Cardiovascular System Overview

Study Notes

The cardiovascular system includes the heart and blood vessels.
The heart is a muscular organ that pumps blood throughout the circulatory system.
It is located in the mediastinum, between the lungs.
It has four chambers: two atria and two ventricles.
The ventricles have thicker musculature than the atria.
The force of heart contraction depends on the muscle.

Arteries are low-resistance tubes that carry blood to tissues under high pressure, acting as pressure reservoirs during ventricular relaxation.
Arterioles are major sites of resistance to blood flow.
Capillaries are sites for nutrient, gas, metabolic end product, and fluid exchange between blood and tissues.
Veins are low-resistance conduits that return blood to the heart; their capacity adjusts to facilitate blood flow.

Although the heart is a single organ, its right and left sides are separate pumps.
The heart has four chambers.
Two upper chambers are atria and two lower chambers are ventricles.
Atria receive blood returning to the heart, transferring it to ventricles that pump blood away from the heart.
Veins return blood to the atria, while arteries carry blood away from the ventricles.
A septum separates the heart's two halves, preventing blood mixing.
The right side receives and pumps oxygen-poor blood, while the left side receives and pumps oxygen-rich blood.

The interventricular septum separates the two ventricles.
Atrioventricular (AV) valves are one-way valves between the atria and ventricles to prevent backflow.
The right AV valve is the tricuspid valve (three flaps) and the left AV valve is the bicuspid (mitral) valve (two flaps).

The heart wall comprises three layers:
- Epicardium: thin, outer layer formed by the visceral pericardium
- Myocardium: middle layer, mainly cardiac muscle, responsible for heart pumping.
- Endocardium: thin, inner layer (endothelium), lining the inside surface of the heart, continuing as the endothelium of blood vessels.

The myocardium forms the bulk of the heart wall.
It is composed of cardiac muscle fibers (myocytes).
Myocardium has three types of muscle fibers:
- Contractile: responsible for the heart's pumping action
- Pacemaker: responsible for initiating heartbeats.
- Conductive: distribute electrical signals of the heart beat throughout.

Rhythmicity: ability to produce its own impulses regularly (autorhythmicity). The sinoatrial node (SA node) is the heart's pacemaker.
Excitability: ability to respond to a stimulus, initiating an electrical action potential followed by contraction.
Conductivity: ability to transmit electrical impulses throughout the heart through a specialized system.
Contractility: ability to contract after receiving a stimulus.

The conductive (junctional) tissues in the heart are modified cardiac muscle fibers.
They rapidly conduct electrical impulses from the SA node to the ventricles.
The main components of the heart's conductive system include:
- SA node
- Intermodal pathways
- AV node
- Bundle of His
- Right & left bundle branches
- Purkinje fibers

Pulmonary Circulation: (lesser circulation)
- Blood is pumped from the right ventricle to the lungs via the pulmonary artery.
- Gas exchange (oxygenation) occurs in the lungs.
- Oxygenated blood returns to the left atrium via pulmonary veins.
Systemic Circulation: (greater circulation)
- Blood is pumped from the left ventricle to the body via the aorta.
- Oxygenated blood is delivered to the body via arteries.
- Deoxygenated blood returns to the right atrium via veins.

The cardiac cycle is the sequence of events during one heartbeat (about 0.8 seconds).
It consists of two major phases:
- Systole: heart contraction and blood ejection.
- Diastole: heart relaxation and filling with blood.
Stroke volume (SV): amount of blood pumped per beat (typically 70 mL).
Cardiac output (CO): blood pumped per minute, calculated as CO = SV × Heart rate.