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)

Sherwood’s Human Physiology 9-5 (9-4 6th Edition)

Human Heart Anatomy

Sherwood’s Human Physiology 9-4 (9-3 6th Edition)

Valve Physiology• Valves are mechanical devices and

function in response to blood flow.

• 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.

P1 P2

P1 P2

Guyton’s Textbook of Medical Physiology 4-1

Ion Concentrations

• Inside a cell

– K+

• Outside a cell

– Na +

– Ca +

– Cl-

Guyton’s Textbook of Medical Physiology 4-2

Cell Membrane Transport

• Passive

– Simple diffusion

– Facilitated diffusion

• Active (needs energy)

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

Sherwood’s Human Physiology 9-11 (9-8 6th Edition)

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.

Guyton’s Textbook of Medical Physiology 10-3

Conduction Delay

• Atrioventricular fibrous tissue– Acts as an insulator

Sherwood’s Human Physiology 9-11 (9-8 6th Edition)

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 9-10 (9-7 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 voltage-gated Ca++ channel.

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

Guyton’s Textbook of Medical Physiology 10-4 Sherwood’s Human Physiology 9-15

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.

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)

Guyton’s Textbook of Medical Physiology 10-2

SA node potentials vs. cardiac cell potentials

• SA node has a higher resting potential than other cardiac muscle cells.

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 contraction1. Conduction (normal and abnormal)2. Pacemakers3. Action potentials4. Refractory period

4. Describe the ECG.1. P Wave, QRS Complex, T Wave, Abnormal ECG