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THE HEART

INTRODUCTION

1. The circulatory system can be subdivided into pulmonary and systemic circuits, each beginning and ending at the heart.

2. The heart has two atria and two ventricles, and their activities are closely coordinated.

GROSS AND FUNCTIONAL ANATOMY

Location and External Appearance

1. The heart lies within the pericardial cavity of the mediastinum.

2. The parietal pericardium forms the outer wall of the pericardial cavity; the visceral pericardium (epicardium) covers the outer surface of the heart.

3. The pericardial cavity contains a small amount of pericardial fluid that prevents friction between the parietal and pericardial membranes.

4.Prominent features of the heart include the base, apex, and the superior, right, left, and inferior borders, and the anterior (sternocostal) and diaphragmatic surfaces.

5.The auricles project above the coronary sulcus that marks the division between the atria and the ventricles. The interventricular sulcus follows the boundary line between the right and left ventricles.

Sectional Anatomy and Organization

1. The atria are separated by the interatrial septum; the ventricles are divided by the interventricular septum.

2. The right atrium receives blood from the systemic circuit via the superior and inferior vena cava.

3. The right atrium communicates with the right ventricle through an opening guarded by the right atrioventricular (AV), or tricuspid valve.

4.The chordae tendinae brace the valves, and the papillary muscles adjust the tension in the chordae tendinae.

5.The internal surfaces of the ventricles are lined with deep grooves and folds, the trabeculae carnae.

6. Blood leaving the right ventricle enters the pulmonary trunk after passing through the pulmonary semilunar valve. The pulmonary trunk divides to form the left and right pulmonary arteries.

7. The left and right pulmonary veins return blood to the left atrium. Blood leaving the left atrium flows into the left ventricle through a valve known as the left atrioventricular, the bicuspid, or the mitral valve.

8. Blood leaving the left ventricle flows through the aortic semilunar valve and enters the ascending aorta.

Functional Anatomy

1. There are characteristic anatomical differences between the ventricles that reflect the different functional demands placed upon them. The wall of the right ventricle is relatively thin, while the left ventricle has a massive muscular wall.

Coronary Circulation

1. The coronary circulation meets the high oxygen and nutrient demands of the cardiac muscle tissue.

2. The coronary arteries originate at the base of the ascending aorta.

3. The right coronary artery follows the coronary sulcus. The marginal branch extends along the right border, and the posterior interventricular branch runs toward the apex in the posterior interventricular sulcus.

4. The circumflex branch of the left coronary artery curves around the coronary sulcus. The anterior interventricular branch (left anterior descending artery) swings around the pulmonary trunk and crosses the anterior surface within the interventricular sulcus.

5. Branches of the left and right coronary arteries interconnect, forming anastomoses that assist in providing a reliable flow of blood to the cardiac muscle tissue.

6. The great and middle cardiac veins carry blood from the coronary capillaries to the coronary sinus. The sinus lies within the posterior portion of the coronary sulcus, and it empties into the right atrium near the base of the inferior vena cava.

Cardiac Histology

1. The bulk of the heart consists of contractile myocardium. The endocardium lines the internal chambers and the epicardium, or visceral pericardium, covers the organ. The fibrous skeleton provides strength, elasticity, and support for the contractile cells and valves of the heart.

2. Cardiac muscle cells, or cardiocytes, are interconnected by intercalated discs that convey the force of contraction from cell to cell and conduct action potentials.

Action Potentials and the Cardiocyte Membrane

1. Cardiocytes obtain energy by the breakdown of glucose and fatty acids. Energy reserves in the form of glycogen and lipid droplets are found in the cytoplasm, and oxygen is bound to myoglobin.

2. Cardiocytes have a prolonged absolute refractory period and therefore cannot be tetanized.The maximum normal rate of contraction is limited to about 200 beats per minute.

The Coordination of Cardiac Contractions

1. Nodal (pacemaker) cells are responsible for setting the pace of cardiac contraction. Conducting fibers convey the stimulus to the general myocardium.

2. Nodal cells depolarize spontaneously. In the normal heart the sinoatrial (SA) node acts as the cardiac pacemaker.

3. The sinoatrial node of the right atrium depolarizes first, and from here the impulse travels to the atrioventricular (AV) node, through the bundle of His, the bundle branches, and the Purkinje fibers before stimulating the myocardial cells of the ventricles.

4. The AV bundle represents the only electrical connection between the atria and the ventricle.

The Extracellular Fluid and Cardiac Function

1. Any factor that affects the permeability of the SA nodal cell membranes can alter the heart rate. Examples include temperature changes, variations in electrolyte composition, and certain drugs.

2. Alterations in calcium and potassium ion concentrations in the interstitial fluid change the rate and strength of cardiac contractions.

The Heart as a Pump

The Cardiac Cycle:

1. The cardiac cycle consists of systole followed by diastole. During systole the heart contracts, and in diastole it fills with blood in preparation for its next contraction.

2. Most of ventricular filling results from the elastic expansion of the myocardial walls and the passive flow of blood from the atria and the major veins.The atria provide the final 30 percent to the volume of the ventricles just before ventricular systole.

Heart Sounds:

1.The closure of valves and the rushing of blood through the heart gives rise to the characteristic heart sounds heard during auscultation. A stethoscope is used to detect these sounds.

The Electrocardiogram (ECG):

1. A recording of the electrical activities of the heart constitutes an electrocardiogram. Important landmarks of an ECG include the P wave, the QRS complex, and the T wave.

Cardiodynamics

1. The amount of blood ejected into the systemic circulation during a single heartbeat represents the stroke volume of the heart.The amount ejected by the left ventricle each minute

represents the cardiac output.

2. Cardiac output can be altered by changing the heart rate or the amount of cardiac contraction. The amount of increase possible represents the cardiac reserve.

The Control of Heart Rate:

1. The basic heart rate is determined by the pace of at the SA node.This rate can be modified by the autonomic nervous system.

2. The heart receives both sympathetic and parasympathetic innervation via the cardiac plexus.

3. Parasympathetic stimulation slows the heart rate because ACh increases the time it takes for the SA nodal cells to reach threshold.

4. Sympathetic stimulation increases the heart rate because the cells depolarize to threshold more quickly.

5. The atrial reflex (Bainbridge reflex) accelerates the heart rate when the walls of the right atrium are stretched.Part of the acceleration results from the stretching of the nodal fibers, and part from a sympathetic reflex involving stretch receptors in the atrial wall.

The Regulation of Stroke Volume:

1. The stroke volume is the difference between the end-diastolic volume and the end-systolic volume. Changing either value affects the stroke volume.

2. The filling time and the venous return interact to determine the end-diastolic volume.

3. Intrinsic regulation involves an automatic response of the heart to changes in the end-diastolic volume. Within normal limits, the greater the EDV, the more powerful the succeeding contraction will be; this has been called Starling's law of the heart .

4. Autonomic innervation primarily affects stroke volume by altering the end-systolic volume. Sympathetic activity produces more powerful contractions, lowering ESV, and parasympathetic stimulation reduces contractile strength, elevating ESV.

5. Stroke volume increases with increasing heart rate up to rates of about 180 bpm. Due to limited filling time, any further increases in heart rate will be accompanied by a reduction in stroke volume.

The Coordination of Autonomic Activity:

1. The cardiac centers (cardioacceleratory and cardioinhibitory) of the medulla represent the autonomic headquarters for cardiac control.

2. The cardiac centers receive inputs from receptors monitoring blood pressure and the concentrations of oxygen and carbon dioxide in the blood and cerebrospinal fluid; reflexes adjust cardiac output to maintain adequate blood flow to peripheral tissues.

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