Chapter 18: Heart textbook notes PDF

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Chapter 18: Heart 18.2 Introduction The heart is one of the body’s most dynamic and resilient organs. Heart disease remains the leading cause of death in Western civilization and is responsible for every 1 out of 3 deaths.

18.3 Organizing Principles of Cardiac Physiology 18.3.1 The Heart is a Pump The human heart is about the size of a fist, beats more than 100,000 times, and circulates more than 7,000 liters of blood per day. Blood is a fluid that is constantly in motion, a process described as flow. In order to create flow, pressure must be applied. According to Boyle’s Law, pressure and volume are inversely related. The pressure created must be greater than resistance, a force that opposed blood flow. Fluids move from areas of higher pressure to lower pressure. Aka a pressure gradient must exist. Valves prevent the backflow of blood and ensure one-directional blood flow. Contraction: how the heart creates pressure inside of your circulatory system, the process in which cardiac muscle becomes shorter. ● When the heart contracts, the volume of its chambers decreases Relaxation: volume of the chambers increase, pressure decreases, and blood flows into the chamber from a region of higher pressure.

There are 2 major divisions to the human circulatory system: ● Pulmonary circulation: sends blood to the lungs for oxygenation and returns oxygenated blood to the heart. The right side is responsible for this. ● Systemic circulation: sends oxygenated blood to the tissues and returns deoxygenated blood to the heart. The left side is responsible for this. The human heart is a double pump that are connected to each other with actions coordinated.

18.4 Anatomy 18.4.1 The Heart is Located in the Thoracic Cavity The heart is located in the anterior portion of the thoracic cavity, nestled between the pleural cavities in a region called the mediastinum. The heart is not with the midsagittal line, instead, it is slightly off with its inferior tip (the

apex) pointing left. The base of the heart is located superiorly and connects to the great vessels.

18.4.2 The Heart is Protected by the Pericardium Pericardial sac: the protective membrane surrounding and stabilizing the heart ● The outermost layer is the parietal pericardium ● The outer layer is the fibrous pericardium, made of dense regular connective tissues ● The inner layer is the serous pericardium, a double-layered, fluid-filled membrane ● The innermost layer is the visceral pericardium which surrounds the surface of the heart and it is continuous with the outermost layer of the heart ○ Located between these 2 layers is the insulating fluid known as pericardial fluid which reduces friction between the serous membranes and protects the tissues from damage. ■ Pericardial effusion: abnormal increases in pericardial fluid

18.4.3 The Heart is a Multi-Layered Organ The heart is made of 3 layers: ● Epicardium: the outermost layer bound together by loose areolar connective tissue ● Myocardium: the middle and thickest layer made of cardiac myocytes (aka cardiac muscle cells) which produce the heart’s pumping action ○ Also contains the cardiac skeleton which serves as an electrical insulator and provides structural scaffolding during development and maintains the overall architecture of the organ ● Endocardium: the innermost layer made of endothelial (simple squamous) tissue ○ Lines the inner chambers of the heart and is continuous with the blood vessels that allow blood to enter and exit these chambers ○ Home of cardiac progenitor cells which are capable of differentiating into the other tissues during cardiac development. These cells are important because cardiac myocytes are non-mitotic and incapable of selfgeneration.

18.4.4 More Than Meets the Eye: Internal Cardiac Anatomy The heart is four-chambered formed from 2 upper chambers known as atria and two lower chambers known as ventricles. The atria is where blood enters the heart. The ventricles send blood out. The left and right sides are separated by the cardiac septum. The left and right atria are separated by the interatrial septum and the ventricles are separated by the interventricular septum. Atria are smaller than ventricles and expand as they fill with blood. They have ear-like sacs called auricles that are on the anterior surface.

Systemic pump: left atrium and left ventricle ● Pumps oxygenated blood Pulmonary pump: right atrium and right ventricle ● Pumps deoxygenated blood

18.4.5 Valves Create One-Directional Blood Flow Blood flows in a single direction. This is because there are 2 sets of valves: ● Atrioventricular (AV) valves: ensure one-directional blood flow from the atria into the ventricles ○ Formed by flaps known as cusps that are anchored to the cardiac skeleton ■ The center-facing region of each cusp is bound by strands of fibrous connective tissue known as chordae tendineae. It anchors the cusps to small muscles in the wall of the ventricle known as the papillary muscles. ■ Tricuspid valve: the valve located between the right atrium and the right ventricle, has 3 cusps ■ Bicuspid valve: aka the mitral valve, the valve between the left atrium and the left ventricle, has 2 cusps ● Semilunar (SL) valves: prevent the backflow of blood from the major vessels into the ventricle. Flow out of the ventricle. Do not require chordae tendineae for stabilization. ○ Aortic valve: located between the left ventricle and the aortic arch ○ Pulmonary valve: located between the right ventricle and the pulmonary trunk.

18.4.6 Blood Flow Through the Heart and Major Vessels Path of blood simplified: ● Blood flow occurs in a single direction due to valves ● Blood flow is a continuous circuit ● The right side of the heart handles deoxygenated blood ● The left side of the heart handles oxygenated blood ● Blood comes into the heart through the atria ● Blood is pumped out of the heart by the ventricles Steps: 1. Oxygen poor blood travels through the inferior vena cava or the superior vena cava or coronary sinus 2. Deoxygenated blood enters the right atrium 3. Blood passes through the tricuspid valves 4. Blood enters the right ventricle 5. Blood moves through the pulmonary valve 6. Blood enters the pulmonary trunk and the pulmonary arteries where the blood is carried to the lungs

7. Blood loses CO2 and gains O2 in the pulmonary capillaries 8. The oxygenated blood enters the pulmonary veins 9. Blood enters the left atrium 10. Blood travels through the mitral valve or the bicuspid valve 11. Blood enters the left ventricle 12. Blood moves through the aortic valve 13. Blood travels through the aorta and systemic arteries 14. Blood loses O2 and gains CO2 in the systemic capillaries 15. Blood enters the veins 16. Repeat

18.4.7 The Coronary Circulation Coronary circulation: contains the vessels that carry blood to and from the heart

The larger vessels are highly interconnected by smaller vessels, forming a vascular network known as the cardiac anastomoses, which are a redundant system of blood flow, where blood can take several different paths to reach the same tissue region. ● They serve as a failsafe by providing many paths for blood to reach various regions of the heart ● A common site of anatomical variation ● The tiny vessels make up the collateral circulation

18.4.8 Histology and Microanatomy of Cardiac Muscle Cardiac muscles: ● Single, centrally located nucleus ● Myofibrils vary in size and are not densely packed ● Resist fatigue more than skeletal muscle ● Will not utilize anaerobic respiration unless under extreme ischemic or hypoxic stress ● Arrangement of sarcoplasmic reticulum in cardiac muscle is less extensive and lacks cisternae ● T-tubules are less abundant, so it lacks the triad organization

The most important distinction between cardiac and skeletal muscle are intercalated discs. In order to generate force, additional fibers must be recruited. Recruitment doesn’t occur in cardiac muscle because of intercalated discs which are fusions in the plasma membranes (sarcolemmas) of adjacent cardiac myocytes. When deprived of oxygen for more than a few minutes, the cardiac myocytes will begin to die. This leads to myocardial infarction aka a heart attack. After too long, there is a disorder known as a reperfusion injury that occurs when blood flow to the heart is restored. It can lead to cell damage, apoptosis, and necrosis.

18.5 Excitability in the Myocardium There are 2 major types of cells that are present within the myocardial layer of the heart: ● Pacemaker cells which are autorhythmic,...