The Cardiovascular System: Heart, Blood Vessels, and Hemodynamics

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The Cardiovascular System: Heart, Blood Vessels, and Hemodynamics

Overview

The cardiovascular system is responsible for transporting blood throughout the body, delivering oxygen and nutrients, and removing waste products. It consists of the heart, blood vessels, and the mechanisms that regulate blood flow (hemodynamics).

Heart Anatomy and Blood Flow

The Travels of a Drop of Blood

Blood enters the heart through the superior and inferior vena cava into the right atrium.
Passes through the tricuspid valve (right atrioventricular valve) into the right ventricle.
Exits via the pulmonary semilunar valve into the pulmonary trunk and pulmonary arteries.
Travels to the lungs (unloads CO2, loads O2).
Returns via pulmonary veins to the left atrium.
Passes through the mitral (bicuspid) valve (left atrioventricular valve) into the left ventricle.
Exits via the aortic semilunar valve into the aorta.
Blood is distributed through arteries, arterioles, and capillaries (where O2 is unloaded and CO2 is loaded).
Returns via venules and veins to the superior/inferior vena cava.

Structure of Cardiac Muscle

Cardiac Muscle Features

Cardiac muscle is striated (contains sarcomeres with A and I bands).
Cells have a single, centrally located nucleus and are branched.
Cells are connected by intercalated discs.

Intercalated Discs

Interdigitating folds: Increase surface area for cell-to-cell contact.
Mechanical junctions: Desmosomes and fascia adherens tightly join cells, providing structural integrity.
Electrical junctions: Gap junctions allow ions to flow between cells, enabling rapid conduction of action potentials so the heart contracts as a unit.

Metabolism of Cardiac Muscle

Relies on aerobic respiration for ATP production.
Contains larger mitochondria than skeletal muscle for increased ATP synthesis.
Acetylcholine (parasympathetic neurotransmitter) inhibits cardiac muscle, while epinephrine and norepinephrine (sympathetic neurotransmitters) excite both cardiac and skeletal muscle.
Chronotropic drugs alter heart rate: positive chronotropes increase HR, negative chronotropes decrease HR.

Position and Coverings of the Heart

Mediastinum

The heart is located in the mediastinum, the central compartment of the thoracic cavity, containing all thoracic organs except the lungs.

Pericardium

Fibrous pericardium: Outer layer, dense irregular connective tissue, anchors and protects the heart.
Serous pericardium: Inner layer, consists of parietal and visceral (epicardium) layers, produces serous fluid to reduce friction.
Epicardium: Visceral layer of serous pericardium, attached to the heart surface.
Myocardium: Thick, muscular middle layer responsible for contraction.
Endocardium: Smooth inner lining of chambers and valves, continuous with blood vessel endothelium.

Heart Problems

Hypertension: High blood pressure; can lead to heart disease, myocardial hypertrophy, and vascular damage.
Pericarditis: Inflammation of the pericardium, often due to infection or lack of serous fluid, causing pain and impaired heart function.
Angina pectoris: Chest pain from reduced blood flow to the heart muscle (ischemia); severe cases may lead to myocardial infarction (heart attack).

Heart Chambers and Valves

Chambers

Right and Left Atria: Receive blood returning to the heart.
Right and Left Ventricles: Pump blood out of the heart (right to lungs, left to body).
Auricles: Ear-like extensions that increase atrial volume.
Pectinate muscle: Found in auricles, increases contraction force.
Trabeculae carneae: Ridges of muscle in ventricles.
Papillary muscles: Anchor chordae tendineae to AV valves.

Septa

Interatrial septum: Wall between right and left atria.
Interventricular septum: Wall between right and left ventricles.
Fossa ovalis: Remnant of fetal foramen ovale in interatrial septum.

Valves

Atrioventricular (AV) valves: Between atria and ventricles; right AV (tricuspid, 3 cusps), left AV (bicuspid/mitral, 2 cusps). Chordae tendineae attach to these valves.
Semilunar valves: Between ventricles and arteries; pulmonary semilunar (right ventricle to pulmonary trunk), aortic semilunar (left ventricle to aorta).

Circuits of the Heart

Systemic circuit: Left heart pumps oxygenated blood to the body; left ventricle wall is thicker due to higher workload.
Pulmonary circuit: Right heart pumps deoxygenated blood to the lungs for gas exchange.
Valves: Ensure unidirectional blood flow.

Cardiac Conduction System

Cardiac muscle is myogenic: contracts without neural input due to pacemaker cells.
Sinoatrial (SA) node: Pacemaker, initiates heartbeat (~100 bpm), sets sinus rhythm (70-80 bpm due to vagus nerve inhibition).
Atrioventricular (AV) node: Receives signal from SA node, delays and relays to ventricles (~50 bpm if acting as pacemaker).
Bundle of His (AV bundle): Pathway from AV node to bundle branches and Purkinje fibers, which stimulate ventricular contraction.
Heart block: Damage to AV node, can cause bradycardia and other symptoms.

Electrical and Contractile Activity of the Heart

Systole: Contraction phase (top number in BP).
Diastole: Relaxation phase (bottom number in BP).
Ectopic focus: Any region other than SA node initiating heartbeat.
Nodal rhythm: AV node sets slower rhythm (40-50 bpm); endurance training can cause healthy bradycardia due to myocardial hypertrophy.

Action Potential

Depolarization: Na+ enters cell, membrane potential rises.
Repolarization: K+ exits cell, membrane potential returns to resting.
Absolute refractory period: No new action potential can be initiated.
Relative refractory period: Exceptionally strong stimulus can trigger another action potential.

Physiology of the SA Node

SA node cells have a resting membrane potential of about -60 mV, which drifts toward threshold (-40 mV) due to Na+ leakage (pacemaker potential).
At threshold, fast Ca2+ channels open, causing depolarization to about +10 mV.
K+ channels open, repolarizing the cell, and the cycle repeats.

Action Potential of Ventricle Contractile Fibers

Depolarization: RMP is -90 mV; voltage-gated fast Na+ channels open, Na+ enters, rapid voltage increase.
Plateau: Slow Ca2+ channels open, Ca2+ enters, K+ channels closed, maintained depolarization.
Repolarization: K+ channels open, K+ exits, membrane potential returns to resting.

Refractory period: Time during which a second contraction cannot be triggered.

Electrocardiogram (ECG)

P wave: Atrial depolarization.
QRS complex: Ventricular depolarization.
T wave: Ventricular repolarization.

The Cardiac Cycle

The cardiac cycle is the sequence of events in the heart from the beginning of one heartbeat to the next.

Atrial Systole/Ventricular Diastole

SA node depolarizes, causing atrial depolarization (P wave).
Atrial systole (contraction) forces blood into ventricles (gravity also contributes).
Continued atrial contraction increases ventricular filling; pressure equalizes between atria and ventricles.
QRS complex marks onset of ventricular depolarization.

Ventricular Systole/Atrial Diastole

Isovolumetric contraction: Ventricles contract, AV valves close, pressure rises but volume unchanged.
Ventricular ejection: Ventricular pressure exceeds aortic/pulmonary trunk pressure, semilunar valves open, blood ejected.

*Additional info: The cardiac cycle also includes isovolumetric relaxation and ventricular filling, completing the cycle of blood flow through the heart.*