Location and Protection of the Heart The heart is located in the thoracic cavity between the lungs and on top of the diaphragm in a region called the Mediastinum. The heart is enclosed by a double layered serous membrane, the Pericardium. The tough, fibrous outer layer of the Pericardium is the Parietal Pericardium.

The inner layer attached to the outer wall of the heart is the Visceral Pericardium. Between these layers is the Pericardial Space. It contains serous fluid and reduces friction between the membranes when the heart moves.

The wall of the heart is composed of 3 distinct layers Epicardium – thin outer layer that corresponds to the Visceral Pericardium. Myocardium – thick middle layer composed of Cardiac Muscle. Endocardium – Inner layer composed of epithelium and connective tissue. The connective tissue contains both collagen and elastin fibers and is continuous with the arteries that leave the heart.

The human heart has 4 chambers and is divided into right and left sides by the Interventricular septum. The right side of the heart receives deoxygenated blood from the body and pumps it under low pressure to the lungs. The left side receives oxygenated blood from the lungs and pumps it under high pressure to the body. 2 upper chambers called atria (atrium) 2 lower chambers called ventricles.

4 one-way valves prevent the backflow of blood and keep it moving in one direction. 1) Tricuspid Valve (3 cusps) – located between the r. atrium and r. ventricle. Allows blood to flow only from the r. atrium into the r. ventricle. Cusps are anchored to the inside wall of the heart by strong, fibrous strings called Chordinae Tendinae. Prevent the valve from swinging back too far when closing. (prolapse)

2) Pulmonary Valve. Located between the r 2) Pulmonary Valve. Located between the r. ventricle and the Pulmonary Artery. Allows blood to enter the Pulmonary Artery and prevents its return to the r. ventricle. 3) Bicuspid or Mitral Valve. Located between the l. atrium and l. ventricle. Allows blood to flow from the l. atrium into the l. ventricle.

4) Aortic Valve. Located between the l. ventricle and the Aorta 4) Aortic Valve. Located between the l. ventricle and the Aorta. Allows blood to enter the Aorta and prevents its return to the l. ventricle.

Path of the Blood Through the Heart Deoxygenated blood enters the Right Atrium from the vena cava. Contraction of the right atrium forces blood through the Tricuspid Valve and into the Right Ventricle. Contraction of the right ventricle forces blood through the Pulmonary Valve and into the Pulmonary Artery.

Blood travels to the Lungs where Oxygen and Carbon Dioxide are exchanged. Oxygen rich blood returns to the heart in the Pulmonary Veins and enters the Left Atrium. Contraction of the left atrium forces blood through the Bicuspid Valve and into the Left Ventricle. Contraction of the left ventricle forces blood through the Aortic Valve and into the Aorta.

The aorta branches many times to supply all body regions with oxygen and nutrients. After the exchange of oxygen and nutrients blood is collected and returned to the heart by the Vena Cava.

Systole and Diastole Atrial Diastole – Atrial walls are relaxed. Blood enters the atria. Atrial Systole – Atrial walls are contracted. Blood is forced into the ventricles. Ventricular Systole – Ventricles contract. Blood flows out of the heart. Ventricular Diastole – Blood flows into the ventricles.

Heart Sounds Lub Dup – Created by vibrations in the heart tissue due to the sudden speeding up of the blood and the closing of the valves. Lub sound – created during ventricular contraction when the Tricuspid and Bicuspid valves are slammed shut. Dup sound – occurs when the ventricles relax and the aortic and pulmonary valves close.

Cardiac Conduction System

Cardiac Conduction System Specialized cardiac muscle tissue whose fibers contain few myofibrils. They don’t contract. Instead they distribute impulses throughout the myocardium. Two units – One makes up the atrial walls and one unit makes up the ventricle walls. Connected only in the floor of the right atrium. Depolarization Change of the electrical polarity of a membrane because of flow of ion in and out of the cell across the cell membranes. In the heart in sinus rhythm, it is caused by electrical impulses arising from the SA node. Diastole Refers to the resting phase of the cardiac cycle when the ventricles relax and fill with blood between contractions (see also blood pressure). Repolarization The restoration of electrical polarity via an increase in ion differential between the cell membranes of the myocardium. It is necessary in preparation for the next heart beat

SA Node – Sinoatrial Node SA Node – Sinoatrial Node. Small mass of cardiac muscle tissue located in the posterior wall of the right atrium. Cells have the ability to excite themselves (initiate impulses without outside stimulation). The SA Node initiates 70 to 80 impulses per minute. Often called the pacemaker. AV Node – Atrioventricular Node. Located in the floor of the right atrium. Impulses from the SA Node are channeled into the AV Node that passes them into the-----

AV Bundle – Group of large fibers located in the interventricular septum. Fibers carry the impulse down the interventricular septum and give rise to----- Purkinje Fibers – Fibers branch off and become continuous with the myocardium of the ventricles from the bottom up. The impulse is slowed by passing through the AV Node creating a delay. This allows time for the ventricles to fill before they contract. The ventricles contract from the bottom up due to the arrangement of the Purkinje Fibers.

