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BIOC33/34 Lec 12.docx

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University of Toronto Scarborough
Biological Sciences
Stephen Reid

BIOC34 Lec 12  Midterm o Anything not covered on exam will not be on exam - anything after this lec o 50-60 MCQ o Calculations will be simple - no calculators allowed  Partial pressure of gases o Partial pressure of a gas: the pressure exerted by a given gas within a gas mixture o Partial pressure of O2 = ~160 mmHg o PCO2 = 0.23 mmHg ~ 0 mmHg  P O2and PCO2 throughout the respiratory system/circulation o Partial pressures throughout the circulatory system (diagram) o Breathe in Patm withO2 of 160 mmHg and PCO2 of 0.23 o Take Patm into lungs and this air enters alveolar gas - alveolar gas exchanges O2 and CO2 with venous blood (mixed venous blood) because returning from right side of heart into lungs o PP of mixed venous blood = 40mmHg O2, 46mmHgCO2 o When looking at PO2 and PCO2 in alveoli, will be in between the values of Patm mixed with blood entering lungs o In alveoli, Po2 = 100mmHg, PCO2 = 40 mmHg  Go from 160mmHg in atm to 100mmHg in alveoli because some O2 has gone to blood  PCO2 which was low in Patm, have gone up to 40 mmHg in alveolar gas o Blood leaves the lung - nothing happens to O2 and CO2 values until blood is pumped to left side of heart o Blood leaves heart with PO2 of 100, PCO2 of 40  By the time the blood has transited through the lungs, the arterial blood leaving the heart is in equilibrium with CO2 and O2 alveolar gas o Blood flows back through pulmonary vein into left atria and then out in systemic circuit o Within cells of tissues, O2 levels are low and CO2 levels are high - if they go down to mitochondrial level, PP of O2 is 1 mmHg  very, very low O2 levels in mitochondria o Arterial blood flows to metabolically active tissues and takes CO2 from those tissues o On other side of capillary beds, oxygen levels dropped from 100 to 40 and PO2 has risen from 40 to 46  oxygen was lost and CO2 was taken into blood o Venous blood goes back into right side of heart and is pumped to lungs o Values of 40 and 46 stay constant until it reaches the pulmonary capillaries o These are the values of PO2 and PCO2 in circulatory system and alveolar gas  O2 uptake and CO2 excretion in the lung are perfusion limited o Looking at uptake of oxygen, see that in humans, uptake is referred to perfusion limited o Uptake of O2 in respiratory organ and transport of oxygen to the blood, transport of CO2 from the tissues takes one of 2 limitations:  Perfusion limitation - see in humans and most mammals  Diffusion limitation - seen in gill-breathing fish o Graphs  Perfusion limitation = amount of O2 that can be delivered to tissues is dependent on blood flow sent to the lungs  cardiac output o Left = PO2 (right = PCO2)  Mixed venous PO2 blood enters lungs with 40 mmHg and leaves with a partial pressure of 100 mmHg  It doesn’t take long for it to go from 40 to 100 mmHg - Rise from venous to arterial level happens very quickly in first third of capillaries  For remainder of capillary in lungs, nothing is happening to oxygen uptake • 2/3 of lungs are not important for oxygen uptake o CO2 - mixed venous blood enters lungs with PCO2 OF 46 and leaves with PCO2 of 40  Decrease occurs within the first 1/3 of the lung o No limitations with oxygen diffusing from lungs into blood or with CO2 from blood into lungs  Because they are moving rapidly, infer that there is no limitation in terms of gas diffusion  Dotted lines show theoretical situation if gases were not diffusing well  Get slow increases in transit times in the lungs o Because we do not get these small changes, there are no diffusion limitations - can enhance blood uptake by pumping more blood  Where term perfusion limitation comes in  o Amount of ventilation depends on oxygen consumption by tissues and amount of production of CO2 o When talking about O2 consumption and CO2 production - talking about mechanisms indicative of metabolic rate o Ratio of CO2 production to O2 consumption = respiratory quotient (RQ)  Amount of production and consumption that is occurring that determines the amount of alveolar ventilation will have  RQ = CO2 Production / O2 Consumption o When looking at control of breathing, for the most part, breathing in humans, mammals etc. is cue to getting rid of CO2 o Eupnea: normal breathing that is sufficient to meet metabolic demands o Hyperpnea: increase in breathing (alveolar ventilation) to meet metabolic demands  Apnea o Apnea - complete cessation of breathing  E.g. sleep apnea  Look at cases that show variables such as the EEG readings (from sleep clinic)  All nasal, chest, abdomen are measures of breathing  Larger movements = person breathing, tiny movements = apnea or no breathing  Oxygen levels in blood are low but increase with breathing - if person is not breathing, oxygen levels fall  As person starts to breathe, heart rate goes up  ECG - measures heart rate  After person takes a few breaths, oxygen levels in blood goes up and BP and HR go up  One of the reasons sleep apnea is dangerous - get cyclical patterns in breathing - periods of breathing have huge increases in HR and BP  puts a lot of pressure on heart and cardiac system - increases risk of stroke o Dyspnea: laboured breathing; shortness of breath  o “A” = alveolar, “a” = arterial o Hyperventilation: increasing breathing  PaO2 > 100 torr; PaCO2 < 40 torr o Hypoventilation: reducing breathing; arterial levels fall (partial pressures);  See PaO2 < 100 torr; PaCO2 > 40 torr o Colloquially, people use these terms in terms of decrease or increase of breathing - won’t be looking specifically at blood-gas status  Daily oxygen consumption o Lungs take in oxygen and deliver to tissues o Oxygen demands/consumption is far more than the amount of oxygen that blood can supply just if looking at oxygen dissolved in the blood o In the blood, oxygen exists as either:  Dissolved oxygen in plasma (gives partial pressure of oxygen in blood)  Oxygen bound to hemoglobin in the red blood cell • This is IMPORTANT to get oxygen to tissues o Typical value for alveoli ventilation = 4.2 l/min o Of 4200 ml of air getting into lungs, 21% is oxygen (882 ml) o Take up about 882 ml into lungs every minute - only 250 ml diffuse into the blood every minute o If multiply 250 ml/min x 24 hrs/day = 360 L/day  Infers that tissues require 360 L of oxygen a day  Oxygen transport in the blood (how do we get 360 L O2/day to the tissues) o Calculations in this slide tells us oxygen in plasma cannot supply 360 L of oxygen a day o Concentration of gas physically dissolved in a solution = partial pressure of gas x solubility coefficient o Thinking s
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