Cardiovascular System

Overview

The heart generates its own electrical impulses and coordinates rhythmic contractions through the Intrinsic Cardiac Conduction System (ICCS). This system ensures the atria contract first, followed by the ventricles, creating efficient blood flow through the heart.

Intrinsic Cardiac Conduction System (ICCS)

• Function:

  • Allows the heart to generate action potentials (APs) independently of the nervous system.

  • Gap junctions between cardiac cells allow rapid spread of APs.

  • Conducting fibers transmit impulses efficiently with fewer myofibrils.

  • Ensures proper timing: atria contract first, then ventricles.

Key Structures

Flow of Impulse: SA Node → Internodal Pathways → AV Node → Bundle of His → R/L Bundle Branches → Purkinje Fibers

Pacemaker Cells

• Found in SA and AV nodes.

• Depolarize spontaneously via automaticity.

• Slow depolarization: simultaneous Na+ influx & K+ efflux.

• Depolarization: Ca2+ influx through voltage-gated Ca2+ channels.

• Repolarization: K+ efflux restores resting potential.

• Intrinsic rate ≈ 100 bpm (modified by ANS).

Pacemaker Phases

1. Na+ in / K+ out — slow depolarization

2. Ca2+ in — depolarization

3. K+ out — repolarization

4. Repeat (to true resting potential)

Contractile Cells

• Make up majority of myocardium (pumping cells).

• Rapid depolarization: Na+ influx.

• Plateau phase: Ca2+ influx + K+ efflux = sustained depolarization.

• Repolarization: K+ out.

• Prolonged refractory period prevents tetany and ensures relaxation.

ECG Correlation

Cardiac Cycle

Main Phases

Valve Associations

• AV Valves: Tricuspid (right), Bicuspid/Mitral (left)

• SL Valves: Pulmonary (right), Aortic (left)

Heart Sounds

Cardiac Output & Regulation

Cardiac Output Formula

Average CO = 5 L/min (70 mL × 75 bpm).

Stroke Volume Influences

• Preload: degree of stretch before contraction

• Contractility: strength of contraction

• Afterload: resistance ventricles must overcome

Nervous System Control

• Medulla Oblongata: controls HR through two centers:

  • Cardioacceleratory (Sympathetic): increases HR & contractility via SA/AV nodes

  • Cardioinhibitory (Parasympathetic): decreases HR via vagus nerve

• SNS: ↑ HR, ↑ force of contraction

• PNS: ↓ HR (no effect on contractility)

Blood Vessels & Circulation

Functions

• Transport blood, deliver nutrients & oxygen, remove waste.

• Arteries → Arterioles → Capillaries → Venules → Veins → Heart

Vessel Wall Structure

Vessel Types

Capillary Types

Capillary Beds

• Terminal arteriole → Capillary network → Postcapillary venule

• Precapillary sphincters regulate flow.

• Metarteriole-throughfare channel: bypasses capillaries when closed (vascular shunt).

• Microcirculation: local blood distribution based on tissue demand.

Quick Reference Tables

• Intrinsic Conduction Flow: SA Node → Atria → AV Node → Bundle of His → R/L Bundle Branches → Purkinje Fibers → Ventricular contraction

• Cardiac Cycle Timing: Filling (low pressure, AV open), Isovolumetric contraction (rising pressure, AV close), Ejection (high pressure, SL open), Isovolumetric relaxation (falling pressure, SL close)

ECG Overview

• P = Atria depolarize

• QRS = Ventricles depolarize

• T = Ventricles repolarize

Key Takeaways

• ICCS controls the timing and coordination of contractions.

• Pacemaker cells depolarize slowly; contractile cells have plateau phases.

• CO = SV × HR defines heart efficiency.

• Sympathetic ↑ HR/force. Parasympathetic ↓ HR.

• Arteries carry blood away; veins return it.

• Capillaries are the exchange site of gases and nutrients.

Blood System

Overview of Blood

• Only liquid connective tissue in the body.

• Bright red = high O2; dark red = low O2.

Functions of Blood

1. Transport

   • Carries O2 from lungs → tissues.

