Biology 212 Anatomy & Physiology I The Heart

Heart: Mass: 250-300 g Located in center of thorax (mediastinum) Anterior to vertebrae Posterior to sternum & 2nd through 6th rib Superior to diaphragm Surrounded by lungs All vessels, nerves, etc. enter or leave superior end ("base")

Layers of Heart: (Outer Surface) Epicardium - Thin, connective tissue Myocardium - Thick, cardiac muscle (Inner Surface) Endocardium - Thin, connective tissue Simple squamous epithelium lines inner surface, next to blood

Myocardium: Cardiac Muscle Cardiac myocytes have the same arrangement of thin myofibrils and thick myofibrils as skeletal myocytes, forming sarcomeres. Although slightly different in arrangement, the transverse tubules and sarcoplasmic reticulum are the same. Just like skeletal myocytes, contraction occurs when calcium ions are released from the sarcoplasmic reticulum, bind onto the thin myofilaments, and allow them to form cross-bridges with the thick myofilaments.

Myocardium: Cardiac Muscle One significant difference: Cardiac myocytes attached end-to-end by intercalated discs which contain both desmosomes (keep the cells from pulling apart) and gap junctions (allow ions to flow directly from one cell to the next). We will return to that structure when we discuss contraction of cardiac myocytes

Heart surrounded by double-layered pericardium Visceral Layer Parietal Layer Serous Pericardium Visceral Layer Parietal Layer Heart Pericardial Cavity

Heart surrounded by double-layered pericardium Fibrous Pericardium Serous Pericardium Visceral Layer Parietal Layer Heart Pericardial Cavity

Anterior View Left Atrium Right Atrium Left Ventricle Right Ventricle

Anterior View Aorta Superior Vena Cava Pulmonary Trunk Inferior Vena Cava

Anterior View Left Atrioventricular Sulcus Right Atrioventricular Sulcus Anterior Interventricular Sulcus

Posterior View Right Atrium Left Atrium Right Ventricle Left Ventricle

Posterior View Left Atrioventricular Sulcus Right Atrioventricular Sulcus Posterior Interventricular Suclus

Posterior View Superior Vena Cava Aorta Pulmonary Arteries Inferior Vena Cava Pulmonary Veins

Left Coronary Artery Circumflex Coronary Artery Right Coronary Artery Anterior Interventricular Coronary Artery Marginal Coronary Artery

Circumflex Coronary Artery Right Coronary Artery Posterior Interventricular Coronary Artery

Anterior Cardiac Veins Great Cardiac Vein Small Cardiac Vein

Great Cardiac Vein Small Cardiac Vein Middle Cardiac Vein Coronary Sinus

Valves of the Heart: Right Atrioventricular Valve (Tricuspid valve) Right Atrium to Right Ventricle

Valves of the Heart: Right Atrioventricular Valve Pulmonary Valve (Right semilunar valve) Right Ventricle to Pulmonary Trunk

Valves of the Heart: Right Atrioventricular Valve Pulmonary Valve Left Atrioventricular Valve (Bicuspid or Mitral valve) Left Atrium to Left Ventricle

Valves of the Heart: Right Atrioventricular Valve Pulmonary Valve Left Atrioventricular Valve Aortic Valve (Left semilunar valve) Left Ventricle to Ascending Trunk

Valves of the Heart: Right Atrioventricular Valve Pulmonary Valve Left Atrioventricular Valve Aortic Valve Note: There are no valves controlling movement of blood a) From superior or inferior vena cavae into right atrium b) From pulmonary veins into left atrium

Sinoatrial Node Atrioventricular Node Atrioventricular Bundle (of His) Bundle Branches Purkinje fibers

Just like skeletal myocytes, the contraction of cardiac myocytes is triggered by changes in the electrical charge (depolarization and repolarization) which moves along the plasma membrane (sarcolemma) as an action potential”. However, the mechanism of that depolarization and repolarization is significantly different.

Recall: Cardiac myocytes attached end-to-end by intercalated discs which contain gap junctions, allowing ions to flow directly from one cell to the next).

The starting point is essentially the same: the sarcolemma is polarized because there are many more positive ions (Na+ and Ca++) outside the myocyte than inside the myocyte (K+); and many more negative ions (proteins, chloride, nucleic acids, phosphates, etc) inside the cell.

