Circulatory System and Heart Anatomy

• The circulatory system transports oxygen, nutrients, hormones, and cellular waste products throughout the body
• It ensures tissues and organs are properly perfused
• The major components are the heart, blood vessels (arteries, veins, capillaries), and blood

Heart Anatomy

• The heart is a muscular organ in the thoracic cavity, between the lungs in the mediastinum
• The heart is enclosed in the double-walled pericardium
• The pericardium has two layers: the outer fibrous pericardium and the inner serous pericardium
• The serous pericardium divides into the parietal layer (lining the fibrous pericardium) and the visceral layer (epicardium) covering the heart surface
• Pericardial fluid, between the parietal and visceral layers, reduces friction during heartbeats
• The heart wall has three layers: epicardium, myocardium, and endocardium
• The epicardium is the outer layer, also known as the visceral layer of the serous pericardium
• The myocardium is the middle and thickest layer, composed of cardiac muscle tissue, responsible for the heart's pumping action
• The endocardium is the inner layer lining the heart chambers and covering the valves, consisting of endothelium and connective tissue

Heart Chambers and Valves

• The heart has four chambers: the right atrium, right ventricle, left atrium, and left ventricle
• The atria are smaller, thin-walled chambers that receive blood returning to the heart
• The ventricles are larger, thick-walled chambers that pump blood out of the heart
• The right atrium receives deoxygenated blood from the superior vena cava, inferior vena cava, and coronary sinus
• The left atrium receives oxygenated blood from the pulmonary veins
• The right ventricle pumps deoxygenated blood to the lungs via the pulmonary artery
• The left ventricle pumps oxygenated blood to the body via the aorta
• Valves ensure unidirectional blood flow through the heart:
• Atrioventricular (AV) valves are located between the atria and ventricles
• The tricuspid valve is on the right side (right atrium to right ventricle)
• The mitral (bicuspid) valve is on the left side (left atrium to left ventricle)
• Semilunar valves are located at the exit of each ventricle
• The pulmonary valve is between the right ventricle and pulmonary artery
• The aortic valve is between the left ventricle and aorta
• Chordae tendineae and papillary muscles prevent the AV valves from inverting during ventricular contraction

Cardiac Blood Supply

• The heart receives its blood supply from the coronary arteries
• The right and left coronary arteries originate from the aorta, near the aortic valve
• The left coronary artery typically branches into the left anterior descending (LAD) artery and the circumflex artery
• The right coronary artery supplies the right atrium, right ventricle, and part of the left ventricle
• Coronary veins drain deoxygenated blood from the myocardium into the coronary sinus, which empties into the right atrium

Cardiac Cycle

• The cardiac cycle comprises all events associated with one heartbeat, including systole (contraction) and diastole (relaxation) of the atria and ventricles
• Atrial systole: Atria contract, pushing blood into the ventricles
• Ventricular systole: Ventricles contract, increasing pressure and ejecting blood into the pulmonary artery and aorta
• Atrial diastole: Atria relax and fill with blood
• Ventricular diastole: Ventricles relax and fill with blood
• The heart sounds ("lub-dub") are produced by the closing of the heart valves
• "Lub" is the sound of the AV valves closing at the start of ventricular systole
• "Dub" is the sound of the semilunar valves closing at the start of ventricular diastole

Blood Vessels

• Arteries carry blood away from the heart
• They have thick, elastic walls to withstand high pressure
• Large arteries branch into smaller arterioles
• Veins carry blood toward the heart
• Veins have thinner walls than arteries and contain valves to prevent backflow of blood
• Small venules merge into larger veins
• Capillaries are the smallest blood vessels
• They form a network connecting arterioles and venules
• Their thin walls allow for exchange of gases, nutrients, and waste products between blood and tissues

Blood Composition

• Blood is composed of plasma and formed elements (cells and cell fragments)
• Plasma is the liquid component, consisting of water, proteins, electrolytes, nutrients, and waste products
• Formed elements include:
  • Red blood cells (erythrocytes): transport oxygen bound to hemoglobin
  • White blood cells (leukocytes): involved in immune responses
  • Platelets (thrombocytes): involved in blood clotting

Blood Circulation

• Pulmonary circulation:
  • Deoxygenated blood flows from the right ventricle to the lungs via the pulmonary artery
  • In the lungs, carbon dioxide is exchanged for oxygen
  • Oxygenated blood returns to the left atrium via the pulmonary veins
• Systemic circulation:
  • Oxygenated blood flows from the left ventricle to the body via the aorta
  • Oxygen and nutrients are delivered to tissues, and waste products are removed
  • Deoxygenated blood returns to the right atrium via the superior and inferior vena cava
• Coronary circulation:
  • Supplies the heart muscle with oxygenated blood via the coronary arteries
  • Drains deoxygenated blood from the heart muscle via the coronary veins

Blood Pressure

• Blood pressure is the force exerted by blood against the walls of blood vessels
• Systolic pressure is the highest pressure during ventricular contraction (systole)
• Diastolic pressure is the lowest pressure during ventricular relaxation (diastole)
• Blood pressure is typically measured in millimeters of mercury (mm Hg)
• Normal blood pressure is around 120/80 mm Hg
• Blood pressure is influenced by factors such as cardiac output, blood volume, and peripheral resistance

Regulation of Heart Function

• The heart's function is regulated by both intrinsic and extrinsic mechanisms
• Intrinsic regulation includes the Frank-Starling mechanism, where increased venous return leads to increased stroke volume
• Extrinsic regulation involves the autonomic nervous system and hormones:
  • Sympathetic stimulation increases heart rate and contractility
  • Parasympathetic stimulation (vagus nerve) decreases heart rate
  • Hormones like epinephrine and norepinephrine increase heart rate and contractility
• The baroreceptor reflex helps regulate blood pressure by detecting changes in arterial pressure and adjusting heart rate and blood vessel diameter accordingly

Electrocardiogram (ECG)

• An ECG is a recording of the electrical activity of the heart
• It is used to diagnose heart conditions such as arrhythmias, ischemia, and infarction
• The ECG waveform consists of several components:
  • P wave: represents atrial depolarization
  • QRS complex: represents ventricular depolarization
  • T wave: represents ventricular repolarization
• Abnormalities in the ECG waveform can indicate various cardiac disorders