The Cardiovascular System: Structure, Function, and Disorders

Version 2

Introduction to the Cardiovascular System

Overview and Homeostasis

The cardiovascular system, also known as the circulatory system, is essential for maintaining homeostasis by transporting blood throughout the body. It consists of the heart (the pump), blood vessels (the distribution network), and is regulated by the nervous system.

• Heart: Pumps blood to all body tissues.

• Blood vessels: Arteries, capillaries, and veins form a branching network for blood flow.

• Nervous system: Controls heart rate and blood distribution.

Blood Vessels: Structure and Function

Types of Blood Vessels

• Arteries: Carry blood away from the heart under high pressure; thick-walled.

• Capillaries: Microscopic vessels where exchange of solutes and water with body cells occurs; walls are one cell thick.

• Veins: Return blood to the heart; thin-walled and serve as blood reservoirs.

Structure of Arteries

• Three layers:

  • Endothelium: Innermost layer of squamous epithelial cells.

  • Smooth muscle with elastic fibers: Middle layer for flexibility and strength.

  • Connective tissue: Outer layer for support.

• Aneurysm: A defect in the arterial wall causing ballooning; may be symptomless until rupture, which can be fatal.

Arterioles and Precapillary Sphincters

• Arterioles: Smallest arteries, regulate blood flow into capillaries.

• Precapillary sphincters: Control blood entry into capillaries.

• Vasodilation: Relaxation increases blood flow.

• Vasoconstriction: Contraction decreases blood flow.

Capillaries: Exchange Sites

• Form extensive networks called capillary beds.

• Selective exchange of substances with interstitial fluid.

• Fluid movement:

  • At the start: Filtration of oxygen and nutrients into tissues (driven by blood pressure).

  • At the end: Reabsorption of fluid (driven by plasma proteins).

Lymphatic System

• Maintains blood volume by returning excess interstitial fluid to the circulatory system.

• Transports large substances (e.g., lipid droplets, microorganisms).

• Major role in immune defense.

• Consists of blind-ended capillaries and lymphatic vessels; lymph is derived from interstitial fluid.

Veins: Structure and Function

• Three layers, but thinner walls and larger lumen than arteries.

• High distensibility (can stretch easily).

• Serve as blood reservoirs.

• Blood return mechanisms:

  • Contraction of skeletal muscles.

  • One-way valves prevent backflow.

  • Pressure changes during breathing assist return.

The Heart: Structure and Function

Heart Anatomy

• Composed mainly of cardiac muscle tissue.

• Pericardium: Fibrous sac surrounding the heart for protection and anchoring.

• Wall layers:

  • Epicardium: Outermost layer.

  • Myocardium: Thick muscle layer; contracts to pump blood.

  • Endocardium: Innermost layer; continuous with blood vessel lining.

Heart Chambers and Valves

• Four chambers:

  • Two atria (upper chambers).

  • Two ventricles (lower chambers).

• Septum: Muscular partition separating right and left sides.

• Four valves prevent backflow:

  • Two atrioventricular (AV) valves: Tricuspid (right), Bicuspid/Mitral (left).

  • Two semilunar valves: Pulmonary and Aortic.

Pattern of Blood Flow

• Two circuits:

  • Pulmonary circuit: Heart → Lungs → Heart (oxygenates blood).

  • Systemic circuit: Heart → Body → Heart (delivers oxygen, removes waste).

• Deoxygenated and oxygenated blood do not mix.

Pulmonary Circuit Steps

1. Deoxygenated blood enters right atrium via vena cava.

2. Passes through right AV valve to right ventricle.

3. Pumped through pulmonary semilunar valve into pulmonary arteries to lungs.

4. Blood is oxygenated in pulmonary capillaries.

5. Returns via pulmonary veins to left atrium.

Systemic Circuit Steps

1. Oxygenated blood flows from left atrium through left AV valve to left ventricle.

2. Pumped through aortic semilunar valve into aorta.

3. Distributed via arteries and arterioles to tissues.

4. Exchange of gases and nutrients in capillaries.

5. Returns via veins to right atrium.

Coronary Circulation

• Coronary arteries: Supply myocardium; can be blocked by atherosclerosis.

• Coronary veins: Return blood from myocardium to right atrium.

The Cardiac Cycle

Phases of the Cardiac Cycle

• Atrial systole (0.1 s): Atria contract, AV valves open, ventricles fill.

• Ventricular systole (0.3 s): Ventricles contract, AV valves close, semilunar valves open, blood ejected.

• Diastole (0.4 s): All chambers relax, semilunar valves close.

Heart Sounds

• "Lub": Closing of AV valves during ventricular systole.

• "Dub": Closing of semilunar valves during ventricular diastole.

• Heart murmurs: Abnormal sounds due to disturbed blood flow, often from defective valves.

Electrical Activity of the Heart

Cardiac Conduction System

• Sinoatrial (SA) node: Pacemaker in upper right atrium; initiates heartbeat.

• Atrioventricular (AV) node: Relays impulse between atria and ventricles.

• AV bundle and Purkinje fibers: Conduct impulses through septum and ventricles.

Electrocardiogram (ECG/EKG)

• Records electrical activity of the heart.

