human physiology by lauralee sherwood ©2007 brooks/cole-thomson learning chapter 9 cardiac...

Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning

Chapter 9Cardiac Physiology

Chapter 9 Cardiac PhysiologyHuman Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning

Circulatory System

• Three basic components

– Heart• Serves as pump that establishes the pressure gradient

needed for blood to flow to tissues

– Blood vessels• Passageways through which blood is distributed from

heart to all parts of body and back to heart

– Blood • Transport medium within which materials being

transported are dissolved or suspended


Circulatory System

• Pulmonary circulation

– Closed loop of vessels carrying blood between heart and lungs

• Systemic circulation

– Circuit of vessels carrying blood between heart and other body systems


Circulatory System

• Anatomy of the heart

– Hollow, muscular organ about the size of a clenched fist

– Positioned between two bony structures – sternum and vertebrae

• Position makes it physically possible to manually drive blood from heart when it is not pumping effectively (CPR)


Circulatory System

• Heart

• Dual pump

– Right and left sides of heart function as two separate pumps

– Divided into right and left halves and has four chambers

• Atria– Upper chambers– Receive blood returning to heart and transfer it to lower

chambers

• Ventricles – Lower chambers which pump blood from heart


Circulatory System

• Heart

– Arteries• Carry blood away from ventricles to tissues

– Veins• Vessels that return blood from tissues to the atria

– Septum • Continuous muscular partition that prevents mixture of

blood from the two sides of heart


Circuit of Blood Flow

• The venae cavae are veins returning blood to the right atrium. Oxygen has been extracted from this blood. Carbon dioxide has been added to it. This blood is pumped from the right ventricle through the pulmonary artery to the lungs.

• The lungs add oxygen to this blood received from the right side of the heart. Carbon dioxide is removed from this blood. This blood flows through pulmonary veins to the left atrium of the heart. This oxygen rich blood is pumped from the left ventricle through the aorta, a large artery.


Blood Flow Through and Pump Action of the Heart


Heart Valves

• Atrioventricular (AV) valves– Right and left AV valves are positioned between

atrium and ventricle on right and left sides– Prevent backflow of blood from ventricles into

atria during ventricular emptying– Right AV valve

• Also called tricuspid valve

– Left AV valve• Also called bicuspid valve or mitral valve

– Chordae tendinae• Fibrous cords which prevent valves from being everted• Papillary muscles


Heart Valves

• Semilunar valves

– Aortic and pulmonary valves

– Lie at juncture where major arteries leave ventricles

– Prevented from everting by anatomic structure and positioning of cusps

• No valves between atria and veins

– Reasons• Atrial pressures usually are not much higher than

venous pressures

• Sites where venae cavae enter atria are partially compressed during atrial contraction


Heart Valves


Heart Wall

• Consists of three distinct layers

– Endothelium• Thin inner tissue

• Epithelial tissue which lines entire circulatory system

– Myocardium• Middle layer

• Composed of cardiac muscle

• Constitutes bulk of heart wall

– Epicardium • Thin external layer which covers the heart


Cardiac Muscle Fibers

• Interconnected by intercalated discs and form functional syncytia

• Within intercalated discs – two kinds of membrane junctions

– Desmosomes

– Gap junctions


• Heart is enclosed by pericardial sac

• Pericardial sac has two layers

– Tough, fibrous covering

– Secretory lining• Secretes pericardial fluid

– Provides lubrication to prevent friction between pericardial layers

• Pericarditis

– Inflammation of pericardial sac


Electrical Activity of Heart

• Heart beats rhythmically as result of action potentials it generates by itself (autorhythmicity)

• Two specialized types of cardiac muscle cells

– Contractile cells• 99% of cardiac muscle cells

• Do mechanical work of pumping

• Normally do not initiate own action potentials

– Autorhythmic cells• Do not contract

• Specialized for initiating and conducting action potentials responsible for contraction of working cells



• Locations of noncontractile cells capable of autorhymicity– Sinoatrial node (SA node)

• Specialized region in right atrial wall near opening of superior vena cava

• Pacemaker of the heart– Atrioventricular node (AV node)

• Small bundle of specialized cardiac cells located at base of right atrium near septum

– Bundle of His (atrioventricular bundle)• Cells originate at AV node and enters interventricular septum• Divides to form right and left bundle branches which travel

down septum, curve around tip of ventricular chambers, travel back toward atria along outer walls

