The Heart

The Heart

1. Structure

   • Valves
     • Atrioventricular
       • These valves link the atria to the ventricles
     • Semilunar
       • These valves link the ventricles to the pulmonary artery and aorta
     • Valves stop blood flowing the wrong way. By: 1) the valves only open one way and whether they're open or closed depends on the relative pressure of the heart chambers 2) if there's higher pressure BEHIND a valve, it's forced open. 3) if the pressure is higher in the FRONT of the valve, its forced shut
   • Chamber walls
     • Each of the four chambers of the heart have a different function. The more work a chamber needs to do , the more muscle it requires therefore the thicker its wall
       • Left ventricle of the heart has a thicker wall than the right ventricle. This is because the left has to contract powerfully to pump blood all the way around the body and overcome systemic resistance. The right only needs to get blood to the lungs which are close to the heart
       • The ventricles have thicker walls than the atria and they have to push blood out of the heart whereas the atria only need to push blood into the ventricles

2. Cardiac Cycle

   • This pumps blood round the body and invloves the constant contraction and relaxation of the atria and ventricles which induce changes in pressure in all the chambers of the heart. This also causes different valves to open and close at the same time.
   • 1. atrial systole
     • This is when the ventricles relax and the atria contracts. The atria fill with blood - pressure in atria increases. increase in pressure causes atrioventricuular valves to open-allows blood to flow into ventricles. Atria then contracts, decreasing their volume and increasing the pressure even more to force the remaining blood out
   • 2. Venctricular systole
     • This is when the ventricles contract and the atria relaxes. The pressure is higher in the ventricles than in the atria so the atrioventricular valves close-prevents backflow. High preessure in the ventricles opens semiluna valves- blood is forces out into the pulmonary artey and aorta
   • 3. Dyastole
     • This is when the atria and ventricles are both relaxed. This increases their volume which therefore lowers the pressure in the chambers. The hight pressure in the arteries cause the semilunar valves to close -preventing backflow. The atria fills with blood again due to the higher pressure in the veins so the cycle then repeats

3. Co-ordination

   • The myogenic heart muscle controls the regular beating of the heart
     • 1) The sino-atrial node (SAN) in the right atrium is where the precess starts
       • 2) The SAN is a natural pacemaker so it sets the rhythm of the heartbeat by sending out waves of excitation to the atrial walls
         • 3) This wave causes both atria to contract at the same time
           • 4) A band of non conducting collagen tissue prevents the wave being passed directly from the atria to ventricles
             • 5) The wave of excitation is transferred from the SAN to the atrioventricular node (AVN)
               • 6) The AVN is responsible for passing the wave to the purkyne tissue in the ventricles walls, however there is a slight delay to ensure than the atria have been empties before the wave is sent
                 • 7) The purkyne tissue carries the waves of ecitation into the msucular walls of the ventricles through the septum and up from the apex which causes then to contract simutaneously from the bottom up