Circulatory System: Heart
Circulatory System: Heart - Chapter 19 Study Notes
Preview/Review
Functions of the Circulatory System:
Essential functions include transportation, protection, and regulation.
Key Structures in the Circulatory System:
Heart: Major organ that pumps blood.
Blood Vessels: Include arteries, veins, and capillaries.
By the Numbers:
Chambers of the Human Heart: 4 chambers
Average Heartbeats per Day: Approximately 100,000 times
Blood Pumped per Beat: About 70 mL
Heart Attack:
Also known as myocardial infarction (MI)
Symptoms include chest pain, shortness of breath, and cold sweat.
Introduction
Structures of the circulatory system include:
Heart
Blood Vessels
Artery: Transports blood away from the heart.
Vein: Transports blood toward the heart.
Capillaries: Responsible for the exchange of gases and nutrients between blood and systemic cells as well as alveoli in lungs.
Functions of Blood:
Transport of gases, nutrients, hormones, and waste products.
Function of the Heart:
Acts as a pump to circulate blood throughout the body.
Function of Blood Vessels:
Vessels provide pathways for blood circulation.
Circulatory System Pathways
Composed of two circuits:
Pulmonary Circuit:
Carries blood to and from the lungs.
Blood flows from the right side of the heart to the lungs and back to the left side of the heart.
Systemic Circuit:
Carries blood to and from systemic tissues.
Blood flows from the left side of the heart to the rest of the body and returns to the right side of the heart.
Blood Flow Through Pulmonary Circulation
Deoxygenated blood enters the right atrium from the venae cavae (superior and inferior vena cavae).
Blood passes through the right atrioventricular (AV) valve (tricuspid valve).
Blood enters the right ventricle.
Blood passes through the pulmonary semilunar valve into the pulmonary trunk.
Blood travels through right and left pulmonary arteries to lungs.
Gas exchange occurs in pulmonary capillaries, oxygenating the blood.
Oxygenated blood enters right and left pulmonary veins.
Blood returns to the left atrium of the heart.
Blood Flow Through Systemic Circulation
Oxygenated blood enters the left atrium from the pulmonary veins.
Blood passes through the left AV valve (bicuspid or mitral valve).
Blood enters the left ventricle.
Blood passes through the aortic semilunar valve into the aorta.
Blood is distributed throughout the body via systemic arteries.
Blood enters systemic capillaries for nutrient and gas exchange.
Deoxygenated blood drains into systemic veins, returning to the heart
Blood enters the right atrium, completing the circuit.
Anatomy of the Heart: Size and Position
Average size: Approximately the size of a fist.
Location:
Lies in the mediastinum, posterior to the sternum and anterior to the vertebral column.
Extends from the level of the 2nd rib to the superior surface of the diaphragm.
Majority located to the left of the body's midline.
Orientation of the Heart
The heart has two parts:
Base: Superior, broader portion where vessels arise, points toward the right shoulder.
Apex: Inferior, narrower portion resting on the diaphragm, points toward the left hip.
Orientation: Right side is more anterior; left side is more posterior.
Chambers of the Heart
The human heart has four chambers:
Two atria: Receiving chambers for blood.
Two ventricles: Pumping chambers for blood.
Heart Coverings and Wall Structure
Pericardium: Surrounds the heart.
Fibrous Pericardium: Composed of dense irregular connective tissue.
Serous Pericardium: Contains two layers:
Parietal Layer: Lines the fibrous pericardium.
Visceral Layer (Epicardium): Covers the heart.
Pericardial Cavity: Contains pericardial fluid, composed of simple squamous epithelium and underlying areolar connective tissue.
Layers of the Heart Wall
Myocardium:
Middle layer, composed predominantly of cardiac muscle.
Thickest layer, responsible for contracting and pumping blood.
Endocardium:
Innermost layer lining the heart chambers, valves, composed of simple squamous epithelium and areolar connective tissue. Continuous with the endothelium (inner lining of blood vessels).
Fibrous Skeleton of the Heart
Composed of dense irregular connective tissue.
Provides structural support, anchors cardiac muscle cells and valves, acts as an electrical insulator between heart chambers.
Important Points About the Heart's Anatomy
Heart functions as a double pump:
Left Side: Handles oxygen-rich blood, pumping it to the body.
Right Side: Handles oxygen-poor blood, pumping it to the lungs.
Blood moves in one direction through the heart due to valves preventing backflow.
The heart is regulated by pressure differences caused by contraction and relaxation of chambers.
Atria: Receive blood and push it into ventricles.
Ventricles: Pump blood out of the heart.
Path of Blood Flow
Arteries: Transport blood away from the heart, branch into arterioles.
Capillaries: Sites of nutrient and gas exchange.
Veins: Transport blood back toward the heart.
