Date post: | 02-Nov-2014 |
Category: |
Education |
Upload: | megan-lotze |
View: | 5 times |
Download: | 0 times |
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.