Study Notes: The Cardiovascular System (Chapter 11)

The Cardiovascular System

Overview and Functions

The cardiovascular system is a closed system consisting of the heart and blood vessels. It is responsible for transporting oxygen, nutrients, cell wastes, and hormones to and from cells throughout the body.

• Heart: Pumps blood throughout the body.

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

• Main Functions: Transport essential substances, remove wastes, and distribute hormones.

Anatomy of the Heart

Location and Structure

The heart is about the size of a human fist and weighs less than a pound. It is located in the thoracic cavity, between the lungs in the inferior mediastinum. The apex points toward the left hip and rests on the diaphragm, while the base points toward the right shoulder.

• Apex: Pointed region directed toward the left hip.

• Base: Area where great blood vessels emerge, directed toward the right shoulder.

Coverings of the Heart

The heart is enclosed by the pericardium, a double-walled sac. The fibrous pericardium is the outer, loose layer, while the serous membrane is deep and consists of two layers: parietal pericardium (outer) and visceral pericardium (inner, also called epicardium). Serous fluid fills the pericardial cavity between these layers.

• Epicardium: Outermost layer, also the visceral pericardium.

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

• Endocardium: Innermost layer, known as endothelium.

Chambers and Associated Great Vessels

Heart Chambers

The heart has four chambers: two atria (right and left) and two ventricles (right and left). The atria are superior receiving chambers, while the ventricles are inferior discharging chambers with thick walls for pumping blood.

• Atria: Receive blood under low pressure from veins.

• Ventricles: Pump blood into circulation.

• Interatrial Septum: Separates the atria.

• Interventricular Septum: Separates the ventricles.

Circulatory Pathways

The heart functions as a double pump:

• Pulmonary Circuit: Right side pumps blood to the lungs and back.

• Systemic Circuit: Left side pumps blood to the body tissues.

Oxygen-poor blood returns to the right atrium via systemic veins, and oxygen-rich blood returns to the left side via pulmonary veins.

Heart Valves

Types and Functions

Heart valves ensure one-way blood flow and prevent backflow. There are two main types:

• Atrioventricular (AV) Valves: Between atria and ventricles (bicuspid/mitral on left, tricuspid on right).

• Semilunar Valves: Between ventricles and arteries (pulmonary and aortic).

AV valves are anchored by chordae tendineae and operate based on pressure changes. Semilunar valves open during ventricular contraction and close during relaxation.

Cardiac Circulation

Coronary Arteries and Veins

The heart has its own blood supply via coronary arteries (branching from the aorta) and cardiac veins. The coronary sinus collects blood from cardiac veins and empties into the right atrium.

• Coronary Arteries: Supply oxygenated blood to the myocardium.

• Cardiac Veins: Drain deoxygenated blood from the myocardium.

• Coronary Sinus: Large vein on the posterior heart.

Physiology of the Heart

Intrinsic Conduction System

The heart contracts spontaneously due to the intrinsic conduction system (nodal system), which ensures regular, continuous contractions. The system includes:

• Sinoatrial (SA) Node: Pacemaker, located in the right atrium.

• Atrioventricular (AV) Node: Junction of atria and ventricles.

• AV Bundle (Bundle of His) and Bundle Branches: In interventricular septum.

• Purkinje Fibers: Spread within ventricle wall muscles.

Impulse starts at the SA node, spreads to AV node, then through the bundle and Purkinje fibers, causing coordinated contraction.

• Tachycardia: Rapid heart rate (>100 bpm).

• Bradycardia: Slow heart rate (<60 bpm).

Cardiac Cycle and Heart Sounds

The cardiac cycle is one complete heartbeat, including contraction (systole) and relaxation (diastole). Average heart rate is 75 bpm, and cycle length is 0.8 seconds.

• Lub: Closing of AV valves.

• Dup: Closing of semilunar valves.

Cardiac Output

Cardiac output (CO) is the amount of blood pumped by each ventricle per minute. It is calculated as:

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

• Heart Rate (HR): Beats per minute (typically 75 bpm).

Formula:

$CO = HR \times SV$

Regulation of Stroke Volume

Stroke volume is influenced by preload (muscle stretch), contractility, and afterload (pressure to overcome). Venous return and relaxation time affect preload.

• Starling’s Law: Greater stretch leads to stronger contraction.

• Contractility: Ability to generate tension.

• Afterload: Pressure ventricles must overcome.

Factors Modifying Heart Rate

• Neural Controls: Sympathetic speeds up, parasympathetic slows down.

• Hormones/Ions: Epinephrine, thyroxine, calcium, sodium, potassium.

• Physical Factors: Age, gender, exercise, temperature.

Blood Vessels

Types and Functions

Blood vessels form a closed system for transporting blood. Types include:

• Arteries: Carry blood away from the heart.

• Arterioles: Small branches of arteries.

• Capillaries: Exchange substances between blood and tissues.

• Venules and Veins: Return blood to the heart.

Microscopic Anatomy

Blood vessels (except capillaries) have three layers:

• Tunica Intima: Endothelium, reduces friction.

• Tunica Media: Smooth muscle and elastic tissue, controlled by sympathetic nervous system.

• Tunica Externa: Fibrous connective tissue, supports and protects.

Arteries have thicker tunica media; veins have thinner media and valves. Capillaries are one cell layer thick for exchange.

Capillary Beds

Capillary beds allow microcirculation. Precapillary sphincters regulate blood flow, and vascular shunts bypass cells when sphincters are closed.

Gross Anatomy of Blood Vessels

Major Arteries

• Aorta: Largest artery, leaves left ventricle.

