Gas Carriage 2

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  • Created by: LBCW0502
  • Created on: 22-03-19 13:20
State the factors which decrease the O2-Hb affinity
Decrease pH, increase pCO2, increase temperature, increase 2,3 DPG, Bohr shift (e.g. exercise, increase metabolic rate, assists O2 unloading)
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State the factors which increase the O2-Hb affinity
Increase pH, decrease pCO2, decrease temperature, decrease 2,3 DPG (e.g. in alveoli)
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What is the pO2 in arterial blood?
~12.5 kPa
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What is the pO2 in venous blood?
~5.3 kPa
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What is anaemia? (1)
A condtions in which there is a reduced content of functional Hb in blood. Defects - Hb production or red cell numbers. Many different causes ranging from defect in synthesis of Hb, gene mutations or in production of RBCs or loss of RBCs
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What is anaemia? (2)
All cases cause a reduction in O2 carrying capacity
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Describe features of the oxygen dissociation curve in anaemia
[Hb] reduced to 75 g/L. Tissues still need to metabolise O2, extracting 5 mL/dL from blood. Venous pO2 lower (3.6 kPa) hence tissue pO2 will also be low (hypoxia) at 3.6 kPa. Limits extraction of O2 during exercise (fatigue)/poor exercise intolerance
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Describe features of carbon monoxide (1)
CO poisoning causes sickness and death. Any incomplete combustion of hydrocarbon fuel results in CO generation. CO has very high affinity for Hb, 240 > O2 binding (same site on haem group as O2)
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Describe features of carbon monoxide (2)
Hb with CO bound forms carboxyhaemoglobin (deep cherry-red colour)
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What are the two negative effects of CO binding to Hb?
Reduces amount of O2 bound to Hb. Shifts O2 binding curve to left (increases affinity of remaining binding sites on Hb) hence decreases unloading of O2 in tissues
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Describe the oxygen dissociation curve in CO poisoning
50% COHb, [Hb] still 150 g/L. Tissues still remove 5 mL/dL. Venous pO2 even lower (2 kPa). Hence tissue pO2 also 2 kPa. Causes severe problems - headache, convulsions, coma, death
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Describe the oxygen dissociation curve of foetal Hb (1)
HbA - mother's Hb has 2 alpha and 2 beta protein chains. HbF - normal foetal Hb has 2 alpha and 2 gamma globin subunits. Different globin increases affinity for haem group for O2. Binds to DPG less
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Describe the oxygen dissociation curve of foetal Hb (2)
Outcome - favours O2 moving from mother's blood to foetal blood across placenta
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Describe features of cyanosis
If deficient O2 supply in tissues, there is an increased content of de-oxyHb in tissue capillaries due to hypoxia. De-oxyHb has blue tinge causing discolouration of tissues (cyanosis)
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What is peripheral cyanosis?
Peripheral cyanosis - reduced blood low, hypoxic tissue - blue tinge in extremities (e.g. hands, feet)
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What are the causes of reduced blood flow?
Cardiovascular shock, low temperature, reduced CO, poor arterial supply. Under these conditions respiration is OK and arterial O2 content probably also normal
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Which is central cyanosis? (1)
Arterial hypoxaemia (reduced O2 content), buccal mucosa/lips in particular (blue tinge also on conjunctiva and ear lobes). If arterial blood contains >1.5-2 g/dL of deoxyHb, cyanosis is observable even in well perfused tissues
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Which is central cyanosis? (2)
Occurs when O2 saturation <85% if [Hb] normal (15 g/dL)
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What causes central cyanosis?
Chronic respiratory disease (COAD) resulting in PaP2 ~ 8 kPa. Right to left heart shunts lead to reduction in O2 saturation
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What are the three ways in which carbon dioxide is carried in the blood?
