Cardiovascular System: Blood, Heart, and Vessels – Study Notes for Microbiology Students

Chapter 18 – Cardiovascular System: Blood

I. Functions and General Composition of Blood

The blood is a vital connective tissue responsible for transportation, regulation, and protection within the body. Its composition and physical characteristics are essential for maintaining homeostasis.

• Functions of blood:

  • Transportation: Carries formed elements, dissolved molecules, ions, nutrients, gases, and wastes throughout the body.

  • Regulation: Maintains body temperature, pH, and fluid balance.

  • Protection: Defends against pathogens and facilitates blood clotting.

• Physical characteristics of blood:

  • Color, volume, viscosity, plasma concentration, temperature, and blood pH.

II. Components of Blood

Blood consists of plasma and formed elements, each with distinct roles in physiology.

• Whole blood (centrifuged):

  • Plasma (~55%): Mostly water, proteins, solutes.

  • Buffy coat (~1%): Leukocytes and platelets.

  • Erythrocytes (~45%): Form hematocrit.

• Hematocrit: Percentage of RBCs; higher in males, varies with hydration and altitude.

III. Composition of Blood Plasma

Plasma is the liquid component of blood, containing proteins and solutes that maintain osmotic pressure and support physiological functions.

• Plasma proteins: Maintain colloid osmotic pressure.

  • Albumins (60%): Transport proteins; regulate osmotic pressure.

  • Globulins (35%): Alpha/beta transport proteins; gamma globulins are antibodies.

  • Fibrinogen (4%): Involved in blood clotting; converts to fibrin.

  • Regulatory proteins (<1%): Enzymes and hormones.

IV. Formed Elements

Formed elements include erythrocytes, leukocytes, and platelets, each with specialized functions.

• Erythrocytes (Red Blood Cells):

  • Biconcave shape; lack nucleus/organelles; filled with hemoglobin.

  • Hemoglobin: 4 globins, 4 hemes with Fe2+ (binds O2); CO2 binds globin.

• Leukocytes (White Blood Cells):

  • Granulocytes:

    • Neutrophils: Phagocytosis of bacteria.

    • Eosinophils: Defend against parasites; role in allergies/asthma.

    • Basophils: Release histamine and heparin.

  • Agranulocytes:

    • Monocytes: Become macrophages; phagocytosis.

    • Lymphocytes: T cells (cell-mediated immunity), B cells (antibody production), NK cells (kill abnormal cells).

• Hematopoiesis: Formation of blood cells from hemocytoblasts in bone marrow.

  • Thrombopoiesis: Megakaryocytes produce platelets.

  • Erythropoiesis: Stimulated by EPO (from kidneys); testosterone enhances.

• Erythrocyte recycling: Lifespan ~120 days; spleen, liver, marrow remove old RBCs; iron recycled, heme → bilirubin.

• Blood groups: ABO (A, B, AB, O) based on surface antigens; Rh factor (D antigen); transfusion reactions: agglutination + hemolysis.

• Platelets: Fragments of megakaryocytes; contain clotting granules; critical in clotting cascade.

V. Hemostasis

Hemostasis is the process of stopping bleeding and involves several coordinated steps.

1. Vascular spasm: Vasoconstriction after injury.

2. Platelet plug formation: von Willebrand factor, ADP, thromboxane A2, serotonin (positive feedback).

3. Coagulation: Intrinsic and extrinsic pathways converge to form fibrin from fibrinogen.

   • $\text{Prothrombin} \rightarrow \text{Thrombin} \rightarrow \text{Fibrinogen} \rightarrow \text{Fibrin}$

4. Clot retraction/repair: Platelets contract; growth factors stimulate healing.

5. Fibrinolysis: Plasmin dissolves clot.

Chapter 19 – Cardiovascular System: The Heart

I. Introduction to the Cardiovascular System

The cardiovascular system transports blood throughout the body, supporting tissue function and homeostasis.

• Components:

  1. Blood Vessels: Arteries, veins, capillaries.

  2. Heart (pump): Chambers, great vessels, valves.

  3. Circulation routes: Pulmonary and systemic circulation.

II. Location of the Heart and Pericardium

The heart is located in the mediastinum and is surrounded by protective layers.

• Pericardial sac: Fibrous pericardium, parietal and visceral layers, pericardial cavity with serous fluid.

• Clinical views: Pericarditis, cardiomegaly.

III. Heart Anatomy

The heart consists of four chambers, valves, and structural features that ensure unidirectional blood flow.

• Chambers: Right & left atria, right & left ventricles.

• Valves: AV valves (tricuspid, mitral), semilunar valves.

• Sulci: Coronary sulcus, interventricular sulci.

• Layers: Epicardium, myocardium, endocardium.

• Coronary circulation: Right & left coronary arteries, veins draining into coronary sinus.

