Associate Degree Nursing Physiology Review

Cardiovascular System (Heart)

Content

Circulatory SystemFunctions of the HeartBlood flow Through the HeartCardiac Muscle CellsIntrinsic Conduction SystemCardiac impulseExcitation-ContractionConduction PathwayElectrocardiogramCardiac CycleHeart SoundsCardiac OutputFactors Affecting Cardiac Output-- Preload-- --Contractility-----AfterloadRegulation of the HeartPrimary control factors of the heartCongestive Heart Failure

Circulatory System

The parts of the circulatory system include:

Functions of the Heart

Pulmonary Circulation

Systemic Circulation

Blood flow Through the Heart

1. Superior/Inferior vena cava and coronary sinus2. Rt Atrium 3. Tricuspid valve 4. Rt ventricle5. Pulmonary semilunar valve 6. Pulmonary trunk 7. Right and left pulmonary arteries 8. Lungs7. Pulmonary veins 8. Lt atrium 9. Bicuspid valve (mitral) 10. Lt ventricle 11. Aortic semilunar valve12. Ascending aorta 13. Coronary arteries 14. Arch of aorta

Click here for an animation that summarizes the flow of blood through the heart.

Cardiac Muscle Cells

Cardiac Cell Histology

Intrinsic Conduction System

Autorhythmic cells depolarize spontaneously but at different rates:

Sinoatrioal (SA) node intrinsic rate

70-80

Atrioventricular (AV) node

40-60

Atrioventricular bundles

20-40

Purkinje fibers

20-40

Sinoatrial (SA) Node

Click here for an animation on the conduction system of the heart.

The animation is followed by practice questions.

Pacemaker Potential

Cycle

1. Normal RMP at -60mV 2. Decreased efflux of K+, membrane permeability decreases between APs, they slowly close at negative potentials3. Constant influx of Na+, no voltage-gated Na + channels4. Gradual depolarization because K+ builds up and Na+ flows inward5. As depolarization proceeds Ca++ channels (Ca2+ T) open before threshold is reached 6. Slow influx of Ca++ further depolarizes bringing to threshold (-40mV)7. At threshold sharp depolarization due to activation of Ca2+ L channels allow large influx of Ca++8. Falling phase at about +20 mV the Ca-L channels close, voltage-gated K channels open 9. Repolarization due to normal K+ efflux 10. At -60mV K+ channels close

Action Potential of Contractile Cardiac Cells

Cardiac impulse

A wave of depolarization that begins in the pacemaker and spreads over the heart (it precedes contraction)

Summary

Why a longer AP in cardiac contractile fibers?

Excitation-Contraction Coupling in Cardiac Contractile Cells

Electrical Signal Flow - Conduction Pathway

Electrocardiogram (ECG or EKG)

Cardiac Cycle - Filling of Heart Chambers

Click here for an animation on the cardiac cycle of the heart.

The animation is followed by practice questions. Click here for even more practice questions.

Heart Sounds

Cardiac Output (CO) and Reserve

Cardiac output (CO) is the amount of blood ejected by the left ventricle into the aorta per minute.

CO = stroke volume (amount of blood ejected with each systole) x beats per minute.

End Diastolic Volume (EDV) End Systolic Volume (ESV) = SV ex. 135- 65 = 70 ml, Residual (about 50%)

Stroke Volume (SV) is the amount of blood pumped out by a ventricle with each beat

Cardiac Output ~ 5L: (70 beats/m X 70 ml/beat = 4900 ml)

Cardiac Reserve - Cardiac reserve is the difference between resting and maximal CO - ratio between the maximum cardiac output a person can achieve and the cardiac output at rest. Normally the figure is four to five times the resting output. maximum CO ~ 25 L/min

Factors Affecting Cardiac Output

1. Preload Stretch - Frank - Starling law - the greater the stretch on cardiac fibers just before they contract (draws myosin fibers closer together) increases their force of contraction, the more blood is ejected from the ventricle the heart is filled during diastole the greater the force of contraction

2. Contractility - strength of a contraction at any give preload. Substances that increase contractility (positive inotropic agents) include hormones (glucagons, thyroxine), catecholemines (epinephrine and norepinephrine), drugs (digitalis) and increased calcium concentration in the extracellular fluid.

3. Afterload - the pressure in the large arteries leaving the heart that must be overcome before the aortic semilunar valve can open. Increased afterload results in decreased stroke volume.

Preload, or degree of stretch, of cardiac muscle cells before they contract is the critical factor controlling stroke volume

Slow heartbeat and exercise increase venous return to the heart, increasing SV

Blood loss and extremely rapid heartbeat decrease SV

Contractility and Norepinephrine - sympathetic stimulation releases norepinephrine and initiates a cyclic AMP second-messenger system

Regulation of the Heart

Intrinsic regulation: Results from normal functional characteristics - Starlings law of the heart

Frank- Starlings Law

Extrinsic regulation:

- Parasympathetic stimulation normally dominates resting heart rate- Parasympathetic nerves include the right and left Vagus (X) nerves. These fibers innervate the SA node, AV node and the atrial myocardium, releasing acetylcholine which decreases heart rate.

There is always a balance between sympathetic and parasympathetic stimulation of the heart, but the parasympathetic dominates at rest.

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Associate Degree Nursing Physiology Review