60% as bicarbonate (in plasma and inside RBC). 30% as Hb-CO2 (carbaminoHb). Dissolved. Solubility of CO2 is 0.52 mL/dL kPa. In normal arterial blood pCO2 is 5.3 kPa. Dissolved CO2 is 2.74 mL/dL ~10% of added CO2
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Describe how carbon dioxide is carried as bicarbonate (1)
First reaction - slow in plasma but fast in RBC due to presence of carbonic anhydrase. CO2 + H2O -> H2CO3 -> H+ and HCO3-. Second reaction - aided by buffering [H+] by deoxy-Hb, bicarbonate formed by reaction 2 diffuses out down conc gradient
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Describe how carbon dioxide is carried as bicarbonate (2)
Via antiporter into plasma in exchange for C- (chloride shift)
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Describe how carbon dioxide is carried as a carbamino compound
CO2 reacts with NH2 groups on proteins to form carbamino compound: RNH2 + CO2 -> RNHCOOH e.g. lysine, arginine side chains. Mostly formed with deoxy-Hb in RBC (little with plasma proteins). Oxygenation of Hb inhibits reaction/aids reversal in lung
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Describe carbon dioxide unloading in the lung (1)
In lung capillaries reactions reverse to expel CO2 from alveoli. CO2 dissolved in blood plasma diffuse down partial pressure gradient into alveoli very rapidly
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Describe carbon dioxide unloading in the lung (2)
CO2 reversibly bound to Hb as carbamino compounds comes off, assisted by oxygenation of Hb and diffuses into alveoli. HCO3- from plasma taken back into RBCs combines with H+ which comes off Hb as O2 binds to Hb forms carbonic acid
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Describe carbon dioxide unloading in the lung (3)
H2CO3 dissociates into CO2 and H2O via carbonic anhydrase with CO2 diffusing into alveoli
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What is the Haldane effect? (1)
At any given pCO2, the quantity of CO2 carried is greater in partially deoxygenated blood (venous) than in oxygenated blood (arterial). Due to - Hb forming carboamino compounds more readily when deoxygenated so carries more CO2
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What is the Haldane effect? (2)
Also, Hb binds to H+ better when deoxygenated, this favours formation of HCO3- increasing CO2 carriage
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Describe the carbon dioxide dissociation curve (1)
Not sigmoid, not plateau, approximately linear over physiological range. Arterial (5.3 kPa, 48 mL/dL). Resting mixed venous (6.1 kPa, 52 mL/dL). More total CO2 carry capacity than O2
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Describe the carbon dioxide dissociation curve (2)
For given pCO2 more CO2 content in venous than arterial blood (Haldane effect). At rest tissues produce 4 mL of CO2 for 100 mL blood passing through
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What is the normal blood pH range?
7.35-7.45
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pH is determined by which factors and which relationship?
[H+], CO2, HCO3- and H+ (inter-related by carbonic acid). Henderson-Hasselbalch equation defines relationship: pH = pKa + log ([HCO3-]/[CO2])
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What is pKa?
-log (dissociation constant of H2CO3) = 6.1. Ratio of [HCO3-]/[CO2] = 20, then pH = 6.1 + 1.3 = 7.4
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What is the [CO2] in arterial blood?
5.3 kPa x 0.23 (solubility [mmol/dL/kPa]) = 1.2 mM (HCO3- = 24 mM)
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How does CO2 affect pH? (1)
Changes in [CO2] lead to changes in pH. Lungs are an important regulator for acid-base balance (short term). Kidneys are acid-base regulators (long term). In exercise, H+ produced lowers pH, increasing ventilation removes CO2
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How does CO2 affect pH? (2)
Reversing carbonic acid reactions and bringing blood pH to normal
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The oxygen uptake in alveoli is usually equal to what?
Amount of oxygen utilised by tissues (250 mL/min)
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What are the two methods for measuring O2 consumption?
Fick's principle and from respiratory measurements
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How do you measure O2 consumption using Fick's principle?
