section a: applied anatomy and physiology 9. structure and function of the heart
TRANSCRIPT
Section A: Applied Anatomy and Physiology
9. Structure and function of the heart
Syllabus • Internal and external structure of the heart, to include the heart
chambers and valves, all blood vessels attached to the heart, the heart wall and pericardium
• Conduction system of the heart, cardiac cycle • Definitions and relationship between cardiac output, stroke
volume, heart rate: differences in values at rest and during exercise • Regulation of heart rate to include, neural, hormonal and intrinsic
factors• Measurement of heart rate response to varying intensities of
workload• Heart rate response during recovery, with a graphical
representation of data
Section A: Applied Anatomy and Physiology
10. Function of the vascular system
Syllabus
• Pulmonary and systemic circulatory systems • Factors linked with venous return • Distribution of cardiac output at rest and during
exercise, to include the vascular shunt mechanism, the role of precapillary sphincters and the role of the vasomotor centre
• Blood flow, blood velocity, blood pressure and the effects of exercise on blood pressure
• Oxygen and carbon dioxide transport
Location of the Heart
• Size of a closed fist • Rests on the diaphragm • Near midline of thoracic cavity • Lies in the MEDIASTINUM – Between the lungs– From sternum to vertebral column
Pericardium
• Pericardium (Fibrous)– Prevents overstretching – Provides protection – Anchors the heart
Layers of the Heart Wall
• Epicardium (slippery texture of outermost surface)– External layer
• Myocardium (cardiac muscle)– Middle layer
• Endocardium (covers valves)– Inner layer
External Structure
Internal Structure
Electrical System of the Heart
• Auto-rhythmic fibres– Cardiac muscle fibres are self-excitable
– They act as a pacemaker – They form the conduction system
The Conduction System Sequence
1. Sino-Atrial node 2. Atrioventricular node3. Bundle of His4. Right and left bundle branches 5. Purkinje fibres
Control of the Heart
• Two mechanisms that help to control the heart rate
– Neural – Hormones
Autonomic Regulation of Heart Rate
• Cardiovascular Centre (Brain) – In the MEDULLA OBLONGATA
• Receives input from sensory receptors and higher brain centres
• Direct appropriate output by increasing or decreasing frequency of nerve impulses
• Cardiac acceleratory centre (sympathetic)• Cardiac inhibitory centre (parasympathetic)
Autonomic Nervous System Divisions
Sympathetic Nervous System• Increases HR by releasing
adrenaline and noradrenaline
• Adrenaline increases strength of contraction
• Noradrenaline aids the spread of the impulse throughout the heart
Parasympathetic Nervous System
• Slows HR by releasing acetylcholine
• Acetylcholine slows the spread of impulses and therefore reduces the heart rate
Cardiac Function of Autonomic Nervous System
Sympathetic Function • Increased HR• Increased strength of
contraction • Vasodilation of arteries
supplying muscles • Some vasoconstriction of
arteries of abdomen etc
Parasympathetic Function• Decreased HR• Decreased strength of
contraction • Vasoconstriction of arteries
supplying muscles • Vasodilation of arteries of
abdomen etc
Neural Factors
• Proprioceptors– Found in muscles, tendons, joints– Informing where movement is occuring
• Chemoreceptors– Located in aorta and carotid arteries– Informing of composition of blood – CO2
• Baroreceptors– Found in aorta, atria and vena cava– Inform of changes in blood pressure
Chemical Regulation of Heart Rate
• Chemical influence– Hypoxia (lowered oxygen levels)– Low OR High pH – ALL DEPRESS CARDIAC ACTIVITY
• Hormones– Epinephrine and norepinephrine increase HR and
contractility
Other Factors affecting Heart Rate
• Temperature– Blood viscosity
• Age – Younger people tend to have higher heart rates
• Gender – Women tend to have higher heart rates
Cardiac Dynamics
• The performance of the heart is largely dependent on two variables:
• Stroke volume• Heart rate
• Multiplying these two variables will provide you with an individual’s CARDIAC OUTPUT
CO = SV × HR
• An average person has a resting heart rate of 70 beats/minute and a resting stroke volume of 70 mL/beat.
• The cardiac output for this person at rest is:
Cardiac Output = 70 (beats/min) X 70 (mL/beat) = 4900 mL/minute.
