Cardiovascular Anatomy and Physiology

Cardiovascular Anatomy and Physiology:

Structure of heart:
The heart pumps blood through circulatory system – heart muscle consists of 2 atria (pumps blood into ventricles) and 2 ventricles (pump blood out of heart)

Coronary circulation:
• distribution of blood to the heart
• left main and the right coronary arteries arise just distal to aortic valve. Left main divides into the left anterior descending (LAD) artery and circumflex artery (LCX)
• LAD  supplies anterior left ventricle, apex and anterior part of septum
• LCX  posterior left ventricle and inferior surface
• right coronary artery  supplies right atrium, right ventricle and inferior aspects of left ventricle
• venous system follows arteries
• vessels receive sympathetic and parasympathetic innervation:
• stimulation of alpha receptors  vasoconstriction
• stimulation of beta-2-adrenoceptors  vasodilation

Electrical conduction system:
• sinoatrial (SA) node is at junction of superior vena cava and right atrium – it is the origin of the impulses for contraction of heart muscle
• depolarisation of SA node  triggers wave of depolarisation through atrium (conduction directly to ventricles is prevented by annulus fibrosis)
• Bundle of His passes through annulus fibrosus  divides into left and right  passes down sides of ventricular septum

Heart valves:
Two atrioventricular (AV) valves – the tricuspid and mitral/bicuspid
Two semilunar valve – pulmonic and aortic

Cardiac cycle:
1) Impulse from SA node  depolarisation and contraction of atria (right atrium contracts just before the left)
2) AV valves open  ventricles filled with blood
3) Impulse propagated through Bundle of His and Purkinjie system (delay allows filling of ventricles)  ventricle contracts from apex of heart towards the base.
4) Pressure in ventricles exceeds that in atria  AV valves close
5) As atria relax, they begin to fill with blood
6) Pressure in ventricles exceeds the pressure in the pulmonary artery and aorta  pulmonary and aortic valves open  blood is pumped into systemic and pulmonary circulation
7) As ventricles relax, pulmonary and aortic valves close
Systole = period of ventricular contraction
Diastole = period of ventricular relaxation

Flow of blood through the heart:
Venous circulation  superior and inferior vena cava  right atrium  tricuspid valve  right ventricle  pulmonary valve  pulmonary artery  lungs  pulmonary veins  left atrium  mitral valve  left ventricle  aortic valve  aorta  arterial circulation

Cardiac output:
• determined by product of stroke volume and heart rate (CO = HR x SV)
• heart rate (HR) is controlled by autonomic nervous system. The rate is increased via the sympathetic branches that act via the beta1-adrenergic receptors in SA node; rate is decreased via the parasympathetic branches that act via the muscarnic receptors in the SA node.
• stroke volume (SV) is dependent on end diastolic pressure (preload), peripheral vascular resistance (afterload) and myocardial contractibility
• preload is determined by the force of venous return (greater force/amount of venous return  greater stretch/tension in ventricles). An increase in preload  increase in stroke volume
• afterload is the arterial pressure that the left ventricle needs to overcome to pump the blood. Afterload is determined by the peripheral resistance, which is determined by constriction or vasodilation of arterioles
• Starling’s law – the force of ventricular contraction is proportional to muscle fibre length