20-1

Chapter 20

Cardiovascular System

The HeartThe Heart

20-2

The Heart• The Heart is two pumps.

– Pulmonary circulation• Carries blood to lungs

• Returns to the left side of heart

– Systemic circulation• Delivers oxygen and

nutrients to the body

• Returns to the right side of the heart

20-3

Functions of the Heart

• Generating blood pressure• Routing blood

– Heart separates pulmonary and systemic circulations

– Ensures oxygenation of blood flowing to tissues.

• Ensuring one-way blood flow– Heart valves ensure one-way flow

• Regulating blood supply– Changes in contraction rate and force match

blood delivery to changing metabolic needs

20-4

Size, Shape, Location of the Heart

• Size of a closed fist• Shape

– Apex: rounded tip of heart

– Base: Flat part at opposite of end of cone

• Located in thoracic cavity within the mediastinum

20-5

Heart Cross Section

20-6

Pericardium

20-7

Heart Wall

• Three layers of tissue– Epicardium: This serous membrane of smooth

outer surface of heart– Myocardium: Middle layer composed of

cardiac muscle cell and responsibility for heart contracting

– Endocardium: Smooth inner surface of heart chambers

20-8

Heart Wall

20-9

External Anatomy• Four chambers

– 2 atria– 2 ventricles

• Auricles• Major veins

– Superior vena cava– Inferior vena cava– Pulmonary veins

• Major arteries– Aorta– Pulmonary trunk

20-10

External Anatomy

20-11

Coronary Circulation

20-12

Heart Valves

• Atrioventricular– Tricuspid

– Bicuspid or mitral

• Semilunar– Aortic

– Pulmonary

• Prevent blood from flowing back

20-13

Heart Valves

20-14

Function of the Heart Valves

20-15

Blood Flow Through Heart

20-16

Systemic and PulmonaryCirculation

20-17

Heart Skeleton

• Consists of plate of fibrous connective tissue between atria and ventricles

• Fibrous rings around valves to support

• Serves as electrical insulation between atria and ventricles

• Provides site for muscle attachment

20-18

Cardiac Muscle

• Elongated, branching cells containing 1-2 centrally located nuclei

• Contains actin and myosin myofilaments

• Intercalated disks: Specialized cell-cell contacts

• Desmosomes hold cells together and gap junctions allow action potentials

• Electrically, cardiac muscle behaves as single unit

20-19

Conducting System of Heart

20-20

Electrical Properties• Resting membrane potential (RMP) present

– Depends on low permiability to Na+ and Ca++ and high permiability to K+.

• Action potential– Depolarization (Fast voltage gated Na+ channels)

– Early partial repolarization (Voltage gated Na+ channels close and a small number of K+ channels open.)

– Plateau phase - Prolonged period of slow repolarization (Slow Ca++ channels are open)

– Rapid final repolarization phase (Voltage gated Ca++ channels close and more K+ channels open)

20-21

Action Potentials inSkeletal and Cardiac Muscle

20-22

SA Node Action PotentialPermiability changes in the pacemaker cells (Autorhythmicity):1. Prepotential

• Small number of Na+ channels open

• Voltage-gated K+ channels are closing

• Voltage-gated Ca++ channels begin to open

2. Depolarization Phase• Voltage-gated Ca++ channels are

open• Voltage-gated K+ channels are

closed.3. Repolarization phase

• Voltage-gated Ca++ channels close.

• Voltage-gated K+ channels open.

Note: Ca++ channel blockers like verapamil are used to treat tachycardia and arrhythmias.

20-23

Refractory Period

• Absolute: Cardiac muscle cell completely insensitive to further stimulation

• Relative: Cell exhibits reduced sensitivity to additional stimulation

• Long refractory period prevents tetanic contractions

20-24

Electrocardiogram

• Action potentials through myocardium during cardiac cycle produces electric currents than can be measured

• Pattern– P wave

• Atria depolarization

– QRS complex• Ventricle depolarization

• Atria repolarization

– T wave: • Ventricle repolarization

20-25

Cardiac Arrhythmias

• Tachycardia: Heart rate in excess of 100bpm• Bradycardia: Heart rate less than 60 bpm• Sinus arrhythmia: Heart rate varies 5%

during respiratory cycle and up to 30% during deep respiration

• Premature atrial contractions: Occasional shortened intervals between one contraction and succeeding, frequently occurs in healthy people

20-26

Alterations in Electrocardiogram

20-27

Cardiac Cycle

• Heart is two pumps that work together, right and left half

• Repetitive contraction (systole) and relaxation (diastole) of heart chambers

• Blood moves through circulatory system from areas of higher to lower pressure.– Contraction of heart produces the pressure

20-28

Cardiac Cycle

20-29

Cardiac Cycle

20-30

Heart Sounds• First heart sound or “lubb”

– Atrioventricular valves and surrounding fluid vibrations as valves close at beginning of ventricular systole

• Second heart sound or “dupp”– Results from closure of aortic and pulmonary semilunar

valves at beginning of ventricular diastole, lasts longer

• Third heart sound (occasional)– Caused by turbulent blood flow into ventricles and detected

near end of first one-third of diastole

• Clinical considerations:– Murmurs: abnormal heart sounds– Incompetent valve - leaks excessively (gurguling,swishing)– Stenosis: abnormally small valve openings (rushing sound before valve closes.– Causes: genetic or due to rhematic fever scaring or myocardial infarction

affecting the papillary muscles.

20-31

Location of Heart Valves

20-32

Mean Arterial Blood Pressure• Stroke volume

– Volume of blood pumped during each cardiac cycle• at rest ~70 ml• can increase to ~200 ml during exercise

• Heart Rate– ~72 BPM and can increase to over 120 BPM during exercise.

• Cardiac Output =stroke volume X heart rate– Resting: 72 beats/min X 70 ml/beat = 5040 ml / min– Exercise: 120 beats/min X 200 ml/beat = 24,000 ml/min.

• Cardiac reserve – max cardiac output - resting cardiac output– = 24,000-5040ml /min = 18960 ml– C.O. is the major factor in determining blood pressure.

• Blood pressure– Blood pressure reflects pressure changes in the aorta not the ventricle– Normal BP 120 systolic / 80 diastolic

20-33

Factors Affecting MAP

20-34

Regulation of the Heart• Intrinsic regulation: Results from normal

functional characteristics, not on neural or hormonal regulation– Starling’s law of the heart– Stretching of the SA node.

• Extrinsic regulation: Involves neural and hormonal control– Parasympathetic stimulation

• Supplied by vagus nerve, decreases heart rate, acetylcholine secreted

– Sympathetic stimulation• Supplied by cardiac nerves, increases heart rate and force of

contraction, epinephrine and norepinephrine released

20-35

Heart Homeostasis• Effect of blood pressure

– Baroreceptors monitor blood pressure

• Effect of pH, carbon dioxide, oxygen– Chemoreceptors monitor

• Effect of extracellular ion concentration– Increase or decrease in extracellular K+ decreases heart

rate

• Effect of body temperature– Heart rate increases when body temperature increases,

heart rate decreases when body temperature decreases

20-36

Baroreceptor and ChemoreceptorReflexes

20-37

Baroreceptor Reflex

20-38

Chemoreceptor Reflex-pH

20-39

Effects of Aging on the Heart

• Gradual changes in heart function, minor under resting condition, more significant during exercise

• Hypertrophy of left ventricle

• Maximum heart rate decreases

• Increased tendency for valves to function abnormally and arrhythmias to occur

• Increased oxygen consumption required to pump same amount of blood