heart beat, contractility and conductivity

What sides of the heart hands venous and arterial blood ?

Right heart handles venous blood

Left heart handles arterial blood

What is the systemic and pulmonary blood flow route ?

RA → RV (pulmonary)

LA → LV (systemic)

What’s the difference between pulmonary and systemic?

Pulmonary:

• low oxygenated blood

• Returns highly oxygenated

• Low resistance

Systemic:

• highly oxygenated blood

• Returns low oxygenated (plus Co2)

• High resistance

What does resistance determine ?

Required pressure

Pressure determines wall thickness

What are the 3 layers of the heart wall?

• pericardium

• Myocardium

• Endocardium

What is the pericardium/pericardial sac and what does it do?

• tough dense connective tissue that protects the heart and maintains position (anchor within thorax)

• Cavity provides lubricatin

• Prevents excessive dilation + reduces friction

What is the epicardium?

Fibroelastic connective tissue

What is the myocardium?

Muscle tissue

What is the composition of the myocardium?

Composed of branching cardiac myocytes arraged in spirals

Spiral arrangement produces ringing contraction (twisting motion that increases efficacy of blood ejection)

What is the endothelium?

• provides smooth endothelial surface

• Minimises turbulence + clot formation

Describe the left ventricular musculature vs the right ventricular musculature

Left musculature:

• systemic circulation

• Much thicker

• Overcome systemic resistance

• High pressure

Right musculature:

• pulmonary circulation

• Reduced thickness

• Lower pulmonary resistance

• Lower pressure

Why is the left ventricular musculature much thicker?

As systemic vascular resistance higher than pulmonary resistance

To eject blood into systemic circulation, left ventricular pressure must exceed aortic pressure

What’s A?

Atrial musculature

Whats B?

Ventricular musculature

Whats A?

Endocardium

What is B?

Myocardium

Whats C?

Pericardial activity

Whats D?

Fibrous pericardium

Whats E?

Parietal layer of serous pericardium

Whats F?

Epicardium (visceral layer of serous pericardium)

Which is contracted which is relaxed?

Relaxed - left

Contracted - right

What does wall thickness within heart reflect?

Laplaces’ law

What is wall stress proportional to?

(Pressure x radius) / wall thickness

What happens if the pressure increases

Thickness must increase to maintain wall stress

Explains hypertrophy in pressure overload situations

What is the role of the atrioventricular valves and what do they include?

• link atria and ventricles

• Bicuspid (mitral) LEFT

• Tricuspid RIGHT

What is the role of the semilunar valves and what do they include?

• link ventricles and aortic and pulmonary arteries

• Aortic

• Pulmonary

When do semilunar valves open?

When ventricular pressure exceeds arterial pressure

What is A?

Tricuspid valve

What is B?

Bicuspid (mitral) valve

Whats C?

Aortic valve

What is D?

Pulmonary valve

Cardiac valves are passive structures, they open and close due to pressure differences, what happens to AV valves when atrial pressure exceeds ventricular pressure?

AV valves open

Cardiac valves are passive structures, they open and close due to pressure differences, what happens to AV valves when Ventricular pressure exceeds atrial pressure?

AV valves close

What follows the contraction of the atrial wall?

• contraction of atrial wall

• Relaxation of chordae tendinae

• Enables downward force on valve flaps

• Opening of valve

• Blood drawn into ventricles from atria

What follows the contraction of the ventricular wall?

• contraction of ventricular wall

• Stretch chordae tendineae

• Mitral valves close

• Pressure of blood on semilunar valves

• Forces blood from ventricles into arteries

Whats A?

Sarolemma

Whats B?

Mitochondria

Whats C?

Myofibrils

Whats D?

Nucleus

Whats E?

T tubule

Whats F?

Terminal cisterna

Whats G?

Triad

What’s H?

Sarcoplasmic reticulum

What is a muscle fibre ?

Individual unit made up of fibrils

What is an intercalated disc?

• link between fibres

• Contain desmosomes for mechanical strength and gap junctions for electrical coupling

intercalated disks contain gap junctions, what effect does this have?

Myocardium behaves as functional unit therefore depolarisation spreads rapidly between cells ensures coordinated contraction

What are T tubules?

Run through muscle to provide electrical and chemical link

What is the sarcoplasmic reticulum ?

Calcium store to aid contraction

Whats a sarcomere ?

Smallest unit of muscle contraction

What are thin filaments ?

