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Transcript
Microgravity and the Circulatory System Mina Iscandar Kevin Morgan Outline Introduction Circulatory System on Earth Changes Due to Microgravity Side effects Upon Return to Earth Possible Countermeasures Ongoing Studies 2 Introduction Exposure to microgravity during space flight affects almost all human physiological systems Musculoskeletal System Neurovestibular System Circulatory System Understanding these affects are of the utmost importance before longer mission and interplanetary travel are possible We will focus on microgravity’s affects on the circulatory system 3 Circulatory System on Earth Terrestrial Life Gravity is always acting on the human body Some Systems work “because of” and some work “in spite of” gravity [1] Circulatory System works “in spite of” gravity blood to flows up from heart to the brain Lower body blood vessels constrict to push blood upwards Blood Pressure normally higher in lower body than in the upper body 5 Terrestrial Life (cont’d) Humans Maintain upright position 70% of blood below heart level [1] Quadrupeds “All fours” body position 70% of blood above heart level [1] [8] 6 Under Pressure Blood Pressure = Cardiac Output (CO) * Total Peripheral Resistance (TPR) Normally 100mmHg at the aortic bulb 2 other components to blood pressure Static pressure (~7 mmHg) Due to elastic characteristic of vascular system Hydrostatic (ρ*g*h) where ρ is blood density A negative value corresponds to pumping above the heart and a positive to pumping below the heart 7 Blood Pressure Regulation Baroreceptors detect blood pressure and send orders to CNS whether to increase or decrease TPR and CO Renin-angiotensin system (RAS) Allows kidneys to compensate for drops in blood pressure by muscle constriction Aldosterone release Release steroid hormone in response to decrease in blood pressure Increase blood pressure in body 8 Factors Affecting Blood Pressure Rate of pumping Volume of fluid The more blood in the body the higher the pressure Resistance Small diameter, higher resistance Viscosity High red blood cell concentration increases viscosity which increases pressure 9 Under Pressure (cont’d) Blood pools in the lower body “because of” gravity causing a pressure gradient Heart must work “in spite of” gravity to return the blood upwards What happens in the absence of the pressure gradient? [7]10 Under Pressure (cont’d) During long periods of bed rest, the heart, lower body, and head are all at the same level The pressure gradient disappears ρ*g*h goes to zero Most of the blood collects in the chest and abdomen What about in microgravity? 11 Changes Due to Microgravity Microgravity As distance from the Earth increases, the affect of the gravity decrease Without the pull of gravity, the blood can no longer pool in the lower legs and feet Instead the blood collects in the head and thorax region of the body Vessels in the legs must no longer constrict to push blood upwards to the rest of the body and become “lazy” 13 “Puffy-Head Bird Legs” Syndrome Sinuses swell because of blood shift Leg muscles weaken due to blood shift and because they are no longer needed to support the body weight Heart increases blood flow per beat causing expanded blood vessels Increased urine excretion by the kidneys [3] 14 Changes in the Circulatory System The change in environment causes an homeostatic response by the Circulatory System Two types of Changes Rapid Response Delayed Adaptive Response 15 Changes in the Circulatory System Rapid Response Decrease of plasma volume Decrease in circulating blood and interstitial fluid volumes Increase mass of red blood cells and hemoglobin Decrease of arterial blood diastolic pressure Blood pressure between heart beats 16 Changes in the Circulatory System Delayed Adaptive Response Further decrease in blood volume Decrease in red blood cell mass and hemoglobin concentration Decrease in blood pressure Decrease in stroke volume Blood pumped from the ventricle in one beat 17 Microgravity Absence of Forces of Gravity Decrease activity of the sympathetic region of the hypothalamus Decrease peripheral resistance Pooling of blood in visceral organs Increase Central Volume Decrease in pressure gradient Increase blood flow and blood pressure in kidneys [1] Adapted Muscle system deconditioning decrease activity of peripheral muscles Increase in excretion of fluids Rapid Decrease plasma and circulating blood Increase in mass of red blood cells Decrease in arterial blood diastolic pressure Delayed Further decrease in blood volume Decreased mass of red blood cells Decreased blood pressure 18 Decrease in stroke volume A New Set Point The Circulatory System establishes a new set point that is different to that on Earth The new set point has a decreased tolerance to gravity The circulatory system is now “deconditioned” with respect to Earth Baroreceptors “reset” themselves to be in line with the new gravitational conditions 19 Circulatory System Adaptation [2] 20 Returning To Earth Returning to Earth Astronauts may not feel the effects of their changing circulatory system in space Upon returning to Earth the changes become very apparent [2] 22 Returning to Earth The Circulatory System has reached a new equilibrium in microgravity Upon returning to Earth a sudden shift in gravity causes a number of effects on the circulatory system It is difficult to quantitatively express these changes as a great amount of factors come into play Duration of space travel Distance form Earth Mission activities 23 Returning to Earth Qualitative Analysis of Returning Astronauts and Cosmonauts Sweaty and pale faces Dizziness Increased Heart rate Inability to assume standing position (Orthostatic intolerance) General impairment due to ineffective control of blood Decreased exercise capacity 24 Return to Earth Quantitative Analysis of Returning Astronauts and Cosmonauts 7 ~ 15% decline in plasma volume 2 ~ 4% decline