ESSAY 32: CV system. Pulmonary and Systemic circulation.

• Cardiovascular (CV) system


  Consists of heart, blood vessels and blood. Its primary role is to transport nutrients and materials to and from all parts of the body and also removes metabolic waste products from cells
• Role of the heart in the CV system

  To pressurise blood and provides the driving force for its circulation
• Circulatory system


  Forms a closed loop in the body so that blood pumped out of one side of the heart returns to back to the heart (via different vessels)
• Types of circulation in the body

  • Systemic circulation- pumping of blood from left ventricle to all organs and tissues under high pressure (120 mmHg)
  • Pulmonary circulation- blood pumped from right ventricle which passes through lungs to be oxygenated and returns to the left atrium (80mmHg)
• Pressure differences in systemic and pulmonary circulation

  Due to the composition of the vessels. In systemic circulation, pressure is higher because there is higher % of SM and smaller radius which leads to increased pressure
• Blood pressure in the pulmonary circulation


  Lower than in the systemic circulation. The walls of the pulmonary capillaries are thinner than those of similar vessels in the systemic circulation
• With ageing


  Less elastic fibers are present and more muscle fibers and there is an increase in production of vasoconstrictors
• Heart


  A muscular organ situated behind sternum and towards left side of body. Its main function is to pump oxygenates blood to tissues and organs and receive deoxygenated blood so that it can be oxygenated via the pulmonary circulation
• Chambers of the heart

  • 2 atria and 2 ventricles which are separated by valves
• Heart valves


  • Atrioventricular valves: Left atrioventricular valve is called bicuspid/mitral valve, right atrioventricular valve is called tricuspid valve
  • Semilunar valves: Aortic valve (between left ventricle and aorta) and the pulmonary valve (between right ventricle and pulmonary trunk)
• Layers of the heart

  • Pericardium- fibrous sac that encloses the heart
  • Myocardium- muscular layer of the heart which consists of cells called cardiac myocytes
  • Endocardium- inner layer of the heart
• Pericardium


  Consists of 2 layers called fibrous and serous pericardium. There 2 are separated by pericardial cavity (20-60ml of plasma ultrafiltrate). Function of this is to act as mechanical protection for heart and big vessels and also as lubrication to reduce friction between heart and surrounding structures
• Myocardium


  Consists of cells called cardiac myocytes. These are arranged in functional syncytium and have the characteristics of contractility, excitability, conductivity and rhythmicity. Function of this layer refers to contractile function of heart. In RV it's made of 2 layers and in LV it's made of 3 layers
• Endocardium


  Serves as a barrier between cardiac muscles and bloodstream. Purkinje fibers are in ct between endo and myocardium. It also produced ANP (stims Na+ excretion). Low Na+=low osmolarity so decrease in extracellular fluid. It has 2 layer- inner lines heart chambers and other layer is continuous with connective tissue of myocardium
• Innervation of the heart

  By sympathetic and parasympathetic fibers from the ANS. There is a network of nerves supplying the heart which is called cardiac plexus. This receives contributions from L/R vagus and sympathetic trunk. R vagus innervates SA node, L vagus innervates AV node. Atrial muscle is innervated by vagal efferents
• Sympathetic stimulation


  Leads to increase in heart rate and increase in myocardial contractility
• Parasympathetic stimulation


  Leads to opposite effects and also vasodilation
• Cardiac plexus


  Supplies conduction system, coronary vessels and also myocardium of atria and ventricles. PS parts receive contributions only from vagus. Preganglionic fibers enter plexus by synapsing with ganglia. S part is composed of fibers form sympathetic trunk, reaches plexus via cardiac nerves
• Afferent fibers in cardiac plexus

  Return to CNS via S cardiac branches and cardiac nerves from L/R vagus. Afferents that pass through vagal cardiac nerves return to corresponding vagus nerve. S afferents relay pain sensation from heart
• Coronary arteries


  Provide the main blood supply to the heart. The coronary arteries also supply the myocardium with oxygen to allow for the contraction of the heart and thus causing circulation of the blood throughout the body. Two main coronary arteries originate from the base of the aorta as it exits the left ventricle: the left and right coronary arteries. These arteries further branch into smaller arteries to supply specific parts of the heart like the atria, ventricles, SA, and AV nodes