layers and Pericardium

The pericardium consists of two main layers: the fibrous pericardium (outer) and the serous pericardium (inner, which itself has parietal and visceral layers).

Layer	Description
Epicardium	Outer layer; also called the visceral pericardium
Myocardium	Middle, muscular layer; responsible for contraction
Endocardium	Inner layer; lines the heart chambers

Major Blood Vessels and Circulation

The heart receives and pumps blood through major vessels:

• Superior and Inferior Vena Cava: Bring deoxygenated blood from the body to the right atrium.

• Pulmonary Arteries: Carry deoxygenated blood from the right ventricle to the lungs.

• Pulmonary Veins: Return oxygenated blood from the lungs to the left atrium.

• Aorta: Distributes oxygenated blood from the left ventricle to the body.

Heart Valves

Valves ensure unidirectional blood flow through the heart:

Valve	Location	Function
Tricuspid	Between right atrium and right ventricle	Prevents backflow into right atrium
Bicuspid (Mitral)	Between left atrium and left ventricle	Prevents backflow into left atrium
Pulmonary	Between right ventricle and pulmonary artery	Prevents backflow into right ventricle
Aortic	Between left ventricle and aorta	Prevents backflow into left ventricle

Coronary Circulation

The heart has its own blood supply via the coronary arteries and veins, which nourish the myocardium.

• Left and Right Coronary Arteries: Branch from the ascending aorta.

• Cardiac Veins: Drain deoxygenated blood from the myocardium into the coronary sinus.

Blood Flow Through the Heart

Blood flows through the heart in a specific sequence:

1. Deoxygenated blood enters the right atrium via the vena cavae.

2. Passes through the tricuspid valve to the right ventricle.

3. Pumped through the pulmonary valve to the pulmonary arteries and lungs.

4. Oxygenated blood returns via pulmonary veins to the left atrium.

5. Passes through the bicuspid (mitral) valve to the left ventricle.

6. Pumped through the aortic valve to the aorta and systemic circulation.

Chamber Functions and Blood Oxygenation

Chamber	Receives Blood From	Pumps Blood To	Oxygenation
Right Atrium	Body (vena cavae)	Right ventricle	Deoxygenated
Right Ventricle	Right atrium	Lungs (pulmonary artery)	Deoxygenated
Left Atrium	Lungs (pulmonary veins)	Left ventricle	Oxygenated
Left Ventricle	Left atrium	Body (aorta)	Oxygenated

Cardiac Muscle & Electrical Activity

Cardiac Muscle Structure

Cardiac muscle is unique in its ability to contract rhythmically and autonomously.

• Intercalated Discs: Specialized connections between cardiac muscle cells containing desmosomes and gap junctions.

• Desmosomes: Provide strong adhesion between cells.

• Gap Junctions: Allow electrical signals to pass rapidly between cells.

Pacemaker Cells and Conduction System

The heart's electrical activity is coordinated by pacemaker cells and the conduction system.

• SA Node: Primary pacemaker, initiates action potentials.

• AV Node: Delays signal before passing to ventricles.

• Bundle of His, Bundle Branches, Purkinje Fibers: Distribute the signal throughout the ventricles.

Action Potential Phases in Cardiac Muscle

1. Depolarization: Rapid influx of Na+ ions.

2. Plateau: Ca2+ channels open, maintaining depolarization.

3. Repolarization: K+ channels open, restoring resting potential.

Electrocardiogram (ECG) Segments

• P wave: Atrial depolarization

• QRS complex: Ventricular depolarization

• T wave: Ventricular repolarization

Cardiac Cycle

Phases of the Cardiac Cycle

The cardiac cycle consists of alternating periods of contraction (systole) and relaxation (diastole) in the atria and ventricles.

• Ventricular Filling: Blood flows from atria to ventricles.

• Isovolumetric Contraction: Ventricles contract, all valves closed.

• Ventricular Ejection: Semilunar valves open, blood ejected.

• Isovolumetric Relaxation: Ventricles relax, all valves closed.

Valve States During Cardiac Cycle

Cardiac Cycle Phase	Atrial State	Ventricular State	State of AV Valves	State of Semilunar Valves
Ventricular Filling	Contracting	Relaxed	Open	Closed
Isovolumetric Contraction	Relaxed	Contracting	Closed	Closed
Ventricular Ejection	Relaxed	Contracting	Closed	Open
Isovolumetric Relaxation	Relaxed	Relaxed	Closed	Closed

Cardiac Physiology

Heart Rate and Cardiac Output

• Heart Rate (HR): Number of beats per minute.

• Stroke Volume (SV): Volume of blood pumped per beat.

• Cardiac Output (CO): Total volume pumped per minute. $CO = HR \times SV$

Factors Affecting Stroke Volume

• Preload: Degree of stretch of cardiac muscle before contraction.

• Contractility: Strength of contraction.

• Afterload: Resistance the heart must overcome to eject blood.

Autonomic Regulation of the Heart

Cardiovascular Center	Function	Type of ANS Stimulation	Neurotransmitter
Cardioacceleratory	Increases heart rate	Sympathetic	Norepinephrine
Cardioinhibitory	Decreases heart rate	Parasympathetic	Acetylcholine

Summary Table: Heart Valves

Valve	Location	Function
Right AV (Tricuspid)	Right atrium and right ventricle	Prevents backflow into right atrium
Left AV (Bicuspid/Mitral)	Left atrium and left ventricle	Prevents backflow into left atrium
Pulmonary	Right ventricle and pulmonary artery	Prevents backflow into right ventricle
Aortic	Left ventricle and aorta	Prevents backflow into left ventricle

Additional info:

• Cardiac muscle cells are autorhythmic, meaning they can generate their own electrical impulses.

• The cardiac cycle is essential for maintaining continuous blood flow and oxygen delivery to tissues.

• ECG interpretation is a key clinical tool for diagnosing heart rhythm and conduction abnormalities.