The Cardiovascular System: Structure, Function, and Regulation

The Cardiovascular System

Overview and Functions

The cardiovascular system is a closed system composed of the heart and blood vessels. Its primary function is to deliver oxygen and nutrients to cells and tissues while removing carbon dioxide and other waste products. The heart acts as a pump, and the blood vessels serve as conduits for blood flow throughout the body.

• Delivers oxygen and nutrients to cells and tissues

• Removes carbon dioxide and metabolic wastes

Structure of the Heart

Location and Orientation

The heart is located in the thorax, between the lungs in the inferior mediastinum. It is about the size of a human fist (approximately 14 cm x 9 cm). The pointed apex is directed toward the left hip, while the base points toward the right shoulder.

Coverings and Walls of the Heart

The heart is enclosed by the pericardium, a double-walled sac that protects and anchors the heart. The pericardium consists of:

• Fibrous pericardium: Outer, tough, and loose layer

• Serous pericardium: Inner, double-layered membrane (parietal and visceral layers)

• Pericardial cavity: Space between layers, filled with serous fluid to reduce friction

The heart wall has three layers:

• Epicardium (visceral pericardium): Outer connective tissue layer

• Myocardium: Middle layer, composed of cardiac muscle

• Endocardium: Inner endothelial lining

Chambers and Associated Great Vessels

Heart Chambers

The heart has four chambers:

• Atria (right and left): Receiving chambers

• Ventricles (right and left): Discharging chambers

The interventricular septum separates the two ventricles, and the interatrial septum separates the two atria.

Pulmonary and Systemic Circulation

The heart functions as a double pump:

• Pulmonary circulation: Right side pumps oxygen-poor blood to the lungs and returns oxygen-rich blood to the left side.

• Systemic circulation: Left side pumps oxygen-rich blood to the body and returns oxygen-poor blood to the right side.

Heart Valves

Types and Functions

Heart valves ensure unidirectional blood flow and prevent backflow. There are four main valves:

• Atrioventricular (AV) valves: Between atria and ventricles

  • Tricuspid valve: Right side

  • Bicuspid (mitral) valve: Left side

• Semilunar valves: Between ventricles and arteries

  • Pulmonary semilunar valve

  • Aortic semilunar valve

Valve Operation

AV valves are anchored by chordae tendineae and open during relaxation, closing during ventricular contraction. Semilunar valves open during ventricular contraction and close during relaxation, responding to pressure changes.

Blood Flow Through the Heart

Pathway of Blood

1. Blood enters the right atrium via the superior and inferior venae cavae.

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

3. Exits through the pulmonary semilunar valve into the pulmonary trunk and arteries to the lungs.

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

5. Passes through the bicuspid valve to the left ventricle.

6. Exits through the aortic semilunar valve into the aorta and systemic circulation.

Intrinsic Conduction System

Setting the Basic Rhythm

The heart's intrinsic conduction system (nodal system) coordinates the heartbeat. Key components include:

• Sinoatrial (SA) node: Pacemaker, initiates heartbeat

• Atrioventricular (AV) node: Junction of atria and ventricles

• AV bundle (bundle of His): In interventricular septum

• Bundle branches: In interventricular septum

• Purkinje fibers: Spread within ventricle walls

Heart Contractions

The SA node initiates the impulse, which spreads to the AV node, then through the AV bundle, bundle branches, and Purkinje fibers, resulting in coordinated contraction of the atria and ventricles.

• Tachycardia: Rapid heart rate (>100 bpm)

• Bradycardia: Slow heart rate (<60 bpm)

• Fibrillation: Uncoordinated, rapid contractions

Cardiac Cycle and Heart Sounds

Phases of the Cardiac Cycle

The cardiac cycle consists of one complete heartbeat, including systole (contraction) and diastole (relaxation). The cycle lasts about 0.8 seconds at a normal heart rate of 75 bpm.

• Mid-to-late diastole: Ventricles fill passively, AV valves open

• Ventricular systole: Ventricles contract, AV valves close ("lub" sound), semilunar valves open

• Early diastole: Semilunar valves close ("dup" sound), ventricles relax

Cardiac Output and Regulation

Cardiac Output (CO)

Cardiac output is the amount of blood pumped by each ventricle in one minute. It is calculated as:

$CO = HR \times SV$

• HR: Heart rate (beats per minute)

• SV: Stroke volume (ml/beat)

Example: If HR = 75 bpm and SV = 70 ml/beat, then $CO = 75 \times 70 = 5250$ ml/min.

Regulation of Heart Rate

• Neural controls: Sympathetic (increases HR), parasympathetic (decreases HR)

• Hormones and ions: Epinephrine, thyroxine, calcium, potassium

• Physical factors: Age, gender, exercise, body temperature

Blood Vessels and Circulation

Types of Blood Vessels

• Arteries: Carry blood away from the heart

• Arterioles: Small branches of arteries

• Capillaries: Sites of exchange between blood and tissues

• Venules: Collect blood from capillaries

• Veins: Return blood to the heart

Microscopic Anatomy of Blood Vessels

Blood vessels (except capillaries) have three layers:

• Tunica intima: Endothelial lining

• Tunica media: Smooth muscle and elastic tissue

• Tunica externa: Fibrous connective tissue

Structural Differences

• Arteries: Thicker tunica media, higher pressure

• Veins: Thinner walls, larger lumen, valves to prevent backflow

• Capillaries: One cell layer thick for exchange

Capillary Beds

Capillary beds consist of vascular shunts and true capillaries. Precapillary sphincters regulate blood flow through the beds, allowing exchange with tissues or bypassing the area as needed.

Blood Pressure

Definition and Measurement

Blood pressure is the force exerted by blood against vessel walls. It is measured in large arteries and expressed as systolic over diastolic pressure (e.g., 120/80 mm Hg).

• Systolic pressure: Peak during ventricular contraction

• Diastolic pressure: Lowest during ventricular relaxation

Blood Pressure Gradient

Blood flows along a pressure gradient, highest in arteries and lowest in veins. This gradient is essential for continuous blood flow.

Factors Affecting Blood Pressure

Blood pressure is determined by cardiac output and peripheral resistance:

$BP = CO \times PR$

• Neural factors: Sympathetic nervous system (vasoconstriction increases BP)

• Renal factors: Kidneys regulate blood volume and release renin

• Temperature: Heat causes vasodilation, cold causes vasoconstriction

• Chemicals: Epinephrine increases BP

• Diet: Low salt, fat, and cholesterol can prevent hypertension

Blood Pressure Variations

• Normal range: 140–110 mm Hg systolic, 80–70 mm Hg diastolic

• Hypotension: Systolic below 100 mm Hg

• Hypertension: Sustained pressure of 140/90 mm Hg or higher

Developmental Aspects and Homeostatic Imbalances

Development

• Heart develops as a simple tube and becomes four-chambered by week 7 of embryonic development

• Congenital heart defects are a major cause of infant mortality from congenital problems

Age-Related Changes

• Weakening of venous valves

• Varicose veins

• Progressive arteriosclerosis

• Hypertension due to loss of vessel elasticity

• Coronary artery disease from fatty, calcified deposits

Healthy lifestyle choices, such as a balanced diet, regular exercise, and avoiding smoking, can help maintain cardiovascular health and reduce the risk of disease.