Understanding the Circulatory System & Heart Anatomy

• The circulatory system is a network of organs and vessels responsible for the flow of blood, nutrients, oxygen, carbon dioxide, and hormones throughout the body.
• It includes the heart, blood vessels (arteries, veins, and capillaries), and blood
• Its primary function is to transport oxygen and nutrients to tissues and remove waste products.

Heart Anatomy

• The heart is a muscular organ located in the chest, slightly to the left of the sternum, within the mediastinum.
• A double-walled sac called the pericardium encloses it, protecting and anchoring the heart, preventing overfilling, and reducing friction.
• The heart wall consists of three layers: the epicardium (outer layer), the myocardium (middle muscular layer), and the endocardium (inner layer).
• The heart has four chambers: two atria (right and left) and two ventricles (right and left).
• The atria are the receiving chambers for blood returning to the heart, and the ventricles are the pumping chambers that eject blood into the pulmonary and systemic circuits.
• 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 into the pulmonary trunk, which carries it to the lungs for oxygenation.
• The left ventricle pumps oxygenated blood into the aorta, which distributes it to the rest of the body.
• The atria are separated from the ventricles by the atrioventricular (AV) valves: the tricuspid valve on the right and the mitral (bicuspid) valve on the left.
• These valves prevent backflow of blood from the ventricles into the atria during ventricular contraction (systole).
• The pulmonary valve is located between the right ventricle and the pulmonary artery, preventing backflow of blood into the right ventricle.
• The aortic valve is located between the left ventricle and the aorta, preventing backflow of blood into the left ventricle.
• The heart's fibrous skeleton provides support, anchors the heart valves, and provides an insertion point for cardiac muscle, also acting as an electrical insulator.

Blood Vessels

• Blood vessels form a network of tubes throughout the body that transport blood to and from the heart.
• There are three main types of blood vessels: arteries, veins, and capillaries.
• Arteries carry blood away from the heart, veins carry blood back to the heart, and capillaries are the site of exchange between blood and tissues.
• Arteries typically carry oxygenated blood, while veins usually carry deoxygenated blood, with the exception of the pulmonary circulation.
• Arteries have thicker walls than veins, with more elastic tissue and smooth muscle, which allows them to withstand the high pressure of blood pumped from the heart.
• The largest artery is the aorta, which receives blood from the left ventricle and branches into smaller arteries that supply blood to the various organs and tissues of the body.
• As arteries get smaller, they branch into arterioles, which regulate blood flow into capillaries through vasoconstriction and vasodilation.
• Veins have thinner walls and larger lumens compared to arteries, and they contain valves to prevent the backflow of blood, especially in the extremities.
• The largest veins are the superior vena cava and inferior vena cava, which drain blood from the upper and lower body, respectively, into the right atrium of the heart.
• Capillaries are the smallest blood vessels, with thin walls consisting of a single layer of endothelial cells, facilitating the exchange of oxygen, carbon dioxide, nutrients, and waste products between blood and tissues.
• Capillaries form dense networks called capillary beds in tissues, maximizing the surface area for exchange.
• Blood flows from arterioles into capillaries and then into venules, which merge into larger veins that carry blood back to the heart.

Blood Composition

• Blood is a specialized connective tissue consisting of plasma and formed elements (red blood cells, white blood cells, and platelets).
• Plasma is the liquid component of blood, making up about 55% of its volume, and consists mostly of water, along with proteins, electrolytes, nutrients, and waste products.
• Red blood cells (erythrocytes) are responsible for transporting oxygen from the lungs to the tissues and carbon dioxide from the tissues to the lungs.
• Red blood cells contain hemoglobin, a protein that binds to oxygen and gives blood its red color.
• White blood cells (leukocytes) are involved in the body's immune response, defending against infection and foreign invaders.
• There are several types of white blood cells, including neutrophils, lymphocytes, monocytes, eosinophils, and basophils, each with specific functions in the immune system.
• Platelets (thrombocytes) are small, irregularly shaped cell fragments that play a critical role in blood clotting (hemostasis).
• When a blood vessel is injured, platelets adhere to the damaged site, aggregate to form a plug, and release factors that initiate the coagulation cascade, leading to the formation of a blood clot.