IV. Microscopic Structure of Cardiac Muscle

Cardiac muscle cells are specialized for continuous, rhythmic contraction.

• Intercalated discs: desmosomes, gap junctions.

• Fibrous skeleton of heart.

• Cardiac metabolism: reliance on aerobic respiration.

V. Anatomic Structures Controlling Heart Activity

Heart activity is regulated by specialized conduction pathways and autonomic innervation.

• SA node, AV node, AV bundle, Purkinje fibers.

• Innervation: sympathetic vs. parasympathetic pathways.

VI. Stimulation of the Heart

Heart rhythm is generated by pacemaker cells and modulated by contractile cells.

• Nodal cells: autorhythmicity, pacemaker potential.

• Cardiac muscle cells: contractile events.

Comparison Table: Nodal Cells vs. Cardiac Muscle Cells

Feature	Nodal Cells (SA/AV nodes)	Cardiac Muscle Cells (Contractile cells)
Resting Membrane Potential (RMP)	~ -60 mV	~ -90 mV
Key Function	Pacemaker – generate action potentials (autorhythmicity)	Contraction – pump blood
Depolarization Channel	Fast voltage-gated Ca2+ channels	Fast voltage-gated Na+ channels
Special Channels	Slow voltage-gated Na+ ("funny") channels	None
Plateau Phase	Absent (short AP)	Present (due to slow Ca2+ influx balanced by K+ efflux)
Repolarization	Voltage-gated K+ channels	Voltage-gated K+ channels
Pumps/Exchangers	Na+/K+ ATPase, Ca2+ ATPase, Na+/Ca2+ exchanger	Na+/K+ ATPase (SR + sarcolemma), Na+/Ca2+ exchanger
Unique Feature	Spontaneous depolarization – autorhythmicity	Long refractory period – prevents tetany, ensures rhythmic contraction

VII. Cardiac Muscle Cells – Electrical & Mechanical Events

Cardiac muscle cells undergo electrical and mechanical events to produce coordinated contractions.

• Electrical events: depolarization, plateau, repolarization.

• Mechanical events: cross-bridge cycling.

• ECG recording: waves, intervals, segments (normal vs. arrhythmias).

VIII. The Cardiac Cycle

The cardiac cycle consists of sequential phases that ensure efficient blood flow through the heart.

1. Atrial contraction (atrial systole): Atria contract, ventricles fill.

2. Isovolumetric contraction (early ventricular systole): Ventricles contract, all valves closed.

3. Ventricular ejection (late ventricular systole): Semilunar valves open, blood ejected.

4. Isovolumetric relaxation (early ventricular diastole): Ventricles relax, valves closed.

5. Ventricular filling (late ventricular diastole): AV valves open, ventricles fill.

IX. Cardiac Output

Cardiac output (CO) is the volume of blood pumped by a ventricle per minute and is a key indicator of heart function.

• Definition: CO = HR × SV

• Equation: $\text{CO} = \text{HR} \times \text{SV}$

A. Heart Rate Regulation – Chronotropic Agents

• Chronotropic agents: Factors that influence heart rate (HR).

• Positive chronotropic agents (↑ HR):

  • Sympathetic nervous system (SNS): NE and EPI bind to β-adrenergic receptors on nodal cells → ↑ Ca2+ influx → faster depolarization.

  • Nicotine: Stimulates sympathetic activity.

  • Caffeine: Prolongs cAMP effects → ↑ Ca2+ entry into nodal cells.

• Negative chronotropic agents (↓ HR):

  • Parasympathetic nervous system (via vagus nerve): ACh opens K+ channels in nodal cells → hyperpolarization → slower depolarization.

  • Beta-blockers: Clinically block β receptors to slow HR.

  • Electrolyte imbalances:

    • ↑ K+ (hyperkalemia) – slows HR by impairing depolarization.

    • ↑ Ca2+ (hypercalcemia) – can decrease HR by prolonging plateau.

B. Stroke Volume Regulation Factors

1. Preload (EDV/stretch of ventricular muscle):

   • Degree of stretch of ventricular walls before contraction; determined by venous return.

   • Frank-Starling Law: ↑ venous return → ↑ preload → stronger contraction → ↑ SV.

   • Excessive stretch (e.g., CHF) = ↓ efficiency.

2. Venous Return (determinant of preload):

   • Affected by:

     • Skeletal muscle pump (muscle contractions squeeze veins).

     • Respiratory pump (inhalation ↑ thoracic pressure → ↑ venous return).

   • Reduced by blood loss, dehydration, or lack of movement.

3. Afterload (resistance ventricles must overcome):

   • Pressure the ventricles must generate to eject blood.

   • Left ventricle: must overcome aortic pressure (~80 mmHg at rest).

   • Right ventricle: must overcome pulmonary arterial pressure (~20 mmHg).

