Heart

Heart

- circulates blood
- size of a fist
250-350 grams
- enclosed in the mediastinum
-2/3 lie left of the midsternal line
- extends from the second rib to the 5th intercoastal rib

Heart coverings

covered in the pericardium
- fibrous pericardium
- serous pericardium

Fibrous pericardium

protects the heart
anchors the heart to surrounding structures
prevents heart from over filling

Serous pericardium

Parietal pericardium
- lines the internal surface of the fibrous pericardium
Pericardial Cavity
- slit like cavity between the parietal and visceral pericardium
Visceral Pericardium
- covers external surface of the heart
- also known as epicardium

Heart layers

Epicardium
Myocardium
Endocardium

Epicardium

- the visceral layer of the pericardium
- often infiltrated with fat

Myocardium

cardiac muscle
- spherical/circular follicles
Fibrous skeleton
- network of elastic and collagen fibers
- serves to provide additional support to the heart / great vessels and the valves
- anchors the cardiac muscle
- serves as an electrical insulator between atria and ventricles

Endocardium

- glistening white sheet of endothelium
- continuous with the endothelial lining of the blood vessels

Heart anatomy

4 chambers
- right atria
- left atria
- on top, receiving chambers
- right ventricle
- left ventricle
- on bottom, bigger, pumping chambers
Separated by septum
- interatrial septum
- interventricular septum

Valves
atrioventricular valve

- located between the atria and ventricles
AV opens when pressure is higher in the atria
AV closes when pressure is higher in the ventricles

Semilunar valve

- located between the ventricles and large
- SV opens when pressure is higher in the ventricles
- SV closes when pressure is greater in the arteries

Cardiac muscle fibers

- cardiomyocytes
- striated and single nucleus
- short, fat, branched and interconnected

cardiac muscle functional

membranes interlock with adjacent fibers - junctions are called intercalated discs
- contain desosomes and gap junction
- myocardium acts as a single coordinated unit

Cardiac vs skeletal means of stimulation

skeletal muscle
- innervated by a nerve
cardiac muscle
- has self excitable tissue that cycle producing automaticity

organ vs motor unit contraction

skeletal
- not all muscle fibers contract with an impulse, only the motor unit that is stimulated
cardiac
- either all do or none (gap junctions)

Refractory period

- in excitable period where na+ are still open or inactivated
cardiac
- lasts about 250ms as long as contraction
skeletal
- 15ms

Electrical events of the heart

controlled both intrinsically
- beating is determined without nervous system innervation
- properties of the heart itself
- intrinsic rate of 100 b/m
and extrinsically
- The autonomic nervous system acts on the heart to alter the intrinsic rate
Together observed normally as 70/80 b/m
- sinus rhythm (average)

Cardiac conduction system

Spontaneous rhythmicity (autorhythmicity)
- Sinoatrial node
- Atrioventricular node
- AV bundle (bundle of his)
- right and left bundle branches
- subendocardial conducting network
From SA node initiation to contraction in .22s
- Electrical signal spreads via gap junctions

Sinoatrial node

initiates contraction signal
- Pacemaker cells in upper posterior RA wall
- Depolarize faster than the rest of myocardium
Signal spreads from the SA node via RA/LA to AV node
- atrial contraction

Atrioventricular node

delays, relays signal to ventricles
- in inferior interatrial septum
- delay (.13s) allows for RA, LA to contract before RV, LV resulting in longer filling time
- relays signal to AV bundle

AV bundle (bundle of his)

relays signal to the RV, LV
- travels along the interventricular septum
- only external connection between atria and ventricles
- divides into ...

Right and left bundle branches

sends signals towards apex of heart

Purkinje fibers (subendocardial conducting network)

send signal into RV, LV
- terminal branches of the bundle branches
- spread throughout entire ventricle walls
- stimulate the rv , lv from contraction towards apex towards atria

Pace maker cells in the heart

- SA node has unstable resting membrane potential
- Spontaneously changing membrane potential = pacemaker potentials

Unstable resting potential "pacemaker potential"

K+ channels closed from previous action potential
Slow depolarization caused by open Na+ channels

Action Potential

Threshold met at -40 mv
Rapid depolarization due to Calcium influx through voltage-gated Ca+ channels

Electrocardiogram (ECG)

typically fires every 0.8 sec
resting rate at 75 bpm

ECG or EKG

recording of hearts electrical activity
- diagnostic tool

Three basic phases

P wave: atrial depolarization
QRS complex: ventricular depolarization
T wave: ventricular repolarization

Cardiac cycle

one complete contraction and relaxation of all four chambers of the heart
Diastole
Systole

Heart sounds
Auscultation

- listening to sounds made by body

First heart sound

- louder and longer "lubb"
- closure of AV valves

Second heart sound

- softer and sharper "dupp"
- closure of semilunar valves
- rarely heard in people over 30

Wiggers Diagram

Cardiac cycle
Actions:
- blood flow direction
- chambers
- valves
Sounds
Electrical activity
Blood Volume
Blood pressure

Ventricular Filling

- ventricles expand and their pressure drops below that of the atria
- av valves open and blood flows into the ventricles

Isovolumetric contraction

- atria repolarize, relax, and remain in diastole for rest of cardiac cycle
- ventricles depolarize, causing QRS complex, and begin to contract
- AV valves close as ventricular bloods surges back against the cusps
* Heart sound occurs at the beginning of this phase
Isovolumetric because although ventricles, they do not eject blood
- aorta/pulmonary trunk pressure still greater than ventricles

Ventricular ejection

Ventricular pressure exceeds arterial pressure - semilunar valves open
- First : rapid ejection -- blood spurts out of ventricles quickly
- Then : reduced ejection -- slower flow with lower pressure
- T wave of ECG occurs late in this phase
- Ventricular repolarization

Ventricles don't expel all blood

stroke volume (SV) is about 70ml
- amount ejected
60 ml remaining blood is end-systolic volume (ESV)
- ESV = EDV - SV

Isovolumetric relaxation

- T wave expands and ventricles began to expand
- Blood from aorta and pulmonary trunk briefly flows backward filling cusps and closing semilunar valves
-- creates pressure rebound that appears as dicrotic notch in graph of artery pressure
-- heart sounds occurs

Isovolumetric because

semilunar valves are closed and AV valves have not opened yet
- ventricles are therefore taking in no blood
when av valves open, ventricular fillings begins again