Heart Anatomy and Physiology

Cardiovascular Physiology

• Cardiovascular physiology studies the structure and function of the heart and blood vessels.
• Objectives include describing cardiac tissue structure and function, identifying blood flow through the heart and body, explaining the cardiac conduction system, and identifying factors influencing cardiac output and blood pressure regulation.
• Blood components and their functions, including clotting, are also key elements.

Medical Terminology

• Medical terminology uses prefixes, suffixes, and combining forms to describe medical conditions, procedures, and anatomical structures related to the cardiovascular system.
• Combining forms like "angi/o" (vessel) and "aort/o" (aorta), prefixes such as "tachy" (rapid) and "brady" (slow), and suffixes like "-itis" (inflammation) and "-ectomy" (excision) are often used.

Cardiac Muscle

• Cardiac muscle is comprised of myocytes, which generate their own action potentials, unlike skeletal muscle.
• Intercalated discs connect adjacent cells, facilitating rapid electrical activity spread within the heart to produce synchronized contractions.
• Cardiac muscle acts as an endocrine tissue, releasing ANP (atrial natriuretic peptide) in response to increased atrial pressure or exercise.

Structure of the Heart

• The heart is located in the mediastinum between the lungs, behind the sternum.

• The base of the heart is superior; the apex (tip) is inferior and leftward.

• The heart is enclosed by pericardial membranes.

• The fibrous pericardium is the outermost layer, a strong, loose-fitting sac.

• Serous pericardium is a folded membrane with two layers: parietal (lines the fibrous pericardium) and visceral (epicardium) pericardium (covers the heart muscle) to reduce friction.

• The heart has four chambers: two atria (receiving chambers) and two ventricles (pumping chambers).

• The atria are separated by an interatrial septum; the ventricles by an interventricular septum.

• The left atrium receives oxygenated blood from the lungs via pulmonary veins; the right atrium receives deoxygenated blood from the body (superior and inferior vena cava).

• The left ventricle pumps blood to the body via the aorta; the right ventricle pumps blood to the lungs via pulmonary arteries.

Structure of the Heart: Pericardial Membranes

• Descriptions of fibrous pericardium and serous pericardium.
• The pericardial cavity between the membranes contains serous fluid to reduce friction as the heart beats.

Structure of the Heart: Layers

• The heart has three main layers:
• Myocardium: the thickest layer, composed of cardiac muscle, responsible for pumping.
• Endocardium: the innermost layer, composed of simple squamous epithelium, which lines the chambers and valves.
  • Also covers heart valves & continues into vessels as their lining.
• Epicardium: also know as visceral pericardium, the outermost layer, and reduces friction.

Valves of the Heart

• Atrioventricular (AV) valves (tricuspid and bicuspid/mitral) prevent backflow from the ventricles to the atria.
• Semilunar valves (pulmonary and aortic) prevent backflow from the arteries to the ventricles.

Structures & Function of the Heart: Other Structures and Features

• Papillary muscles and chordae tendineae prevent inversion of AV valves during ventricular contraction
• The fibrous skeleton of the heart provides support for the valves and separates the atria from the ventricles, insulating them from each other so the atria contract alone.

Coronary Circulation

• The heart receives its blood supply via the coronary arteries branching from the aorta.
• Coronary arteries deliver oxygenated blood to the heart muscle.
• Coronary veins drain deoxygenated blood from the heart muscle into the coronary sinus, which returns blood to the right atrium.

Cardiac Cycle & Heart Sounds

• The cardiac cycle refers to the sequence of events in one heartbeat.
• Systole (contraction) is followed by diastole (relaxation) of heart chambers.
• Heart sounds (lub-dub) are produced by the closure of heart valves.
• The first sound (S1) is created by the closure of AV valves; the second (S2) is by the closure of semilunar valves.

Pathways of Circulation

• The pulmonary circulation circulates blood between the heart and lungs for gas exchange.
  • The pathway begins with the right ventricle pumping blood to the lungs via the pulmonary artery and returning it back via the pulmonary vein.
• The systemic circulation circulates blood from the heart to the body and back.
  • The pathway begins with the left ventricle pumping blood to the body via the aorta and returning it through the vena cava.