   • Increased afterload (e.g., hypertension, aortic stenosis, arteriosclerosis) → harder ejection → ↓ SV.

   • Decreased afterload → easier ejection → ↑ SV.

X. Development of the Heart

The heart develops primitive chambers by week 5 of embryogenesis. Congenital defects include atrial septal defects, patent ductus arteriosus, coarctation of aorta, and tetralogy of Fallot.

Chapter 20 – Cardiovascular System: Vessels and Circulation

Structure and Function of Arteries, Veins, and Capillaries

Blood vessels are classified by structure and function, facilitating efficient circulation and exchange.

• Arteries: Carry blood away from the heart; high pressure, thick walls with smooth muscle and elastic fibers.

• Veins: Carry blood toward the heart; thin walls, large lumen, contain valves to prevent backflow; function as blood reservoirs.

• Capillaries: Microscopic vessels of exchange; composed of endothelium and basement membrane only.

Vessel Wall Tunics

• Tunica intima: Endothelium + basement membrane; smooth surface.

• Tunica media: Smooth muscle + elastic fibers; controls vessel diameter.

• Tunica externa (adventitia): Connective tissue; anchors vessel.

Arteries

• Elastic arteries: Largest, closest to heart (aorta, pulmonary trunk); stretch and recoil to maintain continuous flow.

• Muscular arteries: Medium-sized; distribute blood to organs.

• Arterioles: Smallest arteries; primary regulators of systemic BP by vasoconstriction/vasodilation.

Capillaries

• Continuous: Most common (muscle, skin, brain); tight junctions; least permeable.

• Fenestrated: Found in kidney, intestines, endocrine glands; pores allow larger solutes.

• Sinusoidal: Found in liver, spleen, bone marrow; large gaps, incomplete basement membrane; most permeable.

Veins

• Valves: Prevent backflow of blood.

• Reservoirs: Store 60–70% of blood at rest.

Capillary Beds, Perfusion, Precapillary Sphincters

• Capillary beds: Microcirculatory networks.

• Precapillary sphincters: Smooth muscle rings regulating entry into true capillaries.

• Perfusion: Blood flow per unit time per gram of tissue.

Pathways of Circulation

• Simple: One artery → capillary bed → one vein.

• Arterial anastomosis: Multiple arteries supply same region.

• Venous anastomosis: Multiple veins drain same region.

• Arteriovenous anastomosis: Bypass capillaries entirely.

• Portal system: Two capillary beds connected in series (hepatic portal system).

Bulk Flow: Filtration vs. Reabsorption

Bulk flow describes the movement of fluid across capillary walls, driven by hydrostatic and osmotic pressures.

• Filtration: Out of capillaries; arterial end; driven by hydrostatic pressure.

• Reabsorption: Into capillaries; venous end; driven by osmotic pressure.

• Pressures: Colloid osmotic pressure (COP) is the pulling force from plasma proteins.

• NFP Equation:

  • $\text{NFP} = (\text{HP}_b - \text{HP}_i) - (\text{COP}_b - \text{COP}_i)$

  • Positive NFP = filtration (arterial end)

  • Negative NFP = reabsorption (venous end)

Local Blood Flow

• Vascularity: High in metabolically active tissues.

• Myogenic response: Smooth muscle constriction/dilation in response to pressure.

• Autoregulation: Local chemicals adjust flow to match demand (O2, CO2, H+, adenosine).

Blood Pressure

• Systolic: Peak arterial pressure during ventricular systole.

• Diastolic: Minimum arterial pressure during ventricular diastole.

• Pulse pressure: Systolic – Diastolic.

• MAP (Mean Arterial Pressure): $\text{MAP} = \text{Diastolic} + \frac{1}{3} \text{Pulse Pressure}$

Venous Return

• Aided by:

  • Respiratory pump

  • Skeletal muscle pump

  • Venoconstriction (sympathetic tone)

Resistance Factors

• Viscosity: ↑ hematocrit = ↑ resistance.

• Vessel length: Longer = ↑ resistance.

• Vessel radius: Small radius = dramatically ↑ resistance (1/r4 rule).

Blood Pressure Regulation

• Neural:

  • Baroreceptors (carotid sinuses, aortic arch)

  • Chemoreceptors (carotid, aortic bodies)

• Hormonal:

  • Epinephrine: ↑ HR, contractility, vasoconstriction.

  • ADH: ↑ water retention, vasoconstriction.

  • ANP: Promotes Na+/water excretion.

  • Renin-angiotensin system: Angiotensin II vasoconstriction + aldosterone release.

Long-Term BP Regulation: Renal Mechanisms

• Direct: Alter blood volume via urine output.

Example: In hypertension, increased afterload and resistance can lead to decreased stroke volume and cardiac output, requiring compensatory mechanisms such as increased heart rate or renal excretion of fluid.