Cardiac myocytes in the sinoatrial node (“pacemaker cells”) spontaneously depolarize: 1. Na+ constantly leaks into the cells followed by Ca++, decreasing the voltage until threshold voltage is reached. Ca++ channels then open and Ca++ floods into the cell and depolarize it.

Cardiac myocytes in the sinoatrial node (“pacemaker cells”) spontaneously depolarize: 2. Na+ and Ca++ channels close as K+ channels open and K+ floods out of the cell, repolarizing the membrane. Pumps then return all of the ions to their original positions.

Cardiac myocytes in the sinoatrial node (“pacemaker cells”) spontaneously depolarize: 3. Na+ leakage continues and starts another cycle.

Remember that cardiac myocytes are connected by intercalated discs which contain gap junctions allowing ions to flow directly from cell to cell. Thus, when the myocytes of the sinoatrial node depolarize, they can pass that electrical signal directly to all of the cells with which they have intercalated discs, which can pass it on to other cells, which can pass it on to other cells …...

In these non-pacemaker cells, large amounts of Na+ flood into the cell as their channels open, followed by slower movement of Ca++, causing rapid depolarization (”phase 0”). Just after K+ channels open to begin repolarization (phase 1), additional Ca++ channels open and dramatically slow It down (phase 2). Eventually, those Ca++ channels close. Large amounts of K+ leave the cell as their channels open, repolarizing the myocyte (phase 3). All of these ion channels close and pumps turn on to return all of the ions to their original locations, so the membrane remains polarized (phase 4).

Contraction of the heart (or any one of its chambers) is Systole Relaxation of the heart (or any one of its chambers) is Diastole One systole followed by one diastole is one Cardiac Cycle

Flow of blood through the heart is controlled entirely by changes in pressure. Blood always flows along its pressure gradient, from the area of higher pressure to an area of lower pressure. The open or closed position of a valve depends entirely on the difference in pressure from one side to the other: The higher pressure pushes the valve open or closed,

Assume the chambers of the heart and vessels have the following pressures: Left ventricle = 115 mm Hg Right ventricle = 5 mm Hg Pulmonary trunk = 22 mm Hg Superior vena cava = 2 mm Hg Inferior vena cava = 2 mm Hg Left atrium = 20 mm Hg Right atrium = 10 mm Hg Aorta = 125 mm Hg Which valves of the heart will be open? Which valves of the heart will be closed?

Terms to know: Heart rate: The number of cardiac cycles per minute End diastolic volume: Volume of blood in a ventricle at the end of diastole, just before it begins systole. End systolic volume: Volume of blood in a ventricle at the end of systole, just before it begins diastole. Stroke volume: Volume of blood ejected from a ventricle during a single systole ( = ESV – EDV) Cardiac Output: Volume of blood pumped by a ventricle in one minute (= Heart rate) x (Stroke volume) Cardiac Index: Volume of blood pumped by a ventricle per minute per square meter of body surface

Given the following information: a) Dr. Thompson's total blood volume is 5.8 liters b) His heart ejects 75 ml of blood per contraction c) His kidneys produce 320 ml of urine per hour d) All of his wisdom teeth have been removed e) His heart contracts 70 times per minute f) His systolic blood pressure is 130 mmHg g) His diastolic blood pressure is 80 mmHg h) The pressure in his left ventricle changes between 1 mmHg and 133 mmHg during each cardiac cycle Calculate his Heart Rate Stroke Volume Cardiac Output

Therefore: You can regulate your cardiac output, and therefore your cardiac index, by: a) Increasing or decreasing your heart rate b) Increasing or decreasing your stroke volume In fact: Your ventricles modify both heart rate and stroke volume on a beat-by-beat basis. This depends on how much the cardiac muscle cells are stretched during the preceding diastole, which itself depends on the volume of blood in the chamber = Frank-Starling Law of Cardiac Contraction

Sympathetic stimulation increases heart rate by increasing the frequency with which your sinoatrial node depolarizes. Parasympathetic stimulation decreases heart rate by decreasing the frequency with which your sinoatrial node depolarizes. Increasing the end-diastolic volume and/or decreasing the end-systolic volume will increase stroke volume. Decreasing the end-diastolic volume and/or increasing the end-systolic volume will decrease stroke volume.