Chapter 9

Chapter 9

The Mammalian Heart

mammalian heart The mammalian heart has a mass

of about 300 g (size of fist) composed of cardiac muscle What’s unique about cardiac muscle?

cardiac muscle cardiac muscle made of

interconnecting cells plasma membranes very tight to

facilitate passing of waves of electrical excitation

nucleated cells with striated fibers

Heart Flow

Heart structure largest arching blood vessel –

aorta aorta branches, upwards towards

the head and the mainflow doubling downwards to the rest of the body (descending aorta)

pulmonary artery – blood vessel leaving the heart with two branches leading to each lung

Heart structure cont’d pulmonary veins – bring blood from

the left and right lungs vena cava – two large veins merge,

bringing blood downwards from the head (superior vc) and upwards (inferior vc) from the rest of the body

coronary arteries - branch from the aorta delivering oxygenated blood

Heart structure cont’d septum – wall of muscle that divides

left side and right sides of the heart, blood cannot pass though the septum

four chambers – two on the left, two on the right

atrium (auricle) – upper chamber each side, both receive blood from the veins

Heart structure cont’d right atrium – receive blood from

the vena cavae left atrium – receive blood from

the pulmonary veins ventricles – blood flows into the

ventricles from the atria, then is squeezed out into the arteries

atrio-ventricular valves mitral or bicuspid – between the

left atrium and ventricle tricuspid – between right atrium

and ventricle

The Cardiac Cycle cardiac cycle – sequence of

events that makes up one heart beat

normal heart pulse rate – 70 beats per minute

atrial systole atrial spaces fill with blood and the

muscles of the atrial walls contract low pressure on this contraction forces blood through the atrio-

ventricular valves semilunar valves prevent backflow atrial muscle walls are thin

ventricular systole 0.1 sec after the atria contract the

ventricle contract lasts about 0.3 sec ventricles thick muscle ventricles squeeze inward on the

blood increasing the pressure, pushing it out of the heart

ventricular systole blood leaves the ventricles through the

aorta and pulmonary artery pressure in the ventricle becomes

greater than the atria and pushes the atrio-ventricular valves shut,

papillary muscle – attached to the valves by tendons (Chordae tendineae), prevents the valves from being forced inside out

ventricular diastole

ventricle muscle relax pressure in the ventricles drops blood in the arteries puts pressure on

the cusps of the semilunar valves forcing them shut preventing backflow

diastole whole of the heart muscle relaxes even though the pressure of the blood

in the veins is low, the blood fills the atria as their thin walls distend

diastole some blood can trickle through the atrio-

ventricular valves into the ventricles atrial muscle contract and the cycle

begins again

Why is the left ventricular wall thicker than the right?

The left ventricle must develop sufficient force to push blood around the rest of the body

The right ventricle pushes blood to the lungs, this requires much less pressure, therefore the right ventricle wall is thinner

Control of the Heart Beat myogenic – naturally contracts and

relaxes without receiving nerve impulses

cardiac cells grown in oxygenated nutrient solution will rhythmically contract and relax all by themselves

What if all the cardiac cells of the heart contracted at their own rhythms?

Control of the Heart Beat Heart has its own built-in controlling and

coordinating system Sinoatrial node – SAN – pacemaker –

specialized patch of muscle in the wall of the right atrium

Muscle cells of the SAN – set the rhythm for all the other cardiac cells

Control of the Heart Beat SAN muscles – contract slightly faster

than the rest of the heart muscle Set up a wave of electrical activity –

wave spreads out rapidly over the whole atrial walls

Atrial wall cardiac muscle – responds to this excitation wave by contracting at the same rhythm as the SAN

Control of the Heart Beat Both atria – muscle cells almost contract

simultaneously As the wave spreads Atrio-ventricular node – AVN – patch of

conducting fibers in the septum - the AVN picks up the excitation wave as it

spreads across the atria Atrioventricular fibrous tissue - Band of

fibers between the atria and ventricles that do not conduct the excitation wave

Purkyne tissue (Purkinje fibers) after a delay of 0.1 s, the excitation wave is

passed on to a bunch of conducting fibers that run down the septum, the Purkyne tissue

Purkyne tissue – transmits the excitation wave rapidly to the base of the septum where it spreads out through the ventricle walls

The excitation – causes the ventricle walls to contract from the bottom up, squeezing blood upwards and into the arteries

Healthy Heart atria contract then the ventricles, from the bottom up Lub-dub What if the coordination of contraction

goes wrong?

Fibrillation Fibrillation – heart flutters rather than

contracting as a whole and relaxing as a whole

Must be treated instantly or could be fatal

Electric shock often used

Electrocardiograms (ECG) Electrocardiograms (ECG) – graph of

voltage against time P – represents the wave of excitation

sweeping over the atrial walls Q, R, & S – represent the wave of

excitation in the ventricle walls T – indicates the recovery of the

ventricle walls

Electrocardiograms (ECG)

How to Read an EKG Strip

EKG paper is a grid where time is measured along the horizontal axis.

* Each small square is 1 mm in length and represents 0.04 seconds.

* Each larger square is 5 mm in length and represents 0.2 seconds.

Voltage is measured along the vertical axis.

* 10 mm is equal to 1mV in voltage. * The diagram below illustrates the

configuration of EKG graph paper and where to measure the components of the EKG wave form

Heart rate Heart rate can be easily calculated from the

EKG strip:

* When the rhythm is regular, the heart rate is 300 divided by the number of large squares between the QRS complexes. For example, if there are 4 large squares between

regular QRS complexes, the heart rate is 75 (300/4=75).

Heart rate * The second method can be used with

an irregular rhythm to estimate the rate. Count the number of R waves in a 6 second strip and multiply by 10. For example, if there are 7 R waves in a 6

second strip, the heart rate is 70 (7x10=70).

This dysrhythmia results in the absence of cardiac output.

Almost always occurs with serious heart disease, especially acute MI.

The course of treatment for ventricular fibrillation includes:

* immediate defibrillation and ACLS protocols.

Atrial fibrillation may occur paroxysmally, but it often becomes chronic. It is usually associated with COPD, CHF or other heart disease.Treatment includes: * Digoxin, diltiazem, or other anti-dysrhythmic medications to control the AV conduction rate and assist with conversion back to normal sinus rhythm. * Cardioversion (shocking simultaneously with the QRS) may also be necessary to terminate this rhythm.