2A.body fluids & circulation

• In open circulation, blood flows through vessels and sinuses, while capillaries are absent. Examples include leeches, arthropods, molluscs, echinoderms, and ascidians
• In closed circulation, blood flows through blood vessels, and capillaries are present. Examples include annelida, cephalopoda, cephalochordata, and vertebrata
• The sino-atrial node (SAN) is called the pacemaker because it has the ability to generate action potentials without any external stimuli
• The SAN initiates the heartbeat and is located in the right upper part of the right atrium
• The atrioventricular node is located in the lower left corner of the right atrium close to the interatrial septum. It acts as a relay point for action potentials received from the SAN
• The atrioventricular bundle, or bundle of His, is a bundle of nodal fibers present in the interventricular septum. It carries electrical impulses (action potential) from the AV node to the purkinje fibers located in the lateral walls of the ventricles
• The left atrioventricular aperture is guarded by the bicuspid or mitral valve, while the right atrioventricular aperture is guarded by the tricuspid valve
• The valve of Thebesius is present at the opening of the coronary sinus into the right atrium
• The pulmonary arch arises from the left anterior side of the right ventricle and carries venous blood to the lungs. The systemic arch arises from the right side of the left ventricle and carries oxygenated blood to various parts of the body
• Heart sounds are 'Lub' and 'Dup'
• First heart sound is 'Lub', produced in ventricular systole due to closure of the AV valves
• Second heart sound is 'Dup', produced in cardiac diastole due to closure of semilunar valves
• Cardiac cycle: Cardiac events from the beginning of one heartbeat to the beginning of the next
• Duration of cardiac cycle is about 0.8 sec
• Cardiac output: Volume of blood pumped out by the ventricle per minute
• Cardiac output = stroke volume x No. of beats per minute = 5040 ml/min or approximately 5 liters
• Double circulation: Blood circulation where blood passes through the heart twice in a complete cycle
• Consists of pulmonary circulation and systemic circulation
• Significance: Prevents mixing of oxygenated and deoxygenated blood
• Arteries are more elastic than veins
• Arteries are more elastic to cope with high pressure blood pumped from the heart
• Pressure at major arteries is more than in minor arteries
• Veins have walls with minimum elasticity due to lower pressure