in total body water 15 ~ 36% reduction in stroke volume 8 ~ 11% increase in heart rate Main findings that all of these factors contribute to lower exercise capacity 25 Orthostatic Intolerance In ability to regulate blood pressure On Earth can be felt by standing up or sitting up too fast Astronaut’s and Cosmonaut’s blood vessels have become accustom to being “lazy” In space they are not required to constrict to push blood up to the upper body They stay in a dilated state 26 Orthostatic Intolerance When returning to the Earth g-forces act on the body and all of a sudden the pressure gradient re-appears The blood is then forced from the head and chest to the lower limbs The blood vessels have “forgotten” how to contract and push the blood back towards the brain 27 Orthostatic Intolerance This causes a lack of blood to the brain Results in dizziness and fainting Symptoms duration depends on the length of time subject is exposed to microgravity 4 – 10 day mission requires 10 days recovery period 8 – 12 month mission requires 4 – 5 weeks recovery period Current studies are being performed to better understand factors causing Orthostatic Intolerance 28 Effects on Returning Astronauts Astronauts on short space missions (4 ~ 14 days) Often have trouble walking and keeping balance Must be aided in movements and usually lie down after departing the shuttle Cosmonauts on long missions (6 ~ 12 months) Often capsules land in water because cosmonauts are unable to properly land on the surface Typically faint and have trouble moving 29 Possible Countermeasures Possible Countermeasures Past Countermeasures Drinking Salt Water G – Suits Current Countermeasures Utilization of drugs Midodrine Creation of artificial gravity on the shuttle Twin Bikes System 31 Past Countermeasure Drinking Salt Water Increases the viscosity of the blood Forces heart to work harder causing higher blood pressure G – Suits Rubberized full-body suits Applies pressure on extremities and raises blood pressure Both not sufficient solutions 32 Current Countermeasures Midodrine Improves blood pressure counteracting orthostatic intolerance Binds to alpha-1 receptors on arteries and veins Sympathetic response Constricts blood vessels and increase blood pressure Increase blood pressure for a short period of time 3 – 4 hours approximately depending on dosage 33 Current Countermeasures Creation of Artificial Gravity Twin Bikes System (TBS) Two astronauts riding two bicycles and counterrotating along the inner wall of the cyclical space shuttle [1] Allows astronauts to experience the Earth’s gravity while being in space Enables astronauts to perform physical exercises [1] 34 Current Countermeasures How Artificial Gravity is Generated Any object traveling in a circle will experience an outward acceleration called centrifugal acceleration ( ac ) ac = v2 / R (for space shuttle R = 2 m) ac must be equal to 9.81 m/s2 in order to mimic Earth’s gravity v must be equal to approx. 4.5 m/s 35 Current Countermeasures Centrifugal force due to the mass of the upper body is supported by the bike’s frame This is similar to Earth where the spine supports the mass of the upper body The lower limbs will have a force equal to the mass of the limbs multiplied by ac This is similar to Earth where the lower limbs will be stimulated by their own weight and the force of the muscle while pedaling 36 Current Countermeasures ac = ω2 * R Taking the average acceleration between two points yields ac = ω2 * ( R1 + R2) / 2 Therefore Pa = ρ * [ω2 * ( R1 + R2) / 2] * (R2 – R1) This enables us to measure the total pressure at any desired point in the circulatory system 37 Ongoing Studies Blood Vessel Changes in Rats Dr. Michael Delp at Texas A&M University Animal Enclosure Model (AEM) 8 rats in microgravity 8 control rats Experiment explores the affects of microgravity on blood vessels and smooth muscle 39 Blood Vessel Changes in Rats Results A and C are controls B and D experience microgravity Muscle cells around B and D weaken and shrink This results in the inability to constrict and force blood up to the brain 40 Affects of Nitric Oxide Recent Studies by NASA and the University of California Rats kept in microgravity produce excessive amounts of nitric oxide synthase (NOS) in the heart, blood vessels, kidneys, and brain [4] NOS causes blood vessel dilation Over production of nitric oxide may contribute to orthostatic intolerance Preventing overproduction of nitric oxide may help to treat orthostatic intolerance Aminoguanidine blocks the action of NOS 41 Conclusion Studies on the circulatory system in microgravity have helped further us understand how heart physiology works in gravity Because studies on the affects of microgravity are very limited it is difficult to make progress However, progress must be made before we can venture further out into our solar system 42 Question? Works Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. G. Antonutto, P.E. Prampero, Cardiovascular deconditioning in microgravity: some possible countermeasures. 2003. http://science.nasa.gov/headlines/y2002/25mar_dizzy.htm http://library.thinkquest.org/03oct/01581/SpaceTravelEnglish/dangers/puffy.html http://research.hq.nasa.gov/Spaceline_WEB/SpaceLine/nasa/pdf/Fainting_Astrona uts.pdf T. Trappe, S.Trappe, G.Lee et., Cardiorespiratory response to physical work during and following 17 days of bed rest and spaceflight. Journal of Applied Physiology. pg. 951 – 956. 2006. X. Xiao et. Effect of simulated microgravity on closed-loop cardiovascular regulation and orthostatic intolerance: analysis by means of system identification. Journal of Applied Physiology. pg. 489 – 496. 2004. http://en.wikipedia.org/wiki/Image:Grafik_blutkreislauf.jpg http://www.huntington.org/LibraryDiv/LASKEREXHIBIT/Seabiscuit.jpg http://health.howstuffworks.com/define-heart.htm http://en.wikipedia.org/wiki/Blood_pressure http://www.nasa.gov/vision/earth/livingthings/arterial_remodel.html 44