– Purkinje fibers• Small, terminal fibers that extend from bundle of His and

spread throughout ventricular myocardium


Specialized Conduction System of Heart



• Cardiac impulse originates at SA node

• Action potential spreads throughout right and left atria

• Impulse passes from atria into ventricles through AV node (only point of electrical contact between chambers)

• Action potential briefly delayed at AV node (ensures atrial contraction precedes ventricular contraction to allow complete ventricular filling)

• Impulse travels rapidly down interventricular septum by means of bundle of His

• Impulse rapidly disperses throughout myocardium by means of Purkinje fibers

• Rest of ventricular cells activated by cell-to-cell spread of impulse through gap junctions


Spread of Cardiac Excitation



• Atria contract as single unit followed after brief delay by a synchronized ventricular contraction

• Action potentials of cardiac contractile cells exhibit prolonged positive phase (plateau) accompanied by prolonged period of contraction

– Ensures adequate ejection time

– Plateau primarily due to activation of slow L-type Ca2+ channels



• Ca2+ entry through L-type channels in T tubules triggers larger release of Ca2+ from sarcoplasmic reticulum

– Ca2+ induced Ca2+ release leads to cross-bridge cycling and contraction


Excitation-Contraction

Coupling in Cardiac Contractile Cells



• Because long refractory period occurs in conjunction with prolonged plateau phase, summation and tetanus of cardiac muscle is impossible

– Ensures alternate periods of contraction and relaxation which are essential for pumping blood


Relationship of an Action Potential and the Refractory Period to the Duration of the Contractile Response in Cardiac Muscle


Electrocardiogram (ECG)

• Record of overall spread of electrical activity through heart

• Represents

– Recording part of electrical activity induced in body fluids by cardiac impulse that reaches body surface

• Not direct recording of actual electrical activity of heart

– Recording of overall spread of activity throughout heart during depolarization and repolarization

• Not a recording of a single action potential in a single cell at a single point in time

– Comparisons in voltage detected by electrodes at two different points on body surface, not the actual potential

• Does not record potential at all when ventricular muscle is either completely depolarized or completely repolarized


Electrocardiogram (ECG)

Different parts of ECG record can be correlated to specific cardiac events


Abnormalities in Rate

• Tachycardia

– Rapid heart rate of more than 100 beats per minute

• Bradycardia

– Slow heart rate of fewer than 60 beats per minute


Abnormalities in Rhythm

• Rhythm

– Regularity or spacing of ECG waves

• Arrhythmia

– Variation from normal rhythm and sequence of excitation of the heart

– Examples• Atrial flutter

• Atrial fibrillation

• Ventricular fibrillation

• Heart block


Cardiac Myopathies

• Damage of the heart muscle

– Myocardial ischemia• Inadequate delivery of oxygenated blood to heart tissue

– Necrosis• Death of heart muscle cells

– Acute myocardial infarction (heart attack)• Occurs when blood vessel supplying area of heart

becomes blocked or ruptured


Representative Heart Conditions Detectable Through ECG


Cardiac Output

• Volume of blood ejected by each ventricle each minute

• Determined by

heart rate times

stroke volume


Cardiac Output

• Heart rate is varied by altering balance of parasympathetic and sympathetic influence on SA node

– Parasympathetic stimulation slows heart rate

– Sympathetic stimulation speeds it up


Cardiac Output

• Stroke volume

– Determined by extent of venous return and by sympathetic activity

– Influenced by two types of controls• Intrinsic control

• Extrinsic control

– Both factors increase stroke volume by increasing strength of heart contraction


Frank-Starling Law of the Heart

• States that heart normally pumps out during systole the volume of blood returned to it during diastole


Nourishing the Heart Muscle

• Muscle is supplied with oxygen and nutrients by blood delivered to it by coronary circulation, not from blood within heart chambers

• Heart receives most of its own blood supply that occurs during diastole

– During systole, coronary vessels are compressed by contracting heart muscle

• Coronary blood flow normally varies to keep pace with cardiac oxygen needs


Coronary Artery Disease (CAD)

• Pathological changes within coronary artery walls that diminish blood flow through the vessels

• Leading cause of death in United States

• Can cause myocardial ischemia and possibly lead to acute myocardial infarction

– Three mechanisms• Profound vascular spasm of coronary arteries

• Formation of atherosclerotic plaques

• Thromboembolism


Possible Outcomes of Acute Myocardial Infarction (Heart Attack)

Cardiac Anatomy

Cardiac Cycle

Conduction

human physiology by lauralee sherwood ©2007 brooks/cole-thomson learning chapter 9 cardiac...

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