Anatomy of the Heart: Valves
Valves: Ensure one-way blood flow and prevent backflow.
Atrioventricular (AV) Valves: Between atria and ventricles (tricuspid and bicuspid/mitral valves).
Semilunar Valves: Between ventricles and arteries (pulmonary and aortic valves).
Tendinous Cords: Anchor the AV valves to papillary muscles in ventricles, preventing inversion.
Valves Mechanism
AV Valves: Open during atrial contraction; close during ventricular contraction.
Semilunar Valves: Open during ventricular contraction; close during ventricular relaxation.
Coronary Circulation
Coronary Arteries: Blood vessels that supply blood to the heart wall.
Right coronary artery and left coronary artery arise from the ascending aorta.
Coronary Veins: Great cardiac vein, middle cardiac vein, and small cardiac vein drain into the coronary sinus.
Cardiac Muscle Characteristics
Structure: Striated muscle, shorter than skeletal muscle with one or two centrally located nuclei.
Features for Energy Demand:
Extensive blood supply, myoglobin, high levels of mitochondria.
Ability to utilize multiple energy sources (fatty acids, glucose).
Intercalated Disks
Connections between cardiac muscle cells, providing strength (desmosomes) and allowing electrical continuity (gap junctions).
Autorhythmicity: Some cardiac cells can depolarize spontaneously and generate action potentials.
Cardiac Conduction System
Special cells that generate and propagate electrical impulses controlling contractions:
Components: SA node (pacemaker), AV node, AV bundle, bundle branches, and Purkinje fibers.
Innervation of the Heart
Myogenic initiation of contraction but modulated by the autonomic nervous system:
Parasympathetic: Decreases heart rate via vagus nerves.
Sympathetic: Increases heart rate and contractility.
Physiological Events of the Heartbeat
Sinus Rhythm: Normal heart rate (70-80 bpm).
Electrical Events:
Depolarization, plateau, and repolarization actions for muscle contraction cycle.
Electrocardiography (ECG/EKG)
Recording of electric currents in the heart:
Main waves: P wave (atrial depolarization), QRS complex (ventricular depolarization), T wave (ventricular repolarization).
The Cardiac Cycle
Sequence of events from one heartbeat to the next, involving contraction (systole) and relaxation (diastole).
Heart sounds (S1, S2) associated with valve closure.
Ventricular Volume: End-diastolic volume (EDV) ~ 130 mL, End-systolic volume (ESV) ~ 60 mL, Stroke volume ~ 70 mL.
Factors Influencing Cardiac Output (CO)
Cardiac Output Formula: $CO = HR imes SV$
Varies with activity levels.
Heart Rate and Stroke Volume Influences
Heart Rate (HR): Average 70-80 bpm influenced by multiple factors.
Stroke Volume (SV): Influenced by venous return, inotropic agents, and afterload.
Blood Pressure Dynamics
Preload: Volume of blood in ventricles before contraction, determined by venous return.
Frank-Starling Principle: Greater preload increases contractile strength.
Afterload: Resistance in arteries affects how much blood is ejected.
Conclusion
The circulatory system operates through a complex network of the heart and blood vessels, working efficiently to ensure the continuous flow of blood, maintaining oxygenation and nutrient delivery to all tissues in the body. Understanding the interconnectedness of these systems is essential for grasping human physiology and pathology.
Preview/Review
Functions of the Circulatory System:
Essential functions include transportation (of respiratory gases, nutrients, hormones, metabolic wastes), protection (via immune cells and clotting mechanisms), and regulation (of body temperature, pH, and fluid balance).
Key Structures in the Circulatory System:
Heart: The central muscular pump generating blood pressure and circulating blood.
Blood Vessels: A closed network of conduits, including arteries (carry blood away), veins (carry blood toward), and capillaries (sites of exchange).
By the Numbers:
Chambers of the Human Heart: 4 chambers (two atria and two ventricles).
Average Heartbeats per Day: Approximately 100,000 times, equating to billions over a lifetime.
Blood Pumped per Beat (Stroke Volume): About 70 mL (SV), which means approximately 5-6 liters per minute in a resting adult.
Heart Attack:
Also known as myocardial infarction (MI), resulting from prolonged ischemia (lack of blood flow) to a part of the heart muscle, usually due to a blockage in one or more coronary arteries. This leads to the death of cardiac muscle tissue.
Symptoms include severe chest pain (angina pectoris), often radiating to the left arm, jaw, or back; shortness of breath (dyspnea); cold sweat; nausea; and lightheadedness.
Introduction
Structures of the circulatory system include:
Heart: A muscular organ located in the thoracic cavity, acting as a double pump.
Blood Vessels: A closed delivery system connecting the heart to all body tissues.
