Circulatory System and Heart Structure

• The circulatory system has three main components: blood, the heart, and blood vessels.
• Blood is a connective tissue composed of blood plasma, blood cells, and platelets that acts as a medium of transportation
• The heart is a muscular pump that circulates blood throughout the body
• Blood vessels, including arteries, capillaries, and veins, are connected to the heart and transport blood

Structure of the Heart

• The heart is located between the lungs in the thorax cavity and contains four chambers: the left and right atria, and the left and right ventricles
• The chambers separates by a muscular wall called the septum
• Atria receive blood returning to the heart, while ventricles pump blood out of the heart
• Ventricles have thicker walls and contract more strongly than atria
• The left ventricle has a thicker muscular wall than the right ventricle because it needs to generate greater pressure to pump blood to the whole body, while the right ventricle only pumps blood to the lungs
• The aorta is the main artery transporting oxygenated blood to the body, and the vena cava is the main vein transporting deoxygenated blood back to the heart
• Semilunar valves at the base of the pulmonary artery and aorta prevent blood from flowing back into the ventricles when they relax
• The pulmonary artery transports deoxygenated blood to the lungs, and pulmonary veins transport oxygenated blood from the lungs to the heart
• The bicuspid valve, between the left atrium and ventricle, ensures blood flows into the left ventricle but not back into the left atrium (two leaflets)
• The tricuspid valve, between the right atrium and ventricle, ensures blood flows into the right ventricle but not back into the right atrium (three leaflets)
• The septum separates the left and right sides of the heart, preventing mixing of oxygenated and deoxygenated blood
• Coronary arteries supply oxygenated blood to the heart tissue, while coronary veins transport deoxygenated blood away

Composition of Human Blood

• Human blood consists of 55% plasma and 45% cell components
• Plasma is the medium of transportation
• Blood cells consist of red blood cells (erythrocytes), platelets, and white blood cells (leucocytes)

Plasma

• Blood plasma is 90% water and acts as a medium of transportation and a solvent
• Fibrinogen plays a role in blood clotting
• Albumin controls blood osmotic pressure
• Globulin is a type of antibody involved in the body's defense
• Solutes such as nutrients provide energy, growth and maintenance
• Excretory substances need to be disposed of
• Oxygen is required for respiration
• Hormones and enzymes control physiological activities and metabolic processes

Blood Cell Types

• Erythrocytes, or red blood cells, have an elastic plasma membrane and a biconcave disc shape for efficient gaseous exchange
• Erythrocytes do not have a nucleus at maturity to maximize hemoglobin content being produced in the bone marrow of bones such as the sternum and ribs
• Erythrocytes live up to 120 days and are destroyed in the liver or lymph via phagocytosis
• Each erythrocyte contains hemoglobin, a red pigment that gives blood its red color
• Hemoglobin contains a heme group with iron, the binding site for oxygen
• Hemoglobin combines with oxygen to form oxyhemoglobin in high oxygen conditions
• Oxyhemoglobin releases oxygen in low oxygen conditions and is involved in blood clotting
• Platelets are produced from bone marrow cell cytoplasm fragments and have a lifespan of less than one week
• Leucocytes, or white blood cells, have an irregular shape, contain a nucleus, and lack hemoglobin

Leucocytes

• Leucocytes are produced in bone marrow, have a lifespan of less than five days, and can diffuse out of capillary pores to fight pathogens in tissues
• Granulocytes (neutrophils, eosinophils, basophils) and agranulocytes (lymphocytes, monocytes) are the two types
• Neutrophils have a two-to-five-lobed nucleus and ingest bacterial cells and dead tissues via phagocytosis
• Eosinophils have a two-lobed nucleus and release enzymes to fight inflammation and allergy reactions
• Basophils are the least numerous, containing heparin to prevent blood clotting
• Lymphocytes have a large nucleus and produce antibodies to destroy bacteria and viruses, including antitoxins
• Monocytes are the largest leukocytes, with a spherical nucleus, and ingest bacteria and dead cells or tissues via phagocytosis

Human Blood Vessels

• Arteries transport blood out of the heart, quickly at high pressure due to the heart's pumping action
• The aorta is the main artery leaving the heart, expanding when blood is received and artery walls are elastic which prevents vessel breakage from the high pressure
• Arteries branch into smaller arterioles, leading to capillary networks
• Capillaries have very thin walls (one cell thick) to allow gas exchange between blood and cells via diffusion of nutrients, excretory substances, and hormones
• Capillaries rejoin to form venules, which combine into veins that transport blood back to the heart
• The vena cava is the main vein carrying deoxygenated blood back to the heart

Differences Between Arteries, Capillaries, and Veins

• Artery walls are thick, muscular, and elastic, capillaries are one cell thick, and vein walls are thin and less muscular
• Artery lumens are small, capillary lumens are very tiny, and vein lumens are large
• Arteries lack valves (except for semilunar valves at the aorta and pulmonary artery base), capillaries have no valves, and veins contain valves for unidirectional blood flow
• Blood pressure is high in arteries, low in capillaries, and very low in veins
• Arteries carry blood from the heart to the entire body, capillaries transport blood from arteries to veins, and veins carry blood from the body back to the heart

Overview of the circulatory system's components: blood, heart, and blood vessels. Detailed look at the heart's structure, including its four chambers—atria and ventricles—separated by the septum. Focus on the ventricles' thicker walls and their roles in pumping blood.