   • Carries CO2 and wastes → lungs and kidneys.

   • Transports nutrients, hormones, and heat.

2. Regulation

   • Maintains temperature (vasoconstriction/dilation).

   • Regulates pH via buffers.

   • Maintains fluid volume via plasma proteins.

3. Protection

   • Prevents blood loss (clotting proteins & platelets).

   • Prevents infection (WBCs, antibodies, complement proteins).

Composition of Blood

Hematocrit

• % of RBCs in total blood volume: Men 42–52%, Women 36–48%.

• Indicates oxygen-carrying capacity.

Erythrocytes (Red Blood Cells)

• Biconcave discs, flexible shape for gas diffusion.

• Anucleate, lack organelles.

• Contain structural protein spectrin for shape integrity.

• Packed with hemoglobin (Hb) (~97% of RBC mass).

Function

• Specialized for gas transport of O2 and CO2.

• Use hemoglobin to bind and release gases reversibly.

• Life span ≈120 days; recycled in spleen and liver.

Hemoglobin (Hb)

Structure

• 4 globin subunits (2 alpha + 2 beta chains).

• Each subunit contains a heme group with a Fe2+ ion that binds 1 O2 molecule.

• Each Hb molecule binds up to 4 O2 molecules — oxyhemoglobin.

• Hb can also bind CO2 — deoxyhemoglobin (via amino groups).

Function

• Oxygen transport: lungs → tissues.

• Carbon dioxide transport: tissues → lungs.

• Reversible binding maintains efficient gas exchange.

Summary Equations

(oxyhemoglobin) (carbaminohemoglobin)

Leukocytes (White Blood Cells)

• Defend against pathogens and abnormal cells.

• Migrate through blood and tissues (diapedesis).

• Two main groups: granulocytes and agranulocytes.

A. Granulocytes

B. Agranulocytes

Platelets (Thrombocytes)

• Fragments of megakaryocytes; anucleate.

• Contain granules with factors (ADP, serotonin, thromboxane).

• Circulate in inactive state; activate upon vessel injury → become spiky & sticky.

Process:

Megakaryocyte → Platelet (inactive) → Activated platelet → Platelet plug

Hemostasis (Stopping Bleeding)

A fast, local, and controlled process with three main steps:

Step 1: Vascular Spasm

• Immediate vasoconstriction of damaged vessels.

• Triggered by endothelin release from endothelium & platelets.

• Lasts 20 min–few hours; buys time for next steps.

Step 2: Platelet Plug Formation

1. Adhesion: Platelets bind exposed collagen using von Willebrand factor (vWF).

2. Activation/Degranulation: Platelets release ADP, serotonin, thromboxane A2 to attract more platelets.

3. Aggregation: Platelets stick together → temporary plug forms (positive feedback).

Summary: Adhesion → Activation → Aggregation → Plug

Step 3: Coagulation (Clotting Cascade)

• Reinforces platelet plug with fibrin mesh (molecular glue).

• Requires 30+ reactions, Ca2+, and vitamin K.

Phases

1. Prothrombin Activator Formation

   • Intrinsic pathway: slow, within blood; initiated by negatively charged surfaces (collagen, platelets).

   • Extrinsic pathway: fast, tissue factor (TF) from damaged tissue.

   • Both lead to Factor X → Prothrombin activator.

2. Prothrombin → Thrombin (enzyme activation).

3. Fibrinogen → Fibrin via thrombin.

   • Fibrin forms mesh that stabilizes clot.

PTF/F Sequence: Prothrombin activator → Prothrombin → Thrombin → Fibrinogen → Fibrin

Clot Retraction & Fibrinolysis

Clot Retraction

• Platelet-induced contraction using actin & myosin.

• Pulls wound edges together for repair.

• Releases Platelet-Derived Growth Factor (PDGF) → stimulates vessel healing.

Fibrinolysis (Clot Breakdown)

• Restores normal blood flow after repair.

• Plasminogen → Plasmin (via tPA) → dissolves fibrin mesh.

• Occurs once vessel is healed.

Quick Reference Tables