Artery: Thick-walled, elastic vessels that transport high-pressure blood away from the heart. They branch into smaller arterioles.
Vein: Thinner-walled vessels with larger lumens and often containing valves, transporting low-pressure blood toward the heart. They originate from venules.
Capillaries: Microscopic, thin-walled vessels forming extensive networks within tissues, responsible for the efficient exchange of gases, nutrients, hormones, and waste products between blood and systemic cells, as well as between blood and alveoli in the lungs.
Functions of Blood:
Transport: Carries oxygen (O2) from lungs to tissues (via hemoglobin in erythrocytes), carbon dioxide (CO2) from tissues to lungs, nutrients from the digestive tract, hormones from endocrine glands, and metabolic waste products to kidneys and liver.
Regulation: Helps maintain body temperature (absorbing and distributing heat), normal pH (via buffers), and adequate fluid volume in the circulatory system.
Protection: Prevents blood loss (clotting mechanisms involving platelets and plasma proteins) and infection (housing white blood cells, antibodies, and complement proteins).
Function of the Heart:
Acts as a powerful, rhythmic, and continuous pump, generating pressure to propel blood through the pulmonary and systemic circuits, ensuring unidirectional flow.
Function of Blood Vessels:
Vessels provide a closed, dynamic pathway for blood circulation, distributing blood to tissues and collecting it for return to the heart. They also regulate blood flow and pressure through vasoconstriction and vasodilation.
Circulatory System Pathways
Composed of two distinct, interconnected circuits:
Pulmonary Circuit:
Function: Shunts deoxygenated blood to the lungs for gas exchange and returns oxygenated blood to the heart.
Pathway: Blood flows from the right side of the heart, through the pulmonary arteries to the lungs, where it unloads CO2 and picks up O2 in the pulmonary capillaries, then returns via pulmonary veins to the left side of the heart.
Characteristics: A relatively low-pressure system.
Systemic Circuit:
Function: Delivers oxygenated blood and nutrients to all systemic tissues and removes metabolic wastes and CO_2 from them.
Pathway: Blood flows from the left side of the heart, through the aorta and systemic arteries to the rest of the body's cells, exchanges substances in systemic capillaries, and returns deoxygenated blood via systemic veins to the right side of the heart.
Characteristics: A high-pressure, high-resistance system due to the extensive network of vessels.
Blood Flow Through Pulmonary Circulation
Deoxygenated blood, rich in CO_2, enters the right atrium from the superior vena cava (draining upper body), inferior vena cava (draining lower body), and the coronary sinus (draining the heart wall).
Blood passes through the right atrioventricular (AV) valve, also known as the tricuspid valve, into the right ventricle.
Blood enters the right ventricle, which then contracts to pump the blood.
Blood passes through the pulmonary semilunar valve into the large pulmonary trunk, which then divides into the right and left pulmonary arteries.
Blood travels through the progressively smaller right and left pulmonary arteries, arterioles, and ultimately reaches the pulmonary capillaries within the lungs.
In the pulmonary capillaries, gas exchange occurs across the respiratory membrane: CO2 diffuses out of the blood into the alveoli, and O2 diffuses from the alveoli into the blood, oxygenating it.
Oxygenated blood collects in pulmonary venules, which merge to form the right and left pulmonary veins.
Blood returns to the left atrium of the heart, completing the pulmonary circuit.
Blood Flow Through Systemic Circulation
Oxygenated blood enters the left atrium from the four pulmonary veins (two from each lung).
Blood passes through the left AV valve, also known as the bicuspid or mitral valve, into the left ventricle.
Blood enters the left ventricle, the strongest pumping chamber, which then contracts vigorously.
Blood passes through the aortic semilunar valve into the largest artery in the body, the aorta.
Blood is distributed throughout the entire body via the aorta's major branches (e.g., brachiocephalic, left common carotid, left subclavian arteries) and subsequently through increasingly smaller systemic arteries and arterioles.
Blood enters systemic capillaries within tissues for essential nutrient, hormone, and gas (oxygen delivery, carbon dioxide pickup) exchange with body cells.
Deoxygenated blood, now rich in CO_2 and metabolic wastes, drains from the systemic capillaries into systemic venules, which merge to form progressively larger systemic veins.
These systemic veins ultimately converge into the superior vena cava and inferior vena cava, which return the blood to the right atrium, completing the systemic circuit.
Anatomy of the Heart: Size and Position
Average size: Approximately the size of a person's clenched fist (about 12-14 cm long, 9 cm wide, and 6 cm thick), weighing between 250-350 grams.
Location:
Lies within the mediastinum, the medial cavity of the thorax, specifically its inferior portion.
Positioned posterior to the sternum (breastbone) and anterior to the vertebral column (spinal cord).
