Heart and Blood Vessels

• The heart and blood vessels form a closed system for blood circulation.

Cardiovascular System Components:

• Heart
• Blood vessels

Blood Vessel Types:

• Elastic vessels: arteries
• Resistant vessels: arterioles
• Exchange vessels: capillaries
• Capacitance vessels: veins

The Heart

• The heart is a hollow, muscular organ.
• Location: Primarily on the left side of the thoracic cavity, partly behind the sternum.
• Size: Approximately the size of a man's fist.

Cardiac Valves

• Cardiac valves ensure unidirectional blood flow within the heart.
• They facilitate blood passage from atria to ventricles and from ventricles to the aorta and pulmonary artery.

Types of Cardiac Valves

• Atrioventricular (A-V) Valves:
  • Tricuspid valve: Separates the right atrium from the right ventricle.
  • Mitral valve: Separates the left atrium from the left ventricle.
• Semilunar Valves:
  • Aortic valve: Connects the left ventricle to the aorta.
  • Pulmonary valve: Connects the right ventricle to the pulmonary artery.

Circulation Divisions:

• Systemic (greater or general) circulation: From the heart to body cells and back to the heart.
• Pulmonary (lesser) circulation: From the heart to the lungs and back to the heart.
• Special circulations

Systemic Circulation

• Starts from the left ventricle, which pumps arterial blood into the aorta.
• Sequence: Aorta → large arteries → arterioles → capillaries → venules → veins → superior & inferior vena cavae → right atrium → right ventricle.

Pulmonary Circulation

• Lies in series with the systemic circulation
• Begins in the right ventricle
• The right ventricle pumps venous blood into the pulmonary artery, which goes to the lungs
• Continues to pulmonary capillaries, then four pulmonary veins to the left atrium and then to the left ventricle
• The left ventricle begins the systemic circulation

Cardiac Muscle Fiber Types

• Atrial and ventricular types: Contractile fibers
  • The duration of their contraction longer than in skeletal muscle.
• Rhythmic type: Initiates impulses, e.g., sino-atrial node (SAN) and atrio-ventricular node (AVN).
• Conductive type: Few contractile fibrils; concerned with rapid impulse conduction. They exhibit either automatic rhythmical electrical discharge in the form of action potentials or conduction of the action potentials through.

Properties of Cardiac Muscle

• Electrical activity
• Contractility
• Rhythmicity
• Conductivity
• Excitability

Electrical Activity Components

• Resting membrane potential (RMP)
• Action potential
• Pacemaker potential (prepotential)

Resting Membrane Potential (RMP)

• The potential difference between the outer & inner surfaces of the cell membrane, it is inside negative with respect to outside the cell.
• Normal value: -90 mV.
• Causes: selective permeability of cell membranes to ions and the active Na+-K+ pump

Pacemaker Potential (Prepotential)

• A slow depolarization between each action potential in the pacemaker membrane.
• Decreased membrane permeability to K+ causing decreased K+ efflux initiating depolarization.
• Opening of transient Ca+2 channels (T-channels): Leads to Ca+2 influx.

Action Potential Phases

• Phase 0 (upstroke): Rapid depolarization, overshoot from -90 mV to +20 mV, due to rapid Na+ influx.
• Phase 1 (partial repolarization): Caused by inactivation of Na+ channels and gradually increasing K+ efflux.
• Phase 2 (plateau): Balance between Ca++ influx through voltage-gated Ca++ channels and K+ efflux, maintaining transmembrane potential at about 0 mV.
• Phase 3 (rapid repolarization): Marked increase in K+ efflux and inactivation of Ca++ channels.
• Phase 4 (complete repolarization & restoration of the resting potential):
  • Achieved by increased K+ efflux and the Na+-K+ pump.

Contractility

• The ability of the cardiac muscle to contract via conversion of chemical energy into mechanical energy.
• The contractile response begins just after the start of the depolarization by about 0.02 second

Rhythmicity

• Automaticity: The heart's ability to work independently of any extrinsic stimuli.
• Rhythmicity: The heart's ability to beat regularly.
• Autorhythmicity: The heart's ability to beat regularly independent of any extrinsic stimuli.
• Rhythmicity of different parts of cardiac muscle:
  • SAN (sinus rhythm): 100-110 beat/minute.
  • AVN (nodal rhythm): 45-60 beat/minute.
  • Purkinje fibers (idioventricular rhythm): 25-40 beat/minute

Conductivity

• Internodal pathway (atrial transmission): Conduction velocity is about 1 meter/sec.
• AV nodal transmission: Slow conduction (0.05 meter/second).
• Benefits of slow conduction in the AVN:
  • it allows the excitation waves all over the atria to propagate down to the ventricles.
  • It ensures the Protection of the ventricles from high atrial rhythm (flutter or fibrillation)
• AV bundle (Bundle of His): Conduction velocity is about 3 meter/sec.
• Ventricular transmission: Conduction velocity in Purkinje fibers is 5 meter/sec.

Excitability

• The ability of cardiac muscle fibers to respond to a stimulus.

Stages of Excitability Changes

• Absolute Refractory Period (ARP):
  • No excitability and no response occurs as the excitability is completely lost & reaches 0 %.
  • Coincides with the whole period of systole & the early part of diastole.
• Relative Refractory Period (RRP):
  • Excitability recovers but is still below normal.
  • Only a strong stimulus can produce a weak response.
  • Coincides with the first half of diastole.

Atrial Flutter and Fibrillation

• Pathological conditions are caused by the presence of a pathological focus (foci) that discharges repetitively and continuously.
• Discharges occur more rapidly than the SAN, resulting in a high rate of contraction.

Atrial Flutter

• Slow frequency contractions (200-400 beats per minute).
• The atrium is still functionally contracted, contraction is regular, regular heart rate.
• There is a rhythmic partial block (2:1 or 3:1 rhythm).

Reasons for rhythmic block in atrial flutter:

• The AVN cannot conduct more than 230 impulses/minute impulse/minute.

Atrial Fibrillation

• High-frequency contractions (400-600 beats/min).
• The atrium is dysfunctionally contracted.
• Atrial contraction and heart rate are irregular.
• There is an arrhythmic partial heart block.