The Cardiovascular System: Structure, Function, and Physiology

The Cardiovascular System

Overview and Functions

The cardiovascular system is a closed system consisting of the heart and blood vessels. Its primary function is to transport essential substances throughout the body, maintaining homeostasis and supporting cellular activity.

• Transport: Delivers oxygen, nutrients, hormones, and removes cell wastes.

• Circulation: Blood vessels allow blood to reach all body tissues.

Anatomy of the Heart

Location and Orientation

The heart is approximately the size of a human fist and weighs less than one pound. It is located in the thoracic cavity, between the lungs in the inferior mediastinum, resting on the diaphragm.

• Apex: Directed toward the left hip and rests on the diaphragm.

• Base: Points toward the right shoulder.

Coverings of the Heart

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

• Fibrous pericardium: Loose and superficial outer layer.

• Serous pericardium: Deep to the fibrous layer, composed of two layers:

  • Parietal pericardium: Lines the inner surface of the fibrous pericardium.

  • Visceral pericardium (epicardium): Next to the heart.

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

Wall Structure of the Heart

• Epicardium: Outer layer; same as visceral pericardium.

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

• Endocardium: Inner layer; consists of endothelium lining the heart chambers.

Chambers and Associated Great Vessels

Heart Chambers

• Atria (right and left):

  • Receiving chambers

  • Assist with filling the ventricles

  • Blood enters under low pressure

• Ventricles (right and left):

  • Discharging chambers

  • Thick-walled pumps

  • Propel blood into circulation during contraction

Septum

• Interatrial septum: Separates the two atria.

• Interventricular septum: Separates the two ventricles.

Major Vessels

• Arteries: Carry blood away from the heart.

• Veins: Carry blood toward the heart.

Circulatory Pathways

Pulmonary Circulation

Blood flows from the right side of the heart to the lungs and back to the left side.

• Right ventricle pumps oxygen-poor blood to lungs via pulmonary arteries.

• Oxygen-rich blood returns to the left atrium via pulmonary veins.

Systemic Circulation

Oxygen-rich blood is pumped from the left side of the heart to the body tissues.

• Left ventricle pumps blood into the aorta.

• Blood circulates through systemic arteries to tissues.

• Oxygen-poor blood returns to the right atrium via systemic veins (superior/inferior vena cava).

Heart Valves

Types and Functions

Heart valves ensure unidirectional blood flow and prevent backflow.

• Atrioventricular (AV) valves:

  • Bicuspid (mitral) valve: between left atrium and ventricle.

  • Tricuspid valve: between right atrium and ventricle.

• Semilunar valves:

  • Pulmonary semilunar valve: between right ventricle and pulmonary artery.

  • Aortic semilunar valve: between left ventricle and aorta.

Valve Operation

• Valves open and close in response to pressure changes.

• Chordae tendineae anchor AV valve cusps to papillary muscles, preventing prolapse.

Coronary Circulation

Heart's Blood Supply

• Coronary arteries: Branch from the aorta to supply the myocardium.

• Cardiac veins: Drain deoxygenated blood from the myocardium.

• Coronary sinus: Collects blood from cardiac veins and empties into the right atrium.

Heart Conduction System

Intrinsic Conduction System

The heart's electrical system coordinates contraction and maintains rhythm.

• Sinoatrial (SA) node: Pacemaker, initiates each heartbeat.

• Atrioventricular (AV) node: Delays impulse, allowing atria to contract before ventricles.

• AV bundle (Bundle of His) and bundle branches: Conduct impulses through the interventricular septum.

• Purkinje fibers: Spread impulse through ventricular walls.

Heart Rate Regulation

• Autonomic nervous system:

  • Sympathetic: increases heart rate.

  • Parasympathetic: decreases heart rate.

• Intrinsic conduction system: Sets basic rhythm (about 75 beats/min).

Cardiac Cycle and Heart Sounds

Phases of the Cardiac Cycle

• Systole: Contraction phase.

• Diastole: Relaxation phase.

• Cycle length: ~0.8 seconds at 75 beats/min.

Events

• Atrial diastole: Heart relaxed, AV valves open, blood flows into ventricles.

• Atrial systole: Atria contract, complete ventricular filling.

• Isovolumetric contraction: Ventricles contract, AV valves close, chambers sealed.

• Ventricular systole: Semilunar valves open, blood ejected.

• Isovolumetric relaxation: Ventricles relax, semilunar valves close, AV valves reopen.

Heart Sounds

• Lub: Closing of AV valves.

• Dup: Closing of semilunar valves.