O2 consumption by tissues = CO x (arterial - venous O2 content) = 5000 mL/min x (0.2 mL/mL - 0.15 mL/mL) = 250 mL/min
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How do you measure O2 consumption using respiratory measurements? (1)
Alveolar ventilation = 5 L/min different in O2 content of inspired and expired air if 5% (21% vs 16%) hence O2 uptake is 250 mL/min. CO2 produced in tissues = amount removed in alveoli = 200 mL/min = CO x (venous-arterial CO2 content)
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How do you measure O2 consumption using respiratory measurements? (2)
= 5000 mL/min x (0.52-0.48) mL/mL blood = 200 mL/min
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How do you work out the respiratory quotient?
CO2 produced/O2 utilised (gives a ratio for RQ). Exact value is diet dependent, because different metabolic fuels generate different amounts of CO2 for each O2 consumed
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Alveolar pCO2 is inversely proportional to what?
Alveolar ventilation e.g. at any given rate of CO2 production (metabolic rate) if ventilation increases, CO2 will be blown off faster, decreased pCO2 in alveoli/arterial blood. Decreased ventilation leads to increased pCO2
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What is alveolar ventilation? (1)
Volume of inspired air = tidal volume (Vt) = alveolar volume + dead space volume
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What is alveolar ventilation? (2)
Volume that reaches alveoli where gas exchange can occur that is physiologically useful. Alveolar ventilation = minute ventilation - dead space ventilation = 7500 - (150 x 15) = 5250 mL/min (5.25 L/min)
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How do you calculate minute ventilation?
Tidal volume x frequency e.g. at rest 500 mL x 15 = 7500 mL/min (7.5 L/min)
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What is hyperventilation?
Over-ventilation in proportion to metabolism. Leads to a lowering of arterial pCO2 below normal values. Doesn't mean just increased ventilation (exercise) but matched to increased metabolic rate. No hyperventilation, arterial pCO2/pO2 stays ~constant
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What is hypoventilation?
Under-ventilation in proportion to metabolism. Results in higher arterial pCO2 levels
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What are the causes of hyperventilation?
Hypocapnia, low arterial pCO2 (<5.3 kPa). Low pCO2 reduces [H+] and causes respiratory alkalosis. Due to anxiety, pain, excessive mechanical ventilation, diseases contributing to metabolic acidosis (e.g. renal failure, diabetes)
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What are the consequences of hyperventilation? (1)
Low pCO2 - cerebral vasoconstriction - cerebral hypoxia resulting dizziness and visual disturbances. Alkalosis - reduced plasma free [Ca2+] (more binds to proteins) - increases excitability of excitable cells (VGCC open lower threshold)
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What are the consequences of hyperventilation? (2)
Disturbed sensation (pins and needles in hands/feet) and wanted tetanic muscle contractions (spasms in fingers)
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What are the causes of hypoventilation?
Hypercapnia, high arterial pCO2 (>6 kPa and low arterial pO2). High pCO2 increases [H+] and causes respiratory acidosis. Due to head injury impairing respiration, anaesthetics, drugs, chronic lung disease
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What are the consequences of hypoventilation?
Increasing arterial pCO2 causes peripheral vasodilation, flushed skin, full pulse, extra systoles, very high pCO2 (>10 kPa) depresses CNS function causing confusion, drowsiness, coma, death
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Summarise the gas partial pressures in airways and blood (1)
In kPa (values x 7.5 for mmHg). Inhaled air (pO2 21, pCO2 0). Inspired air in airways (pO2 20, pCO2 0, pH2O 6.3). Alveolus (pO2 13.3, pCO2 5.3). Arterial blood (pO2 12.5, pCO2 5.3). Capillary/tissues (pO2 <5.3, pCO2 >6.1)
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Summarise the gas partial pressures in airways and blood (2)
Mixed venous blood/resting (pO2 5.3, pCO2 6.1). Exhaled air (pO2 16, pCO2 3.5)
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Card 3

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Card 4

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Card 5

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