Human Circulatory System

Human Circulatory System

• Consists of a muscular chambered heart, a network of closed branching blood vessels, and blood
• The fluid which is circulated is blood

Heart

• Located in the thoracic cavity, between the two lungs, slightly tilted to the left
• Size of a clenched fist
• Protected by a double-walled membranous bag called pericardium, enclosing pericardial fluid
• Has four chambers: two atria and two ventricles
• Atria are smaller upper chambers, while ventricles are larger lower chambers
• Inter-atrial septum separates the right and left atria
• Inter-ventricular septum separates the left and right ventricles
• Atrio-ventricular septum separates the atrium and ventricle of the same side
• Openings between the chambers are guarded by valves: tricuspid valve (three muscular flaps) and bicuspid or mitral valve (two muscular flaps)

Cardiac Cycle

• Blood circulates through the four chambers of the heart during one cardiac cycle
• Atrial contraction (systole) occurs while ventricles relax (diastole)
• Atrial diastole is the stage where pulmonary veins and superior vena cava empty into the atria
• Sino-atrial node (SAN) stimulates both atria to contract, increasing pressure inside the atria by about 30%
• Action potential from SAN propagates through AVN and AV bundle, stimulating ventricles to contract
• Ventricular contraction (systole) causes semilunar valves to open, forcing blood to leave the ventricles
• Ventricular diastole coincides with atrial systole, increasing ventricular pressure and causing blood to flow from atria into ventricles

Quiz about the human circulatory system, including the heart and blood vessels.