In-Depth Notes on Cardiovascular System

Chapter 11: The Cardiovascular System

Overview of the Cardiovascular System

• Closed System: Comprises the heart and blood vessels.

• Function:

  • Transport: Oxygen, nutrients, cell wastes, hormones to and from cells.

  • Circulation: Blood is pumped by the heart through blood vessels to all parts of the body.

Anatomy of the Heart

• Size & Shape:

  • Comparable to a human fist, weighing less than a pound.

  • Located in the thoracic cavity, between the lungs (inferior mediastinum).

  • Orientation:

  • Apex directed toward the left hip, resting on the diaphragm.

  • Base points toward the right shoulder.

Coverings of the Heart

• Pericardium: A double-walled sac.

  • Fibrous Pericardium: Loose and superficial layer.

  • Serous Pericardium: Deeper layer, composed of two layers:

  1. Parietal Pericardium: Outside layer lining the fibrous pericardium's inner surface.

  2. Visceral Pericardium (Epicardium): Layer directly attached to the heart.

  • Pericardial Cavity: Space with serous fluid between the layers.

Heart Walls

1. Epicardium: Outermost layer facing the pericardial cavity.

2. Myocardium: Middle layer, primarily cardiac muscle, responsible for contraction.

3. Endocardium: Inner layer; lined with endothelium.

Cardiac Chambers and Associated Vessels

• The heart consists of four chambers:

  • Atria (Right and Left):

  • Function: Receiving chambers that assist in filling the ventricles with blood under low pressure.

  • Ventricles (Right and Left):

  • Function: Discharging chambers, thick-walled pumps that propel blood into circulation during contraction.

• Septums:

  • Interatrial Septum: Separates the two atria.

  • Interventricular Septum: Separates the two ventricles.

• Blood Flow:

  • Heart functions as a double pump

    ▪ Arteries carry blood away from the heart

    ▪ Veins carry blood toward the heart

  • Right Side (Pulmonary Pump): Pumps oxygen-poor blood to the lungs.

  • Left Side (Systemic Pump): Pumps oxygen-rich blood to the body.

Heart Valves

• Functionality: Ensure one-way blood flow to prevent backflow.

• Types of Valves:

  • Atrioventricular (AV) Valves: between atria and ventricles

  • Left AV Valve: Bicuspid (Mitral) Valve.

  • Right AV Valve: Tricuspid Valve.

  • Semilunar Valves: between ventricle and artery

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

  • Aortic Semilunar Valve: Between left ventricle and aorta.

• Valve Mechanics:

  • AV valves open during relaxation and close during contraction.

  • Semilunar valves open during contraction and close during relaxation.

Cardiac Circulation

• Myocardial Nutrition:

  • The heart is nourished by coronary arteries which branch from the aorta.

  • Cardiac Veins: Drain the myocardium and empty into the coronary sinus, which drains into the right atrium.

  • Coronary sinus—a large vein on the posterior of the

    heart; receives blood from cardiac veins

Physiology of the Heart

Intrinsic Conduction System

• Function: Initiates heartbeat independently of nerve impulses.

• ▪ Spontaneous contractions occur in a regular and

  continuous way

  ▪ Atrial cells beat 60 times per minute

  ▪ Ventricular cells beat 20−40 times per minute

  ▪ Need a unifying control system—the intrinsic conduction

  system (nodal system)

• Two systems regulate heart activity

  ▪ Autonomic nervous system

  ▪ Intrinsic conduction system, or the nodal system

  - Sets the heart rhythm

  - Composed of special nervous tissue

  - Ensures heart muscle depolarization in one direction only

• Components Include:

  1. Sinoatrial (SA) Node: Pacemaker located in right atrium. Start each heartbeat

  2. Atrioventricular (AV) Node: Between atria and ventricles.

  3. AV Bundle (Bundle of His), bundle branches, and Purkinje fibers: Spread impulses through ventricle walls.

     • Purkinje fibers spread within the ventricle wall muscles

       - Tachycardia—rapid heart rate, over 100 beats per minute

       - Bradycardia—slow heart rate, less than 60 beats per

       minutes

• Heart Rate Regulation:

  • Normal resting heart rate is approximately 75 beats per minute.

  • Cardiac output (CO) - Amount of blood pumped by each side (ventricle) of

    the heart in 1 minute

  • Stroke volume (SV) - Volume of blood pumped by each ventricle in one

    contraction (each heartbeat)

  • Cardiac output (CO) = Heart Rate (HR) × Stroke Volume (SV).

Cardiac Cycle

• Definition: One complete heartbeat, encompassing contraction (systole) and relaxation (diastole).

