Cardiovascular System: Heart Structure, Function, and Physiology

Cardiovascular System

Functions of the Cardiovascular System

The cardiovascular system is essential for maintaining homeostasis by transporting blood throughout the body. It consists of the heart, blood vessels, and blood, working together to deliver oxygen, nutrients, and remove waste products.

• Generate blood pressure: The heart creates the force necessary to move blood through vessels.

• Routes blood: Directs blood flow through two main circuits: pulmonary (to lungs) and systemic (to body tissues).

• Regulates amount and direction of blood flow: Ensures tissues receive adequate blood supply according to their needs.

• Muscular pump and network of vessels: The heart contracts about 108,000 times per day, sending approximately 14,000 liters of blood through 60,000 miles of vessels.

Heart Anatomy

Location and Structure

The heart is a muscular organ located in the thoracic cavity, specifically:

• Inferior to the diaphragm

• Anterior to the vertebral column

• Posterior to the sternum

• Within the mediastinum and the pericardial cavity

Additional info: The mediastinum is the central compartment of the thoracic cavity, containing the heart, thymus, parts of the esophagus, and trachea.

Chambers of the Heart

The heart is divided into four chambers, each with a specific function in blood circulation:

• 2 Atria (Superior chambers): Receive blood returning to the heart. The right atrium receives deoxygenated blood from the body, and the left atrium receives oxygenated blood from the lungs.

• 2 Ventricles (Inferior chambers): Pump blood out of the heart. The right ventricle sends deoxygenated blood to the lungs, and the left ventricle sends oxygenated blood to the body.

• Divisions: The heart is separated by septums (interatrial and interventricular) and contains four main valves to regulate blood flow.

Heart Circuits

The heart operates two main circulatory circuits:

• Pulmonary Circuit (Right side): Carries deoxygenated blood from the right ventricle to the lungs for gas exchange, then returns oxygenated blood to the left atrium.

• Systemic Circuit (Left side): Delivers oxygenated blood from the left ventricle to the body, then returns deoxygenated blood to the right atrium.

Heart Membranes and Wall Layers

The heart is surrounded and protected by several membranes and consists of three main wall layers:

• Pericardium: Double-walled sac that encloses the heart.

• Fibrous Pericardium: Tough outer layer that anchors the heart and prevents overfilling.

• Serous Pericardium: Composed of parietal (outer) and visceral (inner, also called epicardium) layers, with serous fluid in between to reduce friction.

• Wall Layers:

  • Epicardium: Outer layer, also the visceral pericardium.

  • Myocardium: Middle layer, composed of cardiac muscle responsible for contraction.

  • Endocardium: Inner layer, continuous with the endothelium of blood vessels.

Valves of the Heart

Valves ensure unidirectional blood flow and prevent backflow:

• 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.

Chordae tendineae and papillary muscles anchor the AV valves and prevent inversion during contraction.

Surface Features of the Heart

• Auricles: Muscular pouches attached to the atria, increasing their capacity.

• Sulci: Fat-filled grooves on the heart surface, marking boundaries between chambers and containing coronary vessels.

Coronary Circulation

The heart muscle (myocardium) requires its own blood supply:

• Coronary Arteries: Originate from the base of the aorta; include left and right coronary arteries, circumflex, and interventricular branches.

• Coronary Veins: Collect deoxygenated blood from the myocardium and drain into the coronary sinus.

Cardiac Muscle & Electrical Activity

Types of Myocardial Cells

Cardiac muscle cells are specialized for continuous, rhythmic contraction and electrical conduction:

• Contractile Cells: Most numerous; responsible for the force of contraction.

• Conducting (Autorhythmic) Cells: Initiate and propagate action potentials; include cells in the SA node, AV node, bundle of His, bundle branches, and Purkinje fibers.

Additional info: Cardiac muscle has a long refractory period, preventing tetanus and ensuring rhythmic contractions.

Cardiac Conduction System

The heart's electrical system coordinates contraction:

1. Sinoatrial (SA) Node: Located in the right atrium; acts as the pacemaker, generating impulses 60–75 times per minute.

2. Atrioventricular (AV) Node: Located in the interatrial septum; delays the impulse to allow atria to contract before ventricles.

3. AV Bundle (Bundle of His): Conducts impulses from AV node to bundle branches.

4. Bundle Branches: Carry impulses down the interventricular septum.

5. Purkinje Fibers: Distribute impulses throughout ventricular myocardium, causing coordinated contraction.

Action Potentials in Cardiac Cells

• Autorhythmic cells: Have unstable resting membrane potentials due to Na+ leakage, leading to spontaneous depolarization.

• Contractile cells: Require stimulation from conducting cells; exhibit rapid depolarization, plateau (due to slow Ca2+ channels), and repolarization.

Key Equations:

• Stroke Volume (SV): $SV = EDV - ESV$

• Cardiac Output (CO): $CO = HR \times SV$

Electrocardiogram (ECG/EKG)

An ECG records the electrical activity of the heart, reflecting the sequence of depolarization and repolarization:

• P wave: Atrial depolarization

• QRS complex: Ventricular depolarization (and atrial repolarization)

• T wave: Ventricular repolarization

Cardiac Cycle

Phases of the Cardiac Cycle

The cardiac cycle describes the sequence of events in one heartbeat:

1. Atrial Systole: Atria contract, filling ventricles.

2. Ventricular Systole: Ventricles contract, ejecting blood into arteries.

3. Diastole: Chambers relax and fill with blood.

Blood flows from areas of high pressure to low pressure, and pressure changes drive valve opening and closing.

Heart Sounds

• S1 ("Lub"): Closing of AV valves at the start of ventricular systole.

• S2 ("Dub"): Closing of semilunar valves at the start of ventricular diastole.

Cardiac Physiology

Cardiac Output and Regulation

Cardiac output is the volume of blood pumped by each ventricle per minute. It is determined by heart rate and stroke volume:

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

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

• Cardiac Output (CO): $CO = HR \times SV$

Factors affecting HR and SV include neural, hormonal, and environmental influences.

Factors Affecting Heart Rate and Stroke Volume

Factor	Effect on Heart Rate	Effect on Stroke Volume
Sympathetic stimulation	Increases	Increases (contractility)
Parasympathetic stimulation	Decreases	Decreases
Hormones (epinephrine, thyroid)	Increases	Increases
Preload (EDV)	—	Increases with greater stretch
Afterload (arterial pressure)	—	Decreases with higher resistance
Contractility	—	Increases with stronger contraction

Additional info: The Frank-Starling Law states that increased ventricular filling (preload) leads to increased force of contraction and stroke volume.

Homeostatic Responses to Cardiac Output Changes

Change	Baroreceptor Response	Chemoreceptor Response	Result
CO Decreases	Decreased stretch, increased HR	Decreased O2, increased CO2	Increased HR & SV, restored homeostasis
CO Increases	Increased stretch, decreased HR	Increased O2, decreased CO2	Decreased HR & SV, restored homeostasis

Key Terms and Definitions

• End Diastolic Volume (EDV): Volume of blood in ventricle at end of filling (diastole).

• End Systolic Volume (ESV): Volume of blood remaining after contraction (systole).

• Stroke Volume (SV): $SV = EDV - ESV$

• Cardiac Output (CO): $CO = HR \times SV$

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

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

• Contractility: Strength of contraction at a given preload.

Example Application

If a patient has an EDV of 120 mL and an ESV of 50 mL, their stroke volume is:

$SV = 120\,\text{mL} - 50\,\text{mL} = 70\,\text{mL}$

If their heart rate is 75 beats per minute:

$CO = 75 \times 70 = 5250\,\text{mL/min}$