• Key waves:

  • P wave: Atrial depolarization.

  • QRS complex: Ventricular depolarization.

  • T wave: Ventricular repolarization.

• Detects arrhythmias and other cardiac abnormalities.

Blood Pressure and Its Regulation

Blood Pressure Basics

• Blood exerts force on vessel walls.

• Systolic pressure: Highest pressure during ventricular contraction.

• Diastolic pressure: Lowest pressure during ventricular relaxation.

• Highest in arteries, lowest in veins.

Measuring Blood Pressure

• Measured with a sphygmomanometer (inflatable cuff over brachial artery).

• Reported in mm Hg (millimeters of mercury).

• Normal values:

  • Systolic < 120 mm Hg

  • Diastolic < 80 mm Hg

Table: Systolic and Diastolic Blood Pressure Categories

Category	Systolic (mm Hg)	Diastolic (mm Hg)
Normal	<120	<80
Elevated	120–129	<80
Hypertension Stage 1	130–139	80–89
Hypertension Stage 2	≥140	≥90
Hypertensive Crisis	>180	>120

Additional info: Table inferred from American Heart Association guidelines.

Hypertension and Hypotension

• Hypertension: Sustained high blood pressure; increases risk for cardiovascular disease, damages vessels and heart.

• Hypotension: Abnormally low blood pressure; may cause dizziness or fainting, especially after sudden position changes or blood/fluid loss.

Regulation of the Cardiovascular System

• Homeostasis centers on maintaining constant arterial pressure.

• Regulated by cardiac output and arteriole diameter.

• Local blood flows adjust to tissue needs.

Baroreceptor Reflex

• Baroreceptors: Pressure sensors in aorta and carotid arteries.

• Negative feedback loop maintains blood pressure within narrow limits.

• Increased pressure stretches vessels, activating baroreceptors, which signal the brain to decrease heart rate and dilate vessels, lowering pressure.

• Opposite response if pressure drops.

Hormonal Regulation

• Epinephrine and norepinephrine: Increase heart rate and contraction force.

• Antidiuretic hormone (ADH), aldosterone, renin-angiotensin system, atrial natriuretic hormone (ANH): Influence blood pressure by affecting blood volume and vessel tone.

Local Control of Blood Flow

• Precapillary sphincters adjust blood flow to tissues based on metabolic activity.

• During low blood pressure, vasoconstriction shunts blood to vital organs (brain, heart).

Exercise and Cardiac Output

• Exercise increases blood flow to active muscles and cardiac output.

• Nonathletes: Up to 20–25 L/min; trained athletes: Up to 35 L/min (7x resting output).

Cardiovascular Disorders

Major Disorders

• Hypertension

• Aneurysms

• Atherosclerosis

• Angina

• Heart attack

• Heart failure

• Embolism

• Stroke

Atherosclerosis

• Cholesterol deposits build up in arteries, narrowing them.

• Increases risk of heart attack, stroke, aneurysm, peripheral vascular disease.

• Risk factors: High cholesterol/triglycerides, obesity, inactivity, high-fat diet, smoking, diabetes, hypertension, family history.

• Men under 45 have higher risk than women.

Angina

• Chest pain due to reduced blood flow in coronary arteries.

• Symptoms: Chest tightness, shortness of breath, choking sensation.

• Diagnosis: Angiography.

• Treatment: Medication, balloon angioplasty, coronary artery bypass graft (CABG).

Heart Attack (Myocardial Infarction)

• Blood flow to myocardium is blocked, causing tissue death.

• Symptoms: Intense chest pain, radiating pain, nausea.

• Diagnosis: ECG, blood enzymes.

• Treatment: Arrhythmia control, clot-dissolving drugs, CABG.

Heart Failure

• Heart becomes less efficient; fluid backs up in tissues (congestive heart failure).

• Symptoms: Shortness of breath, swelling.

• Causes: Age, prior heart attacks, valve disease, lung disease, hypertension.

• Treatment: Diuretics, drugs to improve heart function.

Embolism

• Sudden blockage of a vessel by material (clot, fat, air, etc.).

• Types:

  • Pulmonary embolism: Affects lungs.

  • Cerebral embolism: May cause stroke.

  • Cardiac embolism: May cause heart attack.

Stroke

• Brain damage from interrupted blood supply (embolism or vessel rupture).

• Symptoms depend on affected brain area.

• Treatment: Clot-dissolving drugs (embolism), surgery (rupture).

• Recovery may require rehabilitation.

Heart Replacement

• Heart transplants: Average survival 15 years; limited by donor availability.

• Alternatives: Artificial hearts, xenotransplants (animal hearts).

Prevention of Cardiovascular Disease

• Avoid smoking (doubles risk).

• Control cholesterol levels.

• Exercise regularly.

• Monitor and treat hypertension.

• Maintain healthy weight.

• Control diabetes.

• Manage stress.

Key Equations

• Blood Pressure: $\text{Blood Pressure} = \text{Cardiac Output} \times \text{Peripheral Resistance}$

• Cardiac Output (CO): $\text{CO} = \text{Heart Rate} \times \text{Stroke Volume}$

Additional info: Equations provided for academic completeness.