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The Cardiovascular System: Structure, Function, and Circulation

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The Cardiovascular System

Overview and Functions

The cardiovascular system is responsible for transporting oxygen, nutrients, cell wastes, and hormones to and from the cells of the body. It consists of the heart, blood vessels, and blood, working together to maintain homeostasis and support cellular function.

  • Heart: Pumps blood throughout the body.

  • Blood Vessels: Allow blood to circulate to all parts of the body.

  • Main Functions: Transport of oxygen, nutrients, waste products, and hormones.

Heart Anatomy

Location and Orientation

  • Apex: The pointed region of the heart, directed towards the left hip and resting on the diaphragm.

  • Base: The top of the heart, where major blood vessels enter and exit.

Heart Wall Layers

  • Epicardium: The outermost layer, also known as the visceral pericardium.

  • Myocardium: The middle layer, composed mostly of cardiac muscle; responsible for contraction.

  • Endocardium: The innermost layer, also called the endothelium.

Chambers of the Heart

  • Atria (Right & Left): Superior receiving chambers.

  • Ventricles (Right & Left): Inferior chambers that propel blood into circulation during contraction.

  • Interatrial Septum: Separates the two atria longitudinally.

  • Interventricular Septum: Separates the two ventricles longitudinally.

Heart Valves

  • Tricuspid Valve: Between right atrium and right ventricle.

  • Mitral (Bicuspid) Valve: Between left atrium and left ventricle.

  • Pulmonary Semilunar Valve: Between right ventricle and pulmonary artery.

  • Aortic Semilunar Valve: Between left ventricle and aorta.

  • Chordae Tendineae: Tendinous cords anchoring valve cusps to ventricular walls.

Blood Vessels and Circulation

Types of Blood Vessels

  • Arteries: Carry blood away from the heart; generally oxygenated except for pulmonary arteries.

  • Veins: Carry blood toward the heart; generally deoxygenated except for pulmonary veins.

  • Capillaries: Microscopic vessels where exchange of gases (O2 and CO2), nutrients, and wastes occurs.

Major Blood Vessels

  • Aorta: Largest artery in the body; carries oxygenated blood from the left ventricle to the body.

  • Ascending Aorta: Leaves the left ventricle.

  • Aortic Arch: Curves to the left.

  • Thoracic Aorta: Travels downward through the thorax.

  • Abdominal Aorta: Passes through the diaphragm into the abdominopelvic cavity.

  • Superior Vena Cava (SVC): Returns blood from tissues above the diaphragm (head and arms) to the right atrium.

  • Inferior Vena Cava (IVC): Returns blood from tissues below the diaphragm (lower body) to the right atrium.

Circulatory Pathways

  • Pulmonary Circulation: Right ventricle → pulmonary artery (deoxygenated blood) → lungs → pulmonary vein (oxygenated blood) → left atrium.

  • Systemic Circulation: Left ventricle → aorta → body tissues (via systemic arteries) → systemic veins → right atrium.

Exceptions: Pulmonary artery carries deoxygenated blood; pulmonary vein carries oxygenated blood.

Summary Table: Blood Flow Through the Heart

Step

Structure

Blood Oxygenation

1

IVC & SVC

Deoxygenated

2

Right Atrium

Deoxygenated

3

Tricuspid Valve

Deoxygenated

4

Right Ventricle

Deoxygenated

5

Pulmonary Semilunar Valve

Deoxygenated

6

Pulmonary Trunk/Artery

Deoxygenated

7

Lungs

Oxygenation occurs

8

Pulmonary Vein

Oxygenated

9

Left Atrium

Oxygenated

10

Mitral Valve

Oxygenated

11

Left Ventricle

Oxygenated

12

Aortic Semilunar Valve

Oxygenated

13

Aorta

Oxygenated

14

Body (Systemic Capillaries)

Oxygen delivered, CO2 picked up

Cardiac Cycle and Heart Function

Phases of the Cardiac Cycle

  • Systole: Contraction phase of the heart.

  • Diastole: Relaxation phase of the heart.

  • Atrial Systole: Atria contract, ventricles remain in diastole; blood is forced into ventricles.

  • Atrial Diastole: Heart is relaxed, pressure is low, AV valves open, blood flows passively into atria and ventricles, semilunar valves closed.

  • Isovolumetric Contraction: Ventricular systole begins, intraventricular pressure rises, AV valves close, ventricles are closed chambers for a moment.

  • Ventricular Systole: Semilunar valves open, blood is ejected from ventricles.

Heart Sounds and Valves

  • Semilunar Valves: Closed during heart relaxation, open during ventricular contraction.

  • AV Valves: Open during heart relaxation, closed during ventricular contraction.

Cardiac Output and Heart Rate

  • Cardiac Output (CO): Amount of blood pumped by each side of the heart in one minute.

  • Stroke Volume (SV): Volume of blood pumped by each ventricle per contraction (about 70 mL per beat).

  • Heart Rate (HR): Number of heartbeats per minute (typically 75 bpm).

Formula:

Intrinsic Conduction System

  • Sinoatrial (SA) Node: Located in the right atrium; acts as the heart's pacemaker.

  • Atrioventricular (AV) Node: At the junction of atria and ventricles; delays impulse.

  • AV Bundle (Bundle of His) and Bundle Branches: Located in the interventricular septum; conduct impulses to ventricles.

  • Purkinje Fibers: Distribute the impulse throughout the ventricles.

Sequence: SA node → atria contract → AV node (delay) → AV bundle → bundle branches → Purkinje fibers → ventricles contract.

  • Atrial cells: Beat about 60 times per minute.

  • Ventricular cells: Beat about 20-40 times per minute.

Heart Rate Abnormalities

  • Tachycardia: Rapid heart rate.

  • Bradycardia: Slow heart rate.

Blood Pressure

Definitions

  • Systolic Pressure: Pressure in the arteries at the peak of ventricular contraction.

  • Diastolic Pressure: Pressure in the arteries when the ventricles relax.

Comparing Arteries and Veins

Feature

Arteries

Veins

Direction of Blood Flow

Away from heart

Toward heart

Oxygenation

Oxygenated (except pulmonary arteries)

Deoxygenated (except pulmonary veins)

Wall Structure

Heavier, stronger, stretchier tunica

Thinner tunica media

Valves

None

Present to prevent backflow

Additional info:

  • The left ventricle has thicker walls than the right ventricle because it must pump blood throughout the entire body (systemic circuit), whereas the right ventricle only pumps to the lungs (pulmonary circuit).

  • Capillary beds are the sites of exchange between blood and tissues, allowing for the delivery of oxygen and nutrients and the removal of waste products.

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