Cardiovascular System The Heart Chapter 20

Chapter 20
The Heart
• Located in the pericardial cavity, which…
• is in the mediastinum – a space defined by being
between sternum, lungs and spine.
• Also found in mediastinum are the thymus and
vena cava
Pericardial Sac
• Protective membrane surrounding heart – 3 divisions…
– Fibrous Layer – made up of tough, fibrous connective
tissue and irregular dense connective tissue
– Parietal Layer – inner border of fibrous pericardium,
made up of serous membrane
– pericardial space – contains serous fluid (lubrication)
– Visceral Layer – internal to parietal surrounds heart
(Layers of the Serous Pericardium)
Heart Wall
• Epidardium (visceral pericardium)– includes
simple squamous epithelium, loose connective
tissue and fat
• Myocardium – muscle (much of wall thickness)
• Endocardium – a layer of simple squamous
epithelium. (This is continuous with the
endothelium of blood vessels)
• Trabeculae Carnae – raised projections on the
inner surface of the heart wall
Heart “Skeleton”
• Fibrous connective tissue forms supporting
• rings surrounding heart valves
• notice arrangement of muscle attachment to the
rings – contraction causes heart to ‘shorten’ apex
to base
Cardiac Muscle
• Branching/Interconnected/multinucleate muscle
• ‘cells’ join at specialized sites called intercalated
– desmosome – mechanical connection (fibrous
anchoring takes up strain)
– gap junctions – allow ions to flow freely
between cells
– because of this interconnection heart functions
as a syncytium (a fused mass of cells)
Conduction System
• “Pacemakers” of the heart
• SinoAtrial (SA) node – located just toward the
midline from the superior vena cava
• Internodal pathway
• AtrioVentricular (AV) node – medial to the right
AV valve
• AV Bundle
• L/R bundle branches
• Purkinje fibers
Conducting System Function
• SA node pulses causing atria to contract
• Internodal pathway slows this signal ‘just a bit’ to
allow atria time to complete contraction
• Signal reaches AV node causing it to fire an
impulse down the…
• AV bundle (left & right branches)
• Signal reaches heart apex and travels back up
walls of ventricles causing them to contract
• Coordinating beating of Atria & Ventricles
• Heart muscle exhibits autorythmicity – beats on
its own
• Electrical Conduction System regulates beating
for efficiencey
• Since cardiac muscle fibers are interconnected
(remember intercalated disks?) would eventually
all contract.
• Two muscle masses – atria & ventricles (syncitia)
Cardiac Cycle
• Two distinct phases…
• Systole – contraction phase
• Diastole – relaxation phase
Description begins at end of ventricular diastole
with relaxed ventricles
Cardiac cycle … stepwise
• Ventricles are full – AV valves are open, semilunar
valves are closed
• Ventricles begin to contract – pressure rises
• Cuspid (AV) valves will be pushed closed by the
pressure wave
• brief ‘isometric’ contraction period with no blood
• sufficient pressure builds to open semilunar valves
• blood leaves left ventricle through aortic arch and
right ventricle through pulmonary aorta
Cardiac cycle continues
• Isometric relaxation of ventricles – no blood leaves
• Cuspid valves open and blood flows passibely from
atria into ventricles
• About 1/3 way through ventricular diastole atria
begin to contract (atrial systole) and remaining
blood is forced into ventricles.
• After Atrial systole, atria relax (diastole) and atria
fill passively.
Heart Sounds
• Lubb – low pitched sound of valves closing
• Dupp – higher pitch sound of aortic and
pulmonary valves closing
• Slush – (especially audible in thinner people) –
blood flowing turbulently into ventricles
• AP arrives – T-Tubules (transverse tubules) carry
signal from surface of cell to sarcoplasmic
• Loose association between SR and T-tubules
results in slow onset of contraction with
prolonged contraction phase…
• Na+ rushes in via t-tubules to SR causing Ca++ to
be expelled from SR stores
Contraction continued
• Ca++ proceeds to sarcoplasm (cytoplasm) where
it binds to troponin
• troponin changes shape and moves tropmyosin off
receptor sites… excitation occurs
• This portion is referred to as Rising Phase
• A second type of Ca++ channel opens in SR and
Ca++ pump turns off
The End.