Aims
• Introduction to the heart.
• Heart contraction and electrical conduction.
• Readings; Sherwood, Chapter 9
Cardiovascular Physiology
• Cardiac Muscle
– Myocytes (cardiac muscle cells)
– Myocytes are connected to each
other via __Intercalated
Disks__
• Composed of Desmosomes and
Gap Junctions
• Allow waves of action
potentials to spread from one
cell to the next (Syncytium).
Sherwood’s Human Physiology 9-8 (9-6 6th Edition)
Human Heart Anatomy
Sherwood’s Human Physiology 9-5 (9-4 6th Edition)
Valve Physiology
• Valves are mechanical devices and
function in response to blood flow.
P1
P1
P2
P2
• They function on the principle that
they stay open as long as the blood
pressure is greater in the emptying
chamber.
• They close as soon as the blood
pressure is greater in the filling
chamber, thus blood flow is
unidirectional.
Sherwood’s Human Physiology 9-4 (9-3 6th Edition)
Ion Concentrations
• Inside a cell
– K+
• Outside a cell
– Na +
– Ca +
– Cl-
Guyton’s Textbook of Medical Physiology 4-1
Cell Membrane Transport
• Passive
– Simple diffusion
– Facilitated diffusion
• Active (needs energy)
Guyton’s Textbook of Medical Physiology 4-2
Two Types of Cardiac Cells
• Autorhythmic Cells
– __non-contractile_________
– Initiate and conduct action potentials responsible
for contraction.
– Located in the SA node, AV node, Bundle of His,
Purkinje fibers.
• Contractile Cells
– 99% of the cardiac muscle cells
Specialized Conduction System
• Sinoatrial (SA)
node.
• _Pacemaker____
• Cells exhibit
autorhythmicity.
Sherwood’s Human Physiology 9-11 (9-8 6th Edition)
Specialized Conduction System
• Atrioventricular
(AV) node
• Delays electrical
signal due to a
decreased number of
gap junctions.
Sherwood’s Human Physiology 9-11 (9-8 6th Edition)
Conduction Delay
• Atrioventricular fibrous tissue
– Acts as an insulator
Guyton’s Textbook of Medical Physiology 10-3
Specialized Conduction System
• Atrioventricular (AV)
bundle or Bundle of
His.
• Transmits electrical
signal down to the
ventricles.
• Purkinje fibers
• Send action potential
through ventricles.
• Has increased number of
gap junctions.
Sherwood’s Human Physiology 9-11 (9-8 6th Edition)
Pacemaker Potential
• Decreased K+ efflux
and constant Na+
influx via leak
channels.
– Resulting in a
higher resting
potential.
• Slow Ca++ inward
permeability via
transient voltagegated Ca++ channel.
Sherwood’s Human Physiology 9-10 (9-7 6th Edition)
Pacemaker Potential
• Ca++ inward
permeability via
longer lasting
voltage-gated Ca++
channel.
• Resulting in __faster
depolarization___
Sherwood’s Human Physiology 9-10 (9-7 6th Edition)
Pacemaker Potential
• K+ outward
permeability via
voltage-gated
channel.
• Resulting in
repolarization
Sherwood’s Human Physiology 9-10 (9-7 6th Edition)
Pacemakers
• SA node (normal pacemaker)
– 70-80 action potentials per minute.
• Ectopic Pacemakers
• AV node
– 40-60 action potentials per minute.
• Bundle of His and purkinje fibers
– 20-40 action potentials per minute.
Pacemakers
Sherwood’s Human Physiology 9-12 5th Edition only
Abnormal Conduction Pathway
1. Normal
-SA node sets the pace
2. SA Node non-functional
-AV node sets the pace at a slower rate
3. AV Node non-functional
-Atria contract at SA node rate while ventricles
contract at Purkinje fiber rate (much slower)
-Complete heart block that requires an artificial
pacemaker.
4. Purkinje fiber is hyper-excitable
- Called an ectopic focus that causes a
premature beat.
Guyton’s Textbook of Medical Physiology 10-4
Sherwood’s Human Physiology 9-15
Abnormal Conduction Pathway
Guyton’s Textbook of Medical Physiology 10-4
Sherwood’s Human Physiology 9-15
Cardiac Muscle Cell Action Potential
• Depolarization
– Na+ inward
• Plateau
– Ca++ inward
• Repolarization
– K+ outward
Sherwood’s Human Physiology 9-15 (9-11 6th Edition)
SA node potentials vs. cardiac cell
potentials
• SA node has a higher resting potential than other
cardiac muscle cells.
Guyton’s Textbook of Medical Physiology 10-2
ECC in Cardiac Muscle
• The majority of
Ca++ required for
contraction comes
from the
sarcoplasmic
reticulum and not
the ECF.
Sherwood’s Human Physiology 9-16 (9-12 6th Edition)
Refractory Period
• Long refractory
period
important
because it
makes tetanus
impossible.
Sherwood’s Human Physiology 9-17 (9-12 6th Edition)
Requirements for Efficient
Cardiac Contraction
1. Atrial excitation and contraction need to be
complete before ventricular contraction
occurs.
2. Excitation of cardiac muscle fibers should be
coordinated so that each chamber contracts
as a unit.
3. Pair of atria and pair of ventricles should be
coordinated so that both members of the pair
contract simultaneously.
What is an Electrocardiogram
(ECG or EKG)?
• It is not a direct recording of the actual electrical
activity of the heart.
• It measures the portion of the electrical activity
of the heart that is transduced in the body fluids
and reaches the body surface.
• It is a complex recording that represents the
overall activity throughout the heart during
depolarization and repolarization.
– Not a single cell measurement
ECG
• 6 body & 6 chest leads for a total of 12 leads
Sherwood’s Human Physiology 9-18 (9-14 6th Edition)
ECG
•
•
•
•
•
•
P wave- atrial depolarization
PR segment- AV nodal delay
QRS complex- ventricular depolarization and atrial repolarization
ST segment- ventricular contraction and emptying
T wave- ventricular repolarization
TP interval- ventricular filling
Sherwood’s Human Physiology 9-19 (9-15 6th Edition)
Abnormal ECG
• Rate Abnormalities
– Tachycardia (>100
beats/min.)
• Rhythm Abnormalities
(Arrhythmias)
– Extrasystole (premature
beat)
– Ventricular fibrillation
– Atrial fibrillation
– Complete Block
• Cardiac Myopathies
– Myocardial Infarction
Sherwood’s Human Physiology 9-20 (9-16 6th Edition)
Next Time
• Cardiac Cycle
• Cardiac regulation
– Extrinsic vs. intrinsic
• Reading; Sherwood, Chapter 9
Objectives
1. Describe the structure and function of cardiac
myocytes.
2. Describe the anatomy of the heart and how
blood flows through it.
3. Describe cardiac contraction
1.
2.
3.
4.
Conduction (normal and abnormal)
Pacemakers
Action potentials
Refractory period
4. Describe the ECG.
1. P Wave, QRS Complex, T Wave, Abnormal ECG