Rests on the superior surface of the diaphragm.
Extends obliquely from the level of the 2nd rib, inferiorly to roughly the 5th intercostal space.
Approximately two-thirds of its mass (and its apex) is located to the left of the body's midline, with the remaining one-third to the right.
Orientation of the Heart
The heart has two principal parts relative to its orientation in the body:
Base: The superior, broader portion from which the great vessels (aorta, pulmonary trunk, venae cavae, pulmonary veins) emerge or enter. It points towards the right shoulder.
Apex: The inferior, narrower pointed tip formed by the left ventricle, which rests on the diaphragm. It points inferolaterally toward the left hip.
Overall Orientation: The right side (right atrium and most of the right ventricle) is more anterior, while the left side (left atrium and most of the left ventricle) is more posterior. This anterior-posterior tilt is important for understanding clinical examinations.
Chambers of the Heart
The human heart has four distinct chambers, separated by septa to prevent the mixing of oxygenated and deoxygenated blood:
Two Atria: Superior, thin-walled receiving chambers (right atrium, left atrium) that collect blood returning from the body or lungs and push it into the ventricles. They are separated by the interatrial septum.
Two Ventricles: Inferior, thick-walled pumping chambers (right ventricle, left ventricle) that propel blood into the pulmonary and systemic circuits. They are separated by the interventricular septum.
The walls of the ventricles are much thicker and more muscular than the atria, with the left ventricle having the thickest wall to pump blood against high resistance in the systemic circuit.
Heart Coverings and Wall Structure
Pericardium: A triple-layered sac that surrounds and protects the heart, anchoring it within the mediastinum, preventing its overfilling, and reducing friction during activity.
Fibrous Pericardium: The tough, outermost layer composed of dense irregular connective tissue. It protects the heart, anchors it to surrounding structures (like the diaphragm and great vessels), and prevents excessive distension of the heart.
Serous Pericardium: A thin, slippery, two-layered membrane situated deep to the fibrous pericardium.
Parietal Layer: Lines the internal surface of the fibrous pericardium.
Visceral Layer (Epicardium): Also known as the epicardium, it is the outermost layer of the heart wall and covers the external surface of the heart muscle.
Pericardial Cavity: The slit-like space between the parietal and visceral layers of the serous pericardium. It contains a small amount of serous pericardial fluid, which lubricates the membranes, allowing the heart to beat in a relatively frictionless environment.
Layers of the Heart Wall
The heart wall is composed of three distinct layers:
Epicardium: This is the outermost visceral layer of the serous pericardium. It is often infiltrated with fat, especially in older individuals, and contains coronary blood vessels and nerves that supply the heart.
Myocardium: The middle and thickest layer, composed predominantly of cardiac muscle cells arranged in spiral and circular bundles. It is the contractile laver responsible for the heart's pumping action. The fibrous skeleton of the heart anchors the cardiac muscle fibers and provides structural support.
Endocardium: The innermost layer, a slick, thin sheet of simple squamous epithelium (endothelium) overlying a thin layer of areolar connective tissue. It lines the heart chambers, covers the heart valves, and is continuous with the endothelium of the great vessels entering and leaving the heart. Its smooth surface minimizes friction as blood passes through the heart.
Fibrous Skeleton of the Heart
Composed of a network of dense irregular connective tissue, primarily located in the planes between the atria and ventricles and around the great arteries.
This robust framework serves multiple crucial functions:
Structural Support: Provides a rigid framework for the attachment of cardiac muscle fibers and the heart valves.
Anchoring: Acts as an anchor for cardiac muscle cells, giving the muscle cells something to pull against during contraction.
Electrical Insulation: Crucially, it electrically isolates the atria from the ventricles, ensuring that electrical impulses generated in the atria are only transmitted to the ventricles via the AV node, allowing for coordinated contraction.
Preventing Overdilation: Provides limits to the expansion of the heart during maximum filling.
Important Points About the Heart's Anatomy
Heart functions as a double pump:
Left Side (Systemic Pump): Handles oxygen-rich (oxygenated) blood, receiving it from the lungs and vigorously pumping it to the entire body via the aorta. It operates under high pressure to overcome systemic resistance.
Right Side (Pulmonary Pump): Handles oxygen-poor (deoxygenated) blood, receiving it from the body and pumping it to the lungs via the pulmonary trunk. It operates under relatively low pressure to facilitate gas exchange without damaging delicate lung capillaries.
Blood moves in one direction through the heart and circulatory system due to the efficient design and coordinated action of heart valves, which prevent backflow.
The heart's mechanical activity is precisely regulated by cyclical pressure differences caused by the sequential contraction (systole) and relaxation (diastole) of its chambers, ensuring a continuous forward flow of blood.
Atria: Act primarily as receiving chambers and