• Ascending Aorta, Aortic Arch, Thoracic Aorta, Abdominal Aorta: Main regions.

• Branches: Coronary, brachiocephalic, carotid, subclavian, intercostal, celiac trunk, mesenteric, renal, gonadal, lumbar, iliac arteries.

Major Veins

• Superior/Inferior Vena Cava: Enter right atrium.

• Veins: Radial, ulnar, axillary, cephalic, basilic, subclavian, jugular, brachiocephalic, azygos, tibial, saphenous, iliac, gonadal, renal, hepatic portal, hepatic veins.

Arterial Supply of the Brain

The circle of Willis unites anterior and posterior blood supplies, ensuring continuous cerebral circulation.

Hepatic Portal Circulation

Veins from digestive organs, spleen, and pancreas empty into the hepatic portal vein, which carries blood to the liver for processing before returning to systemic circulation.

Physiology of Circulation

Vital Signs

• Pulse: Pressure wave from heart beats, measured at pressure points.

• Blood Pressure: Force exerted by blood against vessel walls.

• Respiratory Rate and Body Temperature: Additional vital signs.

Blood Pressure

Blood pressure is measured as systolic (peak contraction) over diastolic (relaxation) in mm Hg. It is highest in arteries, lower in capillaries, and lowest in veins.

Formula:

$BP = \frac{\text{Systolic}}{\text{Diastolic}}$

Factors Affecting Blood Pressure

• Cardiac Output: Amount of blood pumped per minute.

• Peripheral Resistance: Friction encountered by blood flow.

• Neural Factors: Sympathetic nervous system increases BP.

• Renal Factors: Kidneys regulate blood volume and pressure.

• Temperature: Heat causes vasodilation; cold causes vasoconstriction.

• Chemicals: Epinephrine increases BP.

• Diet: Low salt, saturated fats, and cholesterol help prevent hypertension.

Variations in Blood Pressure

• Normal Range: Systolic 110–140 mm Hg, Diastolic 70–80 mm Hg.

• Hypotension: Low BP, often associated with illness.

• Hypertension: High BP, warns of increased peripheral resistance.

Capillary Exchange

Substances move between blood and tissue cells via capillary walls, driven by concentration gradients. Oxygen and nutrients leave blood; carbon dioxide and wastes enter blood.

• Routes: Direct diffusion, intercellular clefts, pores, vesicle transport.

• Fluid Movement: Blood pressure forces fluid out; osmotic pressure draws fluid in.

At the arterial end, blood pressure is higher; at the venous end, osmotic pressure is higher, reclaiming fluid.

Developmental Aspects of the Cardiovascular System

Embryonic Development

The heart develops as a simple tube and begins pumping blood by week 4 of pregnancy. It becomes a four-chambered organ capable of acting as a double pump within 3 weeks.

Fetal Circulation

The umbilical cord carries nutrients and oxygen from maternal to fetal blood and removes wastes. It contains one umbilical vein and two umbilical arteries. Shunts bypass the liver (ductus venosus) and lungs (foramen ovale, ductus arteriosus).

Age-Related Changes

• Weakening of Venous Valves: Can lead to varicose veins.

• Arteriosclerosis: Progressive hardening of arteries.

• Hypertension: Loss of vessel elasticity increases risk.

• Coronary Artery Disease: Fatty, calcified deposits in vessels.

Key Terms and Definitions

• Myocardium: Cardiac muscle layer responsible for contraction.

• Pericardium: Double-walled sac enclosing the heart.

• Cardiac Output (CO): Blood pumped per minute.

• Stroke Volume (SV): Blood pumped per beat.

• Blood Pressure (BP): Force of blood against vessel walls.

• Arteries/Veins/Capillaries: Types of blood vessels.

• Circle of Willis: Cerebral arterial circle ensuring brain blood supply.

Summary Table: Heart Chambers and Valves

Chamber	Function	Associated Valve
Right Atrium	Receives oxygen-poor blood from body	Tricuspid (AV) Valve
Right Ventricle	Pumps blood to lungs	Pulmonary Semilunar Valve
Left Atrium	Receives oxygen-rich blood from lungs	Bicuspid (Mitral) Valve
Left Ventricle	Pumps blood to body	Aortic Semilunar Valve

Summary Table: Major Blood Vessels

Vessel	Type	Function
Aorta	Artery	Distributes oxygen-rich blood to body
Superior Vena Cava	Vein	Returns blood from upper body
Inferior Vena Cava	Vein	Returns blood from lower body
Pulmonary Arteries	Artery	Carry oxygen-poor blood to lungs
Pulmonary Veins	Vein	Carry oxygen-rich blood to heart

Summary Table: Factors Affecting Blood Pressure

Factor	Effect on BP
Cardiac Output	Increase CO increases BP
Peripheral Resistance	Increase resistance increases BP
Sympathetic Nervous System	Increases BP
Renal Regulation	Alters blood volume, affects BP
Temperature	Heat decreases, cold increases BP
Chemicals	Epinephrine increases BP
Diet	Low salt/fat decreases BP

Example: Cardiac Output Calculation

If heart rate is 75 bpm and stroke volume is 70 mL:

$CO = 75 \times 70 = 5250$ mL/min

This means the heart pumps 5.25 liters of blood per minute.

Example: Blood Pressure Measurement

If systolic pressure is 120 mm Hg and diastolic is 80 mm Hg:

$BP = \frac{120}{80}$ mm Hg

This is considered within the normal range for adults.

Additional info:

• Academic context was added to clarify the physiological mechanisms and clinical relevance of cardiovascular concepts.

• Tables were inferred and expanded for clarity and completeness.