Cardiac Output Facts
• Your entire blood volume flows through your pulmonary and systemic circulations EACH MINUTE
• MILD EXERCISE– HR (100bts.min) and SV (100ml.beat)
• INTENSE EXERCISE (not maximal)– HR (150bts.min) and SV (130ml.beat)
Stroke Volume
• The volume of blood pumped out the heart per beat
• Usually refers to the blood ejected from the left ventricle
• Typically the resting value is 75ml (non-trained individual)
Stroke Volume: Determining Factors
• Venous return – Volume of blood returning to right atrium
• Elasticity of cardiac fibres (Starlings Law)– Also known as pre-load– Degree of stretch prior to contraction
• Contractility of cardiac tissue
Cardiac Dynamics during Exercise
• Heart Rate Response– Extent of increase is dependent on extent of intensity
• Steady state • Anticipatory rise
• Stroke Volume Response– At 40-60% of maximum effort it plateaus – Able to increase for 2 reasons
• Increased venous return (due to muscle pump)• Frank-Starling mechanism (stretch more/contract more)
• Cardiac Output Response – During maximum exercise, Q may reach values of 8 times that
of resting
Stroke Volume Response to Exercise
Resting Stroke Volume
Submaximal Exercise
Maximal Exercise
Trained 80-110ml 160-200ml 160-200mlUntrained 60-80ml 100-120ml 100-120ml
Cardiac Output Response to Exercise
Resting Cardiac Output
Submaximal Exercise
Maximal Exercise
Trained 5L/min 15-20L/min 30-40L/minUntrained 5L/min 10-15L/min 20-30L/min
A Trained Heart
• Cardiac hypertrophy– Endurance athletes tend to display larger ventricular cavities– High-resistance strength training will display thicker ventricular
walls
• Bradycardia– Reduction in the resting heart rate that accompanies training
• Ejection fraction – Proportion of blood actually pumped out of the left ventricle per
contraction – SV divided by EDV (end diastolic volume)
Blood
• Consists of cells and cell fragments surrounded by plasma
• Average male has between 5-6L
• Average female has between 4-5L
Functions of Blood
• Transportation of nutrients (glucose and oxygen)
• Protection and fighting disease through interaction with lymphatic system
• Maintenance of homeostasis (temp regulation, enzyme and hormone action, pH balance)
Blood Viscosity
• The relative thickness of blood
• Ratio of blood cells to plasma (dehydration)
• Training brings an increase in the total blood volume and therefore an increase in the number of red blood cells. Plasma volume increases more facilitating flow!
Arteries & Arterioles
• High pressure vessels
• Constant subdivision decreases diameter
• As network subdivides, blood velocity decreases
• Arterioles have more smooth muscle than elastic tissue in the tunica media– Allows vasodilation and vasoconstriction – Regulate blood pressure – Enable efficient delivery and exchange of gases
Functions of Arteries & Arterioles
• Act as conduits
• Cushion and smooth out the pulsatile flow of blood from the heart
• Help control blood pressure
Veins and Venules
• Low pressure vessels
• Possess less smooth muscle and elastic tissue
• Veins gradually increase in thickness the nearer the heart
• Thinner walls often distend and allow blood to pool in them (pocket valves)
• Up to 70% of blood volume is found in venous system at any one time at REST
Capillaries
• Functional units of vascular system
• Single layer of endothelial cells
• Pre-capillary sphincters – controls and regulates volume of blood entering capillary bed
Venous Return Mechanism
• Venous return – blood returning to right side of heart via veins
• Several mechanisms aid this;– Muscle pump – Pocket valves – Respiratory pump– Smooth muscle – Gravity
Blood Pressure
• The force exerted by the blood against the walls of the vessels
• BLOOD PRESSURE = CARDIAC OUTPUT MULTIPLIED BY RESISTANCE
• Systolic pressure – heart pumps• Diastolic pressure – heart relaxing/filling
Exercise and BP
• Steady aerobic exercise– Systolic increases as a result of increase cardiac output – Diastolic remains as blood feeds into muscles due to
increased ateriole dilation
• High-intensity isometric and anaerobic exercise – Both rise due to increased resistance of blood vessels– Increased peripheral resistance (veins) – Increased intra-thoracic pressure due to contraction of
abs
Q at Rest and during Exercise ORGAN AT REST (CM3) % BLOOD
FLOW MAX EFFORT (CM3)
% BLOOD FLOW
Skeletal muscle 1000 20 26000 88
Coronary vessels
250 5 1200 4
Skin 500 10 750 2.5
Kidneys 1000 20 300 1
Liver/gut 1250 25 375 1.25
Brain 750 15 750 2.5
Whole body 5000 100 30000 100
Transport of Oxygen
• Approx. 97% of oxygen is carried by RBCs (haemoglobin)
• Each molecule of haemoglobin can combine with 4 molecules of oxygen (1.34ml)
• Concentration of Hb is about 15g per 100ml• Each 100ml can transport up to 20ml of oxygen
• Haemoglobin + Oxygen = Oxyhaemoglobin Hb + O2 = HbO2
Transport of Carbon Dioxide
• Approx. 8% is dissolved in blood plasma
• Up to 20% combines with Hb to form carbaminohaemoglobin
• Up to 70% is dissolved in water as carbonic acid