• Atrial diastole (ventricular filling)

  ▪ Heart is relaxed

  ▪ Pressure in heart is low

  ▪ Atrioventricular valves are open

  ▪ Blood flows passively into the atria and into ventricles

  ▪ Semilunar valves are closed

• Atrial systole

  ▪ Ventricles remain in diastole

  ▪ Atria contract

  ▪ Blood is forced into the ventricles to complete

  ventricular filling

• Isovolumetric contraction

  ▪ Atrial systole ends; ventricular systole begins

  ▪ Intraventricular pressure rises

  ▪ AV valves close

  ▪ For a moment, the ventricles are completely closed

  chambers

• Ventricular systole (ejection phase)

  ▪ Ventricles continue to contract

  ▪ Intraventricular pressure now surpasses the pressure in

  the major arteries leaving the heart

  ▪ Semilunar valves open

  ▪ Blood is ejected from the ventricles

  ▪ Atria are relaxed and filling with blood

• Isovolumetric relaxation

  ▪ Ventricular diastole begins

  ▪ Pressure falls below that in the major arteries

  ▪ Semilunar valves close

  ▪ For another moment, the ventricles are completely

  closed chambers

  ▪ When atrial pressure increases above intraventricular

  pressure, the AV valves open

• Average Length: 0.8 seconds, with heart sounds resulting from valve closure.

  • S1 (Lub): Closure of AV valves. Longer, louder heart sound

  • S2 (Dub): Closure of semilunar valves. Short, sharp heart sound

• Summary of events occurring during the cardiac cycle

  1. Atrial diastole (ventricular filling)

  2. Atrial systole

  3. Isovolumetric contraction

  4. Ventricular systole (ejection phase)

  5. Isovolumetric relaxation

• Starling’s law of the heart

  ▪ The critical factor controlling SV is how much cardiac

  muscle is stretched

  ▪ The more the cardiac muscle is stretched, the stronger

  the contraction

• Venous return is the important factor influencing the stretch of heart muscle

• Factors modifying basic heart rate

  • Neural (ANS) controls

    • Sympathetic nervous system speeds heart rate

    • Parasympathetic nervous system, primarily vagus nerve

    fibers, slow and steady the heart rate

  • Hormones and ions

    • Epinephrine and thyroxine speed heart rate

    • Excess or lack of calcium, sodium, and potassium ions

      also modify heart activity

  • Physical factors

• Blood Vessels - Blood vessels form a closed vascular system that

  transports blood to the tissues and back to the

  heart

  ▪ Arteries and arterioles - Vessels that carry blood away from the heart

  ▪ Capillary beds - Vessels that play a role in exchanges between tissues

  and blood

  ▪ Venules and veins - Vessels that return blood toward the heart

• Three layers (tunics) in blood vessels (except the

  capillaries)

  ▪ Tunica intima forms a friction-reducing lining

  • Endothelium

  ▪ Tunica media

  • Smooth muscle and elastic tissue

  • Controlled by sympathetic nervous system

  ▪ Tunica externa forms protective outermost covering

  • Mostly fibrous connective tissue

  • Supports and protects the vessel

• Structural differences in arteries, veins, and capillaries

  ▪ Arteries have a heavier, stronger, stretchier tunica

  media than veins to withstand changes in pressure

  ▪ Veins have a thinner tunica media than arteries and

  operate under low pressure

  ▪ Capillaries

  • Only one cell layer thick (tunica intima)

  • Allow for exchanges between blood and tissue

  • Form networks called capillary beds that consist of: A vascular shunt and True capillaries

  ▪ Blood flow through a capillary bed is known as

  microcirculation

Blood Pressure

• Definition: Force exerted by blood against vessel walls, measured in mm Hg.

• Key Measurements:

  • Systolic Pressure: Peak pressure during ventricular contraction.

  • Diastolic Pressure: Pressure during ventricular relaxation.

  • Normal: About 120/80 mm Hg.

• Factors Influencing Blood Pressure:

  • Cardiac Output and peripheral resistance.

  • Neural factors (sympathetic and parasympathetic nervous system).

  • Hormonal influences (e.g., norepinephrine, epinephrine).

Developmental Aspects

• Embryonic Development: Heart develops as a tube by week 4 and becomes a four-chambered structure in three weeks.

• Fetal Circulation: Unique shunts such as the foramen ovale and ductus arteriosus bypass lungs and liver.

Aging Changes

• Common conditions include varicose veins, arterial stiffening (arteriosclerosis), and hypertension.

• Coronary artery disease can develop due to fatty deposits in vessels.

Summary of Major Arteries and Veins

• Major Arteries: Aorta (ascending, arch, thoracic, abdominal), coronary arteries, and arteries supplying limbs and organs.

• Major Veins: Superior and inferior vena cava, brachiocephalic vein, renal veins, and hepatic portal vein.