Cardiac Output and Regulation

Definitions and Equations

• Cardiac output (CO): Amount of blood pumped by each ventricle per minute.

• Stroke volume (SV): Volume of blood pumped per beat (~70 ml).

• Heart rate (HR): Beats per minute (~75 bpm).

Formula:

$CO = HR \times SV$

Example: $CO = 75 \text{ beats/min} \times 70 \text{ ml/beat} = 5250 \text{ ml/min} = 5.25 \text{ L/min}$

Factors Affecting Heart Rate

• Neural: Sympathetic increases, parasympathetic decreases rate.

• Hormonal: Epinephrine, thyroxine increase rate.

• Ions: Calcium, sodium, potassium levels affect activity.

• Physical: Age, gender, exercise, temperature.

Blood Vessels: Structure and Function

Types of Blood Vessels

• Arteries: Carry blood away from the heart.

• Capillaries: Site of exchange between blood and tissues.

• Veins: Carry blood toward the heart.

Microscopic Anatomy

• Tunica intima: Endothelium, reduces friction.

• Tunica media: Smooth muscle, elastic tissue, controls diameter.

• Tunica externa: Fibrous connective tissue, supports vessel.

Structural Differences

• Arteries: Thick tunica media, withstand high pressure.

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

• Capillaries: One cell layer thick, allow exchange.

Major Vessels of Systemic Circulation

Arteries

• Aorta: Largest artery, branches into regions (ascending, arch, thoracic, abdominal).

• Coronary arteries: Supply the heart.

• Other branches: Carotid, subclavian, intercostal, renal, gonadal, iliac, femoral, tibial arteries.

Veins

• Superior vena cava: Drains head and arms.

• Inferior vena cava: Drains lower body.

• Other veins: Jugular, subclavian, brachiocephalic, saphenous, iliac, renal, hepatic veins.

Hepatic Portal Circulation

• Veins from digestive organs drain into the hepatic portal vein.

• Blood is processed in the liver before entering systemic circulation.

Physiology of Circulation

Vital Signs

• Includes pulse, blood pressure, respiratory rate, body temperature.

• Pulse: Pressure wave from heart contraction, palpable at superficial arteries.

Blood Pressure

• Definition: Pressure exerted by blood on vessel walls.

• Gradient: Highest in arteries, lowest in veins.

• Measurement: Systolic/diastolic (e.g., 120/80 mm Hg).

Factors Affecting Blood Pressure

• Cardiac output (CO): Amount of blood pumped per minute.

• Peripheral resistance (PR): Friction encountered by blood.

Formula:

$BP = CO \times PR$

• Neural: Sympathetic increases BP via vasoconstriction.

• Renal: Kidneys regulate blood volume; renin-angiotensin-aldosterone system increases BP.

• Temperature: Heat causes vasodilation (lowers BP), cold causes vasoconstriction (raises BP).

• Chemicals: Epinephrine increases BP.

• Diet: Low salt, fat, cholesterol helps prevent hypertension.

Blood Pressure Ranges

• Normal systolic: 110–140 mm Hg

• Normal diastolic: 70–80 mm Hg

• Hypotension: Systolic below 100 mm Hg

• Hypertension: Sustained pressure above 140/90 mm Hg

Capillary Exchange

Mechanisms of Exchange

• Exchange occurs due to concentration gradients.

• Oxygen and nutrients move from blood to tissues; carbon dioxide and wastes move from tissues to blood.

• Routes:

  • Direct diffusion through membranes

  • Diffusion through intercellular clefts

  • Diffusion through pores (fenestrated capillaries)

  • Transport via vesicles

Summary Table: Heart Wall Layers

Layer	Location	Main Components
Epicardium	Outer	Serous membrane
Myocardium	Middle	Cardiac muscle
Endocardium	Inner	Endothelium

Summary Table: Types of Blood Vessels

Type	Main Function	Wall Structure
Artery	Carry blood away from heart	Thick tunica media
Vein	Carry blood toward heart	Thin walls, valves
Capillary	Exchange with tissues	Single cell layer

Summary Table: Cardiac Cycle Phases

Phase	Event
Atrial diastole	Ventricular filling
Atrial systole	Atria contract
Isovolumetric contraction	Ventricles contract, AV valves close
Ventricular systole	Blood ejected
Isovolumetric relaxation	Ventricles relax, semilunar valves close

Additional info: These notes expand upon the original slides and text, providing definitions, context, and examples for each major topic in the cardiovascular system, suitable for college-level Anatomy & Physiology study.