1215 Week 17

1215 Week 17

1. Lecture 1

   • Compensatory Mechanisms
     • all mechanisms increase CO and/or TPR which increase MAP
     • Baroreceptors
       • a decrease in MAP reduces the firing rate of baroreceptors
       • there is an increase in sympathetic activity and a decrease in parasympathetic activity
       • increased sympathetic activity increases SV by increasing contractility, heart rate and EDV
       • vasoconstriction increases TPR
     • Chemoreceptors
       • detect hypoxia, hypercapnia and acidosis
       • occurs below 60mmhg
       • causes vasoconstriction, increased rate and depth of breathing which increases heart rate and venous return
     • Cerebral Ischemia
       • occurs when MAP falls below 40 mmhg
       • decreased blood flow to the brain activates the sympathoadrenal system
       • causes intense vasoconstriction, increases in contractility and MAP
       • in extreme causes parasympathetic stimulation
     • Reabsorption of Tissue Fluids
       • decrease in MAP, vasoconstriction and reduced venous pressure all cause a decrease in hydrostatic pressure
       • lowered hydrostatic pressure causes reabsorption of interstitial fluid into the capillaries
       • 1 L/hour
       • the loss of interstitial fluid causes the movement of intracellular fluid into the to interstitial space
       • cortisol is important in the restoration of plasma volume
     • Endogenous Vasoconstrictors
       • NA and adrenaline increase sympathetic nervous activity 50 times
       • ADH and angiotensin 2 increase TPR and MAP
       • ADH and aldosterone stimulate NaCl and water
       • increased sympathetic activity and decreased MAP decrease urinary secretion of NaCl and water
   • Decompensatory Mechanisms
     • positive feedback loops - all are unwanted
     • Cardiac Failure
       • unclear how much it affects shock occurring
       • lowers MAP
     • Acidosis
       • due to lack of CO2 removal
       • no acid secretion depresses cardiac function
       • loss of cardiac function means less blood flow etc
     • CNS Depression
       • decreases MAP
       • opiods released by shock
       • opiods depress the CNS
       • a depressed CNS means less sympathetic activity increasing shock etc
     • Aberrations in Blood Clotting
       • hypercoagulability - clots form within minutes
       • thromboxane A2 aggregates platelets causing more thromboxane release
       • hypocoagulability then accurs
     • Mononuclear Phagocytic System
       • system is depressed
       • phagocytic activity is controlled by an opsonic protein - causes bacteria to be more susceptible to phagocytes
       • opsonic activity decreases causing antibacterial and antitoxin defense mechanisms to fail
       • endotoxins can then accumulate in the circulation causing vasodilation decreasing MAP
   • Control of Blood Volume
     • homeostasis
       • the epithelial barrier to the external environment includes the skin, lungs, GIT and kidney tubules
       • regulatory mechanisms must be able to detect the variable, relay the changes to the integrating centre and cause the desired effect
       • the barareflex controls MAP at about 95mmhg and response to changes occurs in 1-2 seconds
     • increase in blood volume
       • increases pressure
         • increases venous return
           • increases EDV
             • increases SV
               • increases CO
                 • increases MAP
                   • increases losses of Na+ and water
                     • decreases plasma volume
                       • decreases blood volume
     • long term changes in blood volume occur by changes in plasma protein concentration or changes in the number of formed elements present
     • haemorhage - bleeding or abnormal blood flow
     • shock - a condition brought about by a sudden drop in blow flow
     • losses of more than 10% of total blood volume have marked effects on MAP

