Chapter 5 - Cardiac Function

Chapter 5 - Cardiac Function

1. Structure of the Heart

   • The heart is a muscular, cone-shaped organ, and is approximately the size of a clenched fist
   • The heart is divided into two parts and each part contains a ventricle and an atrium
     • The left side of the heart has thicker walls as it needs to pump blood all around the body
   • Blood Vessels:
     • VENA CAVA - carries deoxygenated blood from the body to the right atrium
     • PULMONARY ARTERY - carries deoxygenated blood from the right ventricle to the lungs
     • PULMONARY VEIN - delivers oxygenated blood from the lungs to the left atrium
     • AORTA - delivers oxygenated blood from the left ventricle to the body
   • Valves:
     • They ensure that blood flows in one direction only and prevent the backflow of blood

2. The Conduction System

   • Myogenic - it creates its own impulse
   • An electrical impulse originates in the SA node
     • It spreads through the walls of the atria, causing them to contract and forcing blood into the ventricles
       • The signal then passes through the AV node
         • There is a 0.1 second delay to enable the atria to contract fully
           • Then passes through the bundle of His
             • Then to the purkyne fibres causing the ventricle to contract

3. The Cardiac Cycle

   • Cardiac Cycle - describes the emptying and filling of the heart
   • Diastole Phase - the relaxation phase of the heart where the chambers fill with blood
   • Systole Phase - the contraction phase of the heart where blood is forced around the heart or out of the heart
   • Each cardiac cycle takes approximately 0.8 seconds
   • The flow of blood is controlled y the conduction system

4. Cardiac Dynamics

   • Stroke Volume
     • The amount of blood pumped out by the left ventricle per beat
     • Determined by:
       • Venous return
         • Increase in VR = increase in SV
       • The elasticity of the cardiac fibres
         • A greater force of contraction = greater stroke volume
       • The contractility of the cardiac tissue
         • The greater the force = the greater the contraction
     • Increases as exercise intensity increases - up to 40-60%
   • Heart Rate
     • In response to exercise:
       • HR increases with exercise, depends on the intensity of the exercise
       • Anticipatory rise - due to hormonal action of adrenaline
     • The number of times the heart beates per minute
     • Average = 72 bpm
   • Cardiac Output
     • In response to exercise:
       • Bradycardia - a decrease in resting heart rate to below 60 beats per minute
     • The amount of blood pumped out of the left ventricle per minute
     • CO = SV x HR

5. Control of Heart Rate

   • Heart rate increases during exercise to ensure that the working muscles receive more oxygen
   • Neural Control Mechanism
     • Sympathetic system - stimulates the heart to beat faster
     • Parasympathetic - returns the heart to its resting level
     • Receptors (e.g. chemo,baro, propria) send impulses to the cardiac control centre
       • Sends an impulse through the sympathetic nervous system to the SA node to increase heart rate
     • When exercise stops receptors send impulses to the cardiac control centre.
       • An impulse is sent through the parasympathetic system which stimulates the SA node and heart rate decreases
   • Hormonal control mechanism
     • Exercise causes stress-induced adrenaline response which results in:
       • Stimulation of the SA node
       • Increase in blood pressure
       • Increase in blood glucose levels

6. CV Drift

   • A progressive decrease in stroke volume and arterial blood pressure and a rise in heart rate
   • Occurs after round 10 minutes
   • To minimise CV drift it is important to maintain high fluid consumption

7. Effects of training on the heart

   • VO2 max - the maximum amount of oxygen that can be taken in and used by the body in 1 minute
   • Athletes heart - enlarged hear caused by repeated strenuous exercise
   • Hypertrophy - the heart gets bigger and stronger which results in bradycardia
   • Maximum Cardiac Output uncreases
   • Increased cappilarisation of the muscle
   • Increased contractility