Cardiac Conduction System

• The cardiac conduction system regulates the heart's rhythmic contractions.
  • Nerve impulses are not required for contraction to occur.
• The SA node (sinoatrial node) is the heart's natural pacemaker, located in the right atrium.
  • Depolarizes (initiates heartbeat) 60-80 times per minute.
• The AV node (atrioventricular node) relays electrical signals between the atria and ventricles.
• The bundle of His (AV bundle) transmits the signal through the interventricular septum to bundle branches.
• The bundle branches then transmit impulses to the Purkinje fibers, which stimulate the ventricular contraction.

Resting Heart Rate (HR)

• Healthy adult resting HR is 60-80 bpm.
• Parasympathetic input (vagus nerves) decreases HR; sympathetic input increases it.
• Well-trained individuals may have lower resting heart rates due to greater heart efficiency

Heart Rate

• Heart rate is primarily regulated by the autonomic nervous system, either increasing or decreasing it, via parasympathetic and sympathetic inputs.
• If the SA node malfunctions, the AV node can take over, but at a slower rhythm resulting in overall slower heart beat.

Cardiac Physiology

• Cardiac output (CO) is the volume of blood ejected by a ventricle per minute, calculated as CO = stroke volume (SV) × heart rate (HR).
• Stroke volume (SV) is the volume of blood pumped out by a ventricle per contraction.
• Ejection fraction is the percentage of blood ejected by the ventricle per beat.
• Preload is the force that stretches cardiac muscle cells before contraction.
• Afterload is the force required to eject blood from the ventricles.

Regulation of Heart Rate

• Cardiac control centers in the medulla oblongata regulate heart rate.
• The accelerator center increases HR and contractility, while the inhibitory center decreases HR.
• Baroreceptors and chemoreceptors detect changes in blood pressure and blood oxygen, respectively.
• Sensory nerves carry this information to the cardiac control centers.

Vascular Physiology

• Arteries and arterioles carry blood away from the heart.
• Veins and venules return blood to the heart.
• Capillaries are the sites of material exchange between blood and tissues.

Blood

• Blood is a crucial body fluid, responsible for transporting essential nutrients.
• Blood volume is 4-6 Liters, composed of plasma (~55%) and formed elements (cells, ~45%).
• Plasma is mostly water, containing essential proteins, nutrients, various waste products, electrolytes and hormones.
• Blood cells include erythrocytes (red blood cells, crucial for oxygen transport), leukocytes (white blood cells, involved in the immune response), and thrombocytes (platelets, vital for blood clotting).

Capillaries

• Capillaries are structures with walls composed only of a single layer of endothelial cells.
• Extensive capillary networks in body tissues facilitate rapid gas exchange, fluid transfer, nutrient delivery, and waste removal.

Capillary Exchange

• Capillary exchange is the process by which materials are exchanged between the blood in the capillaries and the interstitial fluid surrounding body tissues.
• This process occurs via diffusion or filtration.

Velocity of Blood Flow

• Blood velocity in the vascular system varies.
• In capillaries, velocity is the lowest allowing more efficient exchange of material.
• Velocity is fastest in arteries, becoming slower in arterioles and then capillaries, before increasing again in the venules and veins.

Blood Pressure

• Blood pressure is the force exerted by blood against the walls of blood vessels.
• Blood pressure is influenced by cardiac output, blood volume, peripheral resistance, and elasticity.

Factors Affecting Blood Pressure

• Various factors are involved in maintaining blood pressure, including:
  • Venous return
  • Cardiac contractility
  • Peripheral resistance (the resistance that blood vessels offer to flow)
  • Elasticity of large arteries
  • Viscosity of blood
  • Hormones

Summary of Hormones in Blood Pressure Regulation

• Hormones like epinephrine and norepinephrine increase heart rate and contractility, leading to increased blood pressure.
• Hormones such as ADH (antidiuretic hormone; vasopressin) and aldosterone increase blood volume, and thus, increase blood pressure.
• ANP (atrial natriuretic peptide) is a hormone that reduces blood volume and blood pressure.

Renin-Angiotensin Mechanism

• This mechanism is a crucial regulatory system that helps maintain blood pressure by adjusting peripheral resistance.
• In response to lowered blood pressure, the kidneys release renin.
• This activates the conversion of angiotensinogen into angiotensin II.
• Angiotensin II causes vasoconstriction to increase blood pressure and the release of aldosterone to increase blood volume.

Test your knowledge of heart structure and function, including heart wall layers, valves, and cardiac cycle phases. Explore the role of ANP, mitral valve prolapse, and effects of drugs that disrupt adipocyte conversion. Learn the impact of toxins on intercalated discs.