Circulatory Pathways

• The circulatory system consists of two main circuits: the pulmonary circuit and the systemic circuit.
• The pulmonary circuit carries blood between the heart and the lungs for gas exchange.
• Deoxygenated blood is pumped from the right ventricle into the pulmonary trunk, which divides into the right and left pulmonary arteries, carrying blood to the lungs.
• In the lungs, blood flows through capillaries surrounding the alveoli, where oxygen is picked up and carbon dioxide is released.
• Oxygenated blood then returns to the left atrium of the heart via the pulmonary veins.
• The systemic circuit carries blood between the heart and the rest of the body.
• Oxygenated blood is pumped from the left ventricle into the aorta, which branches into smaller arteries that supply blood to the various organs and tissues of the body.
• In the tissues, blood flows through capillaries, where oxygen and nutrients are delivered, and carbon dioxide and waste products are picked up.
• Deoxygenated blood then returns to the right atrium of the heart via the superior vena cava (from the upper body) and inferior vena cava (from the lower body).
• The coronary circulation is a part of the systemic circulation that supplies blood to the heart muscle itself.
• The coronary arteries branch from the aorta and provide oxygenated blood to the myocardium.
• The coronary veins drain deoxygenated blood from the myocardium and empty into the coronary sinus, which then empties into the right atrium.

Cardiac Cycle

• The cardiac cycle refers to the sequence of events that occur during one complete heartbeat, including atrial and ventricular contraction (systole) and relaxation (diastole).
• The cardiac cycle consists of two main phases: diastole (relaxation) and systole (contraction).
• During diastole, the heart chambers relax and fill with blood.
• The atrioventricular (AV) valves are open, allowing blood to flow from the atria into the ventricles.
• The semilunar valves (aortic and pulmonary) are closed, preventing blood from flowing back into the ventricles from the aorta and pulmonary artery.
• During atrial systole, the atria contract, pushing any remaining blood into the ventricles.
• During ventricular systole, the ventricles contract, increasing pressure inside the ventricles.
• The AV valves close to prevent backflow of blood into the atria.
• The semilunar valves open, allowing blood to be ejected from the ventricles into the aorta and pulmonary artery.
• After ventricular systole, the ventricles relax, and the cardiac cycle begins again.
• The sinoatrial (SA) node, located in the right atrium, is the heart's natural pacemaker and initiates each heartbeat by generating electrical impulses.
• The impulses from the SA node spread through the atria, causing them to contract.
• The impulses then reach the atrioventricular (AV) node, which delays the signal slightly to allow the atria to finish contracting before the ventricles contract.
• From the AV node, the impulses travel down the bundle of His and then through the Purkinje fibers, which spread the impulses throughout the ventricles, causing them to contract.
• The electrical activity of the heart can be recorded using an electrocardiogram (ECG or EKG), which provides information about the heart rate, rhythm, and conduction.

Regulation of Circulation

• The circulatory system is regulated by both intrinsic and extrinsic mechanisms to maintain adequate blood flow and blood pressure.
• Intrinsic mechanisms include the Frank-Starling mechanism, which states that the force of ventricular contraction is proportional to the end-diastolic volume (the amount of blood in the ventricles at the end of diastole).
• Extrinsic mechanisms include neural and hormonal controls.
• The nervous system regulates circulation via the autonomic nervous system, which includes the sympathetic and parasympathetic branches.
• The sympathetic nervous system increases heart rate, contractility, and vasoconstriction, leading to increased blood pressure and blood flow.
• The parasympathetic nervous system decreases heart rate and contractility, leading to decreased blood pressure and blood flow.
• Hormones, such as epinephrine (adrenaline) and norepinephrine (noradrenaline), released from the adrenal medulla, have similar effects as the sympathetic nervous system, increasing heart rate, contractility, and vasoconstriction.
• The kidneys also play a role in regulating blood pressure by controlling blood volume through the release of hormones such as renin and aldosterone.
• Baroreceptors, located in the aorta and carotid arteries, detect changes in blood pressure and send signals to the brainstem to adjust heart rate, contractility, and vasoconstriction to maintain blood pressure within a normal range.
• Chemoreceptors, located in the aorta and carotid arteries, detect changes in blood oxygen and carbon dioxide levels and send signals to the brainstem to adjust ventilation and circulation to maintain adequate gas exchange.