Overview of the Circulatory System
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- Created by: amazingemilyjones
- Created on: 23-04-19 10:35
Overview of the Circulatory System
Overview of the Circulatory System
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What does the cardiovascular system do?
- Tisusue perfusion
- Immune system
- Hormones
- Waste
- Temperature control
- pH control
- Carbon dioxide
- Oxygen
- Nutrients
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Cardiovascular System
Heart
Blood Vessels
Blood Vessels
- Tunica intima
- Endothelium
- Tunica media
- Smooth muscle
- Elastic fibres
- Tunica adventitia
- Collagen fibres
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Pressure and Pulsation from Heart to Tissue
Why does pressure and pulsation decrease?
- Pressure falls because of resistance to flow
- Resistance is inversely proportional to vessel diameter
- Blood vessels get smaller as they get further from the heart, so resistance increases and pressure energy is lose
- Flow is less pulsatile due to increased blood vessel compliance
- Blood vessel walls get thinner as they get further from the heart
- The thinner the wall the more stretchy (compliant) the vessel is
- Result is the pulsation is damped by movement of blood vessel wall
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Blood Vessel Size
Heart Contraction
Foetal Circulation
- Maternal circulation supplies nutrients and oxygen
- Lungs not required for gaseous exchange: bypass via ductus arteriosus
- Digstion not required: bypassed via ductus venosus
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Foetal Circulation
Cardiac Output
- Cardiac output = stroke volume x heart rate
- Stroke volume = volume ejected in 1 contraction cycle
- Heart rate = number of contraction cycle per unit of time
- Resting cardiac output: 4-7 l/min (depends on body size)
- Continuously adjusted
- sleep reduced by 10%
- standing reduced by 20%
- stress increased by 25%
- pregnancy increased by 40%
- heavy exercise increased by 500%
- Changes in carbon monoxide usually involve changes in heart rate and stroke volume
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Stroke Volume
- Stroke volume is affected by
- diastolic stretch - depends on venous filling pressure
- force of contraction
- arterial pressure
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Diastolic Stretch
- Frank-Starling's law of the heart: the volume entering at the beginning of a contraction cycle is the volume ejected at the end of the cycle
Venous Return
- Blood returns to the heart via the vena cava
- The rate of venous return determines cardiac output
- The circulation is a closed look so venous return = cardiac output
- Factors affecting venous return
- skeletal muscle activity
- gravity
- sympathetic nervous activity
- breathing or compression of thoracic vena cava
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Skeletal Muscle Pump
- Veins contain non-return valves
- Contraction displaces blood into next compartment
- Mechanism very important for extremeties
Overcoming Gravity
- Changes in posture impact upon forces affecting venous return
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Gravity - Postural Hypotension
- Standing causes pooling of blood in the feet and lower legs
- Without compensation would lead to precipitous fall in blood pressure
- Compensation mechanisms
- baroreceptor reflex: vasoconstriction of veins
- local myogenic response
- skeletal muscle pumping
- breathing
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Autonomic Sympathetic Effect on Venous Return
- Activation of the sympathetic nervous sytem has multiple effects on cardiovascular system function
- In terms of affecting venous return...
- veins are normally very compliant (stretchy)
- activation of the sympathetic nervous system causes a decrease in venocompliance
- a decrease in compliance leads to an increase in venous pressure and hence venous return
- increased venous return leads to increased cardiac output
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Control of Venous Return - Breathing
- Breathing cycle affects intra-thoracic pressure and thus venous return
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