AU Biology Dept Spring 2008

Anatomy & Physiology II Lecture Outline

Text Saladin, K.S. 2007. Anatomy & Physiology: The Unity of Form and Function. 4th ed. McGraw-Hill, NY.

Chapter 19 The Circulatory System: The Heart

Memory Verse: "And I will give them a heart to know Me, for I am the Lord; and they will be My people, and I will be their God, for they will return to Me with their whole heart" NASB

A. Topics

1) Gross Anatomy of the Heart

2) Cardiac Muscle and the Cardiac Conduction System

3) Electrical and Contractile Activity of the Heart

4) Blood Flow, Heart Sounds, and the Cardiac Cycle

5) Cardiac Output

B. Lecture Outline

Introduction

a. Circulatory system: heart, blood and lymph vessels, blood and lymph.

b. Cardiovascular system: heart and blood vessels (fig 19.1).

2 major divisions:

i.Pulmonary circuit - right side of heart.

carries blood to lungs for gas exchange.

ii. Systemic circuit - left side of heart.

supplies blood to all organs of the body excl lungs.

1) Gross Anatomy of the Heart

Heart location - in the thoracic cavity within the mediastinum, between the lungs (fig 19.2).

Heart shape - Base - broad superior portion of heart.

Apex - inferior end, tilts to the left, tapers to point.

Heart size - 3.5 in. wide at base, 5 in. from base to apex and 2.5 in. anterior to posterior; weighs approx 300 g.

a. The heart is enclosed in a sac called the pericardium

The pericardium - allows heart to beat without friction:

1. Parietal pericardium – outer CT layer, thin inner serous layer.

2. Pericardial cavity – filled with 5-30 ml pericardial fluid.

3. Visceral pericardium (or epicardium of heart wall) – thin serous layer.

b. The heart wall consists of 3 layers: (fig 19.3)

1. Epicardium – visceral pericardium.

2. Myocardium – cardiac muscle and CT fibrous skeleton.

3. Endocardium – thin inner membrane continuous with endothelium.

c. The heart has 4 chambers: (fig 19.7)

1. Right and left atria - 2 superior, posterior chambers, receive blood returning to heart.

2. Right and left ventricles - 2 inferior chambers, pump blood into arteries.

Interatrial septum - wall that separates atria.

Interventricular septum - wall that separates ventricles.

d. The heart has 4 valves to ensure unidirectional blood flow: (figs 19.8 and 19.9)

1. Right AV valve (tricuspid).

2. Left AV valve (mitral, bicuspid).

Chordae tendineae - cords connect AV valves to papillary muscles.

3. Pulmonary semilunar valve.

4. Aortic semilunar valve.

e. The coronary circulation.

Blood vessels of heart wall nourish cardiac muscle.

Myocardial infarction (MI) is the sudden death of heart tissue caused by interruption of blood flow (ischemia) from narrowing or occlusion of a blood vessel.

2) Cardiac Muscle and the Cardiac Conduction System

a. Structure of Cardiac Muscle

Composed of short, thick, branched cells called myocytes, intercalated discs join myocytes end to end.

b. Cardiac Conduction System: (fig 19.12)

Myogenic - heartbeat originates within heart.

Autorhythmic - depolarizes spontaneously 70-80 times/min.

1. SA node – pacemaker potential, initiates depolarization, sets heart rate.

2. Fibrous skeleton insulates atria from ventricles.

3. AV node – electrical gateway to ventricles.

4. AV bundle and bundle branches – pathway for signals from AV node.

5. Purkinje fibers – spread upward from apex throughout ventricular myocardium.

3) Electrical and Contractile Activity of the Heart

Systole = contraction.

Diastole = relaxation.

a. Signal Conduction and Systole

Each depolarization creates one heartbeat.

SA node at rest fires at 0.8 sec, about 75 bpm = results in atrial systole.

AV node delays signal 100 msec, allows ventricles to fill.

AV bundle and Purkinje fibers distributes signal to ventricles = results in ventricular systole.

b. Electrocardiogram (ECG) (figs 19.16 and 19.17)

Composite of all action potentials of myocardial cells; detected, amplified, and recorded by electrodes on arms, legs, and chest.

1. P wave - SA node fires, atrial depolarization.

2. QRS complex - AV node fires, ventricular depolarization, atrial repolarization (signal obscured).

c. T wave - ventricular repolarization.

An ECG can reveal abnormalities in conduction pathways, MI, heart enlargement, and electrolyte and hormone imbalances.

4) Blood Flow, Heart Sounds, and the Cardiac Cycle

a. Heart Sounds

Auscultation - listening to sounds made by body.

First heart sound (S1), "lubb", occurs with closure of AV valves.

Second heart sound (S2), "dupp", occurs with closure of semilunar valves.

b. Phases of the Cardiac Cycle (fig 19.20)

1. Ventricular filling – AV valves open, atrial systole.

End-diastolic volume (EDV) of about 130 ml/ventricle.

2. Isovolumetric contraction of ventricles.

3. Ventricular ejection.

Stroke volume of about 70 ml/ventricle.

4. Isovolumetric relaxation of ventricles.

c. Rate of Cardiac Cycle

Atrial systole = 0.1 sec

Ventricular systole = 0.3 sec

Quiescent period = 0.4 sec

Total 0.8 sec, heart rate 75 bpm.

5) Cardiac Output (CO)

Volume of blood ejected by each ventricle in 1 minute:

CO = Heart rate (HR) x Stroke volume (SV)

Resting values, CO = 75 beats/min x70 ml/beat = 5,250 ml/min, usually about 4 to 6L/min.

Vigorous exercise ≠ CO to 21 L/min for fit person and up to 35 L/min for world class athlete.

a. Heart Rate

Infants have HR of 120 beats/minute (bpm) or more.

Young adult females avg. 72 - 80 bpm.

Young adult males avg. 64 to 72 bpm.

Tachycardia: persistent, resting adult HR > 100; stress, anxiety, drugs, heart disease or ≠ body temp.

Bradycardia: persistent, resting adult HR < 60; common in sleep and endurance trained athletes (≠ SV).

Chronotropic Effects

Positive chronotropic agents raise HR and negative chronotropic agents lower HR.

The cardiac center of the medulla oblongata holds a cardioacceleratory center (sympathetic) and a cardioinhibitory center (parasympathetic).

1. Cardioacceleratory center (sympathetic):

Stimulates cardiac accelerator nerves to SA node, AV node, and myocardium.

These nerves secrete norepinephrine.

CO peaks at HR of 160 to 180 bpm.

2. Cardioinhibitory center (parasympathetic):

Stimulates vagus nerves to SA node and AV node.

These nerves secrete acetylcholine.

Maximum vagal stimulation Ø HR as low as 20 bpm.

Inputs to cardiac center include higher brain centers, proprioreceptors, baroreceptors, chemoreceptors, drugs.

b. Stroke Volume

Stroke volume of the ventricles is governed by three factors:

1. Preload

≠ preload causes ≠ SV.

The amount of tension in ventricular myocardium before it contracts determined by venous return (exercise ≠ venous return, stretches myocardium).

Frank-Starling law of heart - SVµ EDV.

2. Contractility

≠ contractility causes ≠ SV.

Contraction force for a given preload.

3. Afterload

≠ afterload Ø SV.

Pressure in arteries above semilunar valves opposes opening of valves.

Any impedance in arterial circulation ≠ afterload.

The Cutting Edge:

Engineering heart valves