• Actin

• Slide to provide muscle contraction

What are thick filaments ?

• myosin

• Provide the mechanical force

What happens during contraction?

Myosin heads bind to actin, ATP hydrolyses power stroke, filaments slide past each other and sarcomere shortens

Filaments do not shorten, they slide - generates force

Whats A?

Nucleus

Whats B?

Muscle fibre

Whats C?

Mitochondria

Whats D?

Sarcolemma

Whats E?

Light I band

Whats F?

Dark A band

Whats G?

Myofibril

What’s H?

Sarcomere

Whats I?

Sarcoplasmic reticulum

Whats J?

Thin (actin) filament

Whats K?

Z disc

Whats L?

H zone

Whats M?

Z disc

Whats N?

Thick (myosin) filament

What’s O?

I band

Whats P?

A band

Whats Q?

I band

What’s R?

M line

How does contraction occur (simple explanation)

• myosin head on thick filament binds to actin

• Myosin head pivots (power stroke)

• Thin filaments pulled towards the midline

• Myosin head detaches from the actin

• Cycle repeats

Describe the process of contraction using the diagram

1) acetylcholine released from the axon terminal binds to receptors on the sarcolemma

2) an AP is generated and travels down the T tubule

3) Ca2+ is released from the sarcoplasmic reticulum in response to the change in voltage

4) Ca2+ binds troponin. Cross-bridges form between actin and myosin

5) acetylcholinesterase removes acetylcholine from the synaptic cleft

6) Ca2+ is transported back into the sarcoplasmic reticulum

7) Tropomyosin binds active sites on actin causing the cross-bridge to detach

Acetylcholine stimulates post synaptic receptors on cardiac muscle, what follows this ?

Electrical depolarisation opens voltage gated Ca2+ channels

Membrane depolarisation spreads via the T Tubules

Small calcium influx triggers release of larger calcium fr sarcoplasmic reticulum

Calcium initiates myosin head binding and contraction

Cycstoic calcium binds to troponin C , what does this cause?

• troponin changes confirmation

• Tropomyosin moves

• Myosin binding sites exposed

• Cross-bridge cycling begins

• Contraction occurs

Relaxation requires calcium removal. Why and how ?

• reuptake to SR (sarcoplasmic reticulum)

• Extrusion via sodium calcium exchanger

• Without proper Ca2+ handling = contraction fails

What is sinoatrial (SA) node?

Primary pacemaker

What is bachmans bundle?

Links left and right atria

What is the atrioventricular (AV) node?

Secondary pacemaker

Whats the bundle of his ?

Links AV node to intraventricular septum

What is the right and left bundle branches?

Innervate ventricles

What is the role of the SA node?

Specialist cells that spontaneously depolarise to allow atrial contraction to complete before ventricular contraction begins

How does the SA node allow atrial contraction to complete before ventricular contraction begins ?

• specialist cells that spontaneously depolarise

• These impulses spread through atrial myocardium, reaches atrioventricular node, has a delay (approx 100milliseconds)

• This delay allows atrial contraction to complete before ventricular contraction begins

Why is rapid coordinated activation important?

Ensuresheart has efficient ejection of blood

Where does the impulse travel after it allows atrial contraction to complete before ventricular contraction begins ?

Down bundle of his, left and right bundle branches + Purkinje fibres , distribute depolarisation rapidly through the ventricles

How long is the delay that lows atrial contraction to complete before ventricular contraction?

Approx 100 milliseconds

Whats A?

Arch of aorta

Whats B?

Bachmans bundle

What’s C?

Left atrium

Whats D?

Atrioventricular (AV) bundle (bundle of his)

Whats E?

Left ventricle

Whats F?

Right and left bundle branches

Whats G?

Purkinje fibres

What’s H?

Sinoatrial (SA) node

Whats I?

Anterior internodal

Whats J?

Atrioventricular (AV) node

Whats K?

Middle internodal

Whats L?

Posterior internodal

Whats M?

Right atrium

Whats N?

Right ventricle

Describe how ventricular contraction begins

1) SA node and the remainder of the conduction system are at rest

2) SA node initiates the action potential which sweeps across the atria

3) after reaching the atrioventricular node, there is a delay of approximately 100ms that allows the atria to complete pumping blood before the impulse is transmitted to the atrioventricular bundle

4) the impulse travels through the atrioventricular bundle and bundle branches

5) the impulse spreads to the contractile fibres of the ventricle

6) ventricular contraction begins