2. Lecture 2

   • Diving
     • 1 ATA/760mmHg is the pressure exerted by 24 miles of air
     • air is compressible - half an ATA is exerted by 3.41miles
     • 10m of water also exerts 1 ATA of pressure
     • water is not compressible so every 10m adds 1 ATA
     • Boyles law - at a constant temperature the volume of gas is inversely proportional to the pressure to which it is exerted
     • Pressure 1 X Volume 1 = Pressure 2 X Volume 2
     • Dalton's Law - the pressure exerted by a mixture of gases is equal to the sum of the pressures which each would exert if it alone occupied the space filled by the mixture - partial pressures
     • Henry's Law - the weight of gas absorbed by a given weight of liquid with which it does not combine chemically is directly proportional to the partial pressure of gas about the liquid - the greater the pressure the more gas disolved
     • the absolute amount of gas dissolved in equilibrium is determined by the solubility of the gas in the liquid as well as the partial pressure of the gas
     • when the pressure is reduced the gas will come out of solution
     • dry/wet suits aren't enough so heated suits are used to protect against hypothermia - the heat also warms the inspired gases
     • saturation diving - the diver remains at the working pressure for an extended duration and is called saturation diving because helium saturates the body
   • Freediving
     • diving in 1 breath
     • if arterial pO2 falls below the level required for consciousness the diver will pass out - 76 mmHg
     • ear pressure
       • the ear drum can rupture
       • middle ear pressure has to be equalised with the valsalva manoeuvre
     • mask squeeze
       • the pressure in the mask can suck and squeeze the eye balls causing damage
       • the pressure can be equalised by breathing through the nose
     • mammalian diving relfex
       • found on all mammals
       • manifests as bradycardia
       • when the face is immersed the CV centre slows heart rate to conserve oxygen
     • lung squeeze, blood shift and peripheral vasoconstriction
       • as depth increases the lung's airspace decreases which could collapse a lung, rupture capillaries, internal bleeding
       • blood shift - blood moves into the lungs to resist lung collapse and blood is redirected
     • factors that increase maximum breathe time
       • diving response
       • breathing movements
       • valsalva
       • central voluntary suppression of respiratory drive
       • swallowing
       • larger lung volume
       • respiratory isometric exercise
     • factors that decrease maximum breathe hold time
       • cold shock
       • low oxygen
       • high CO2
       • chest wall affewrent
   • SCUBA
     • self contained underwater breathing apparatus - allows longer diving time
     • SCUBA tanks are pressurised to 200 ATA which is then reduced to 10 ATA about ambient at the first demand valve and the to ambient pressure at the second demand valve
     • problems with SCUBA
       • air embolism
         • as the diver descends the air in the lungs expands
         • excessive stretching of the alveoli forces micro bubbles into circulation where they aggregate and can lodge in the brain/organs
         • spontaneous pneumothorax - tearing of the alveoli so that air leaks into the pleura
         • interstitial emphysema - tearing through both the alveoli and pleura so that air leaks into the tissue
       • nitrogen narcosis
         • nitrogen dissolves into the blood and tissues and acts like an anaesthetic
         • 20-30m causes intoxication
         • >70m causes unconsciousness
         • is influenced by conditions e.g.darkness, cold, fatigue
       • decompression sickness
         • as dive depth and time increases the amount of dissolved nitrogen also increases
         • nitrogen reaches equilibrium slowly and dissolves more into fat
         • nitrogen also leaves the body slowly so if a diver ascends too quickly the nitrogen forms bubbles in the tissues
         • symptoms normally occur 4-6 hours after diving
         • severe violations can result in paralysis and death in minutes
         • symptoms
           • dizziness
           • itchy skin
           • joint pain
           • bubbles form in venous/arterial vascular beds in the CNS causing lesions in brain/spinal cord
           • bubbles in lungs cause asphyxia
         • treatment
           • oxygen therapy
           • decompression chambers to repressurise and then safely reduce pressure
         • divers have to stop on ascent to allow safe decompression and the time stopped is determined by the slowest tissue to eliminate nitrogen
         • air travel following diving increases the risk of decompression sickness
       • oxygen toxicity
         • at sufficient pressure and time oxygen will cause functional impairment
         • oxygen causes the chemical destruction of cells - it forms free radicals 02- and H2O2
         • oxygen is normally produced at unharmful levels
         • high levels of oxygen free radicals damages cellular components and membranes
         • inspiring a gas with pO2 > 2ATA increases oxygen toxicity risk, divers shouldn't breathe pure oxygen below depths >7m
         • at extreme depths special gas mixtures are used
         • helium is substituted for nitrogen as it is inert and more easily eliminated
         • low density helium reduces breathing resistance and can increase heat loss

3. Lecture 3

   • Space Travel
     • gravity - 9.8ms-2 - at 250 miles gravity is 89% full strength
     • in orbit everything appears weightless - microgravity is 1X10-6 g
     • microgravity causes an array of problems
   • Neurovestibular Disturbances/ Psychosocial Effects
     • motion sickness
       • occurs 1-2 days after arrival in space and return
       • nausea/vomiting caused by neurovestibular mismatch
       • treated with time and antinausea medication
     • sleep deprivation
       • acoustic noise
       • disrupted circadian cycles
       • treated with sleeping medication
     • isolation
       • treated with rigorous selection procedures
   • Immune Dysregulation
     • reduced lymphocyte activity - decreased NKCs
     • decreased activation of T cells
     • impaired mediated immunity - may be associated with cell cytoskeletal function which is activated by mechanical forces
     • high emotional and physical stress supress immune system
     • reactivation of latent Herpes virus
     • treatment - nutritional supplements
   • Fluid Redistribution
     • fluid moves to face and torso
     • 10 less in legs
     • distension of baroreceptors reduces angiotensin-renin - release of ANP increases water and salt excretion which reduces plasma volume
     • first 24 hours leads to 17% reduction in plasma volume increasing haematocrit levels which decreases EPO to help return haematocrit levels to normal
     • blood volume reduces by 10%
     • post flight aerobic capacity decreases due to a reduced SV
     • post flight there is an inability to stand as blood pools in the lower limbs causing low BP
     • treated with negative pressure suits in space and positive pressure suits on earth
   • Muscles Effects
     • loss of mass and strength in postural muscles
     • caused by absence of gravitational loading which normally triggers protein synthesis
     • protein synthesis decrease - 15% - degradation increases and may be caused by stress and sub optimal nutrition
     • reduction in muscle fibres - 20-50%
     • treated with exercise on elastic resistance machines and dietary supplements
   • Bone Effects
     • bone density reduction - load determines turnover rate
     • higher CO2 levels cause acidosis which is countered by bicarb and phosphate from bone
     • bone reabsorption markers in the urine
     • parathryroid hormone decreased
     • lack of vitamin D3
     • increased loss of calcium causes increased risk of kidney stones
     • loss of bone - 1-2%/month
     • after 6 months 8-12% loss if too much spongy bone may prevent repair on return
     • treated unsuccessfully with vitamin K and D and resistance training