Structure of Arteries and Veins

Arteries Strong elastic vessels. Carry blood away from the heart under high pressure. Wall of the artery is composed of 3 distinct layers. 1) Tunica Intima – Innermost layer composed of simple squamous epithelium. Also called endothelium. Very smooth. Reduces friction between blood and the wall of the artery.

2) Tunica Media – Middle layer. Composed of Smooth muscle. Thickest layer. Innervated by sympathetic fibers of the autonomic nervous system. Contraction of these muscle fibers reduces the diameter of the vessel. Called Vasoconstriction. When the fibers relax the diameter increases. Vasodilation.

3) Tunica Adventitia (Externa) – Thin outer layer composed of connective tissue. Arteries divide into smaller and smaller branches until they are Arterioles – the smallest arteries that connect to the capillaries.

Capillaries Smallest diameter blood vessel. Consist of a single layer of endothelium. Allows for easy diffusion of O2 and CO2 between the blood and the fluid surrounding cells. The exchange of gases, nutrients, and metabolic by-products between the blood and the tissue fluid surrounding the body cells occurs through the capillaries.

Tissues with high metabolic rates (muscles, stomach) have extensive capillary networks. Tissues with low metabolic rates (skin, cartilage) have very few capillaries. Entrances to capillary networks are encircled by a smooth muscle called a Precapillary Sphincter. These muscles close a capillary network by contracting or open it by relaxing to meet the oxygen demand of the tissues.

Veins Carry blood back to the heart under little or no pressure. Walls of veins are composed of the same 3 layers as the arteries. Middle layer is greatly reduced in thickness. Veins also have valves that prevent blood flow away from the heart.

Major Arteries of the Body Aorta – Largest artery in the body. Coronary Arteries – arise immediately after the Aorta exits the heart. Supplies the Myocardium. 3 major arteries originate from the aortic arch. 1) Brachiocephalic (Innominate) – Divides to form the Right Common Carotid (Brain) and the Right Subclavian (Arm)

Branches of the Descending Aorta 2) Left Common Carotid (Brain) 3) Left Subclavian (Arm) Branches of the Descending Aorta Celiac Artery – Stomach and Spleen Renal Artery – Kidneys Superior Mesenteric Artery – Small Intestines. Inferior Mesenteric Artery – Large Intestines

Major Veins of the Body Most veins share names with the arteries in the same region with some exceptions. Inferior and Superior Vena Cava – Largest Veins in the body. Empties deoxygenated blood from the body into the r. atrium. Internal & External Jugular – Drain blood from the brain.

Portal Vein – Collects blood from the abdominal organs (stomach, small and large intestines). The blood then passes through the liver and then through the Hepatic Vein which empties into the Inferior Vena Cava. Median Cubital Vein – Site of venipuncture. Great Saphenous Vein – Longest vein in the body

Blood Pressure Pressure the blood exerts on the inner wall of the blood vessels. Arterial Pressure is greatest when the ventricles contract. Systolic Pressure. Lowest when the ventricles relax. Diastolic Pressure. Blood pressure is measured in millimeters of Mercury (mm Hg) with the systolic pressure written first. 120/80 mm Hg is considered normal. Below 100 mm Hg is considered low. 140/90 too high.

Factors Affecting Blood Pressure 1. Heart Action – Volume of blood entering the arterial system with each ventricular contraction. Stroke volume = 70 ml. Cardiac output = Stroke volume X Heart rate Example: 70 ml X 72 bpm = 5040 ml/min. 2. Blood Volume – Any blood loss reduces pressure. 3. Peripheral Resistance – Friction between the blood and the wall of the vessel. Friction must be overcome by increased blood pressure.

4. Viscosity – increased viscosity increases blood pressure. Blood pressure is most influenced by stroke volume and peripheral resistance. Any factor that influences these two factors greatly influences blood pressure.