Lecture 21: Gas Diffusion And Gas Transport Flashcards

0
Q

Again what is Dalton’s law of partial pressure?

A

The total pressure of a mixture of gases is a sum of the partial pressures exerted by each gas.
Partial pressure is the pressure exerted by an individual gas in a mixture

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1
Q

What is the composition of air?

Like what’s in it and what %?

A

-78% nitrogen
-21% oxygen
-0.033% carbon dioxide
Water vapour dilutes gases in air.
Each gas in the mixture contributes to the total pressure. It’s contribution yo the total pressure is called ‘partial pressure’

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2
Q

Partial pressure of gases?
What is it?
What does it depend on?
How do you calculate it?

A

The partial pressure is a gas (mmHg) is the proportion of total air pressure contributed by that gas. It depends on:
1. Total pressure exerted by air
2. % of that gas within the air
P(gas) = percent composition (gas) x total pressure (mixture)
Eg nitrogen
Composition = 78 %
P(N) = 0.78x 760mmHg

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3
Q

Gases in liquids. Gases also exert pp when dissolved in liquid.
What does it depend on?

A

-The amount of gas dissolved in the liquid depends on both the partial pressure gradient and the gas solubility.
-when Gas is in contact with a liquid, net movement of gas will occur between Compartements
-this will occur until equilibrium PO2 in air = PO2 in liquid is reached. This is equilibrium.
BUT equilibrium doesn’t imply that no. Of gas molecules in liquid = no. In air.
-the final concentration of gas in a liquid at equilibrium will depend in its solubility.
-CO3 is 20x more soluble than O2 a the same partial pressure gradient.
Slide 10

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4
Q

Respiratory gas exchange

Gas diffusion

A

O2 and CO2 diffuse between alveolar air and blood across the respiratory membrane

  • diffusion occurs because there is a partial pressure gradient for both CO2 and O2.
  • each gas diffuse from high partial pressure to low partial pressure
  • so O2 diffuses from alveoli into the blood, and CO2 diffuses from blood into alveoli
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5
Q

Tell me about the partial pressure gradients in the lungs?

A
  • O2 diffuses from alveoli to blood down a PP gradient (100-40mm Hg)
  • CO2 diffuses from blood to alveoli down PP gradient (46➡40mmHg)
  • gas exchange complete when equilibrium is complete between blood and alveoli
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6
Q

Give me three reasons as to why P(O2) is lower and P(CO2) is higher in the alveoli than in the atmosphere.

A
  1. Gas exchanges continuously between alveoli and blood
  2. atmospheric air mixes with dead space gas (⬆CO2 and ⬇O2)
  3. Air in alveoli is saturated with water
    Slide 15
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7
Q

What are the Factors that affect the rate of gas diffusion

A
  1. Partial pressure
  2. Surface area of membrane
  3. Permeability of membrane
  4. Diffusion distance
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8
Q

What is hyperventilation and what is its consequences?

A
  • alveolar ventilation exceeds demands of tissues
  • excess CO2 is removed and excess O2 is inspired for bodies requirement.
  • P(CO2) 100mmHg in alveoli and arterial blood
  • Hypocapnia (low PCO2) causes vasoconstriction in the brain leading to reduced O2 delivery to brain and dizziness
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9
Q

What is hypoventilation and what are its consequences?

A
  • Alveolar ventilation is insufficient to meet tissues demands
  • could be in disease were normal ventilation is difficult eg asthma, overdose of sleeping tablets
  • cells continue to produce Co2 and consume O2 yet ventilation unable to keep up with demand
  • P(CO2) increases > 40mmHg in alveoli and arterial blood
  • PO2 decreases < 100mmHg in alveoli And arterial blood
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10
Q

What are some diseases etc that could cause diffusion problems?

A
  • ⬇Surface area eg emphysema -breakdowns of the alveoli reduces surface area for diffusion/gas exchange.
  • ⬇permeability of the respiratory membrane eg fibrosis-gases diffuse more slowly through fibrous scar tissue
  • ⬆diffusion distance eg pulmonary oedema➡ fluid build up in lungs due to increased pressure in pulmonary capillaries (congestive heart failure, lung infections etc. this excess interstitial fluid ➡ greater diffusion distance ➡decreases exchange
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11
Q

Gas transport in blood

Oxygen transport in blood. Tell me how da shit it works

A

-Oxygen is poorly soluble in plasma
-most oxygen is transported bound to haemoglobin
-dissolved oxygen is important for tissue supply
-O2 dissolved in the plasma determines the PO2 I’d the blood
Slide 23

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12
Q

Tell me about haemoglobin?

A

Hb is a protein found in RBC
Has 4 subunits each consisting of:
-a protein (globin) chain
-a heme unit (one Fe2+ and one protoporphyrin molecule)
-O2 binds reversibly to the Fe2+ of the heme
Hb + O2 ⬅➡Hb x O2
-each Hb molecule can bind up to 4 molecules of O2
-binding of O2 is reversible
-the amount of O2 bound to Hb depends on the PO2
-carbon monoxide competes with O2

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13
Q

Describe and explain the Hb: O2 dissociation curve

Slide 25

A
  • P(O2) of surround fluid is primarily a determinant of Hb O2 saturation.
  • Oxygen saturation= % of Hb binding sites occupied by O2
  • relationship between PO2 and Hb saturation described by the hemoglobin-oxygen dissociation curve
  • ability of Hb to bind to O2 depends on how much O2 is already bound
  • O2 uptake in lungs, O2 release in tissues
  • in alveolar capillaries, high PO2 in the plasma promotes O2 binding to Hb
  • in tissues capillaries, low PO2 in the plasma promotes O2 unbinding from Hb.

At rest Hb carries much more oxygen than tissues repaire

  • Hb in blood returning from tissues in systemic veins still 75% saturated with O2
  • O2 reserves that can be drawn in if needed eg during exercise
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28
Q

What are the factors affecting Hb: O2 binding affinity

A
  • The PO2 is the most important regulator of binding by Hb
  • 3 other factors can change Hb: O2 binding affinity
  • they shift Hb: O2 dissociation curve to left or right

Temp, PCO2/H and 2,3 diphosphoglycerate all affect affinity

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29
Q

Effect of temperature on Hb structure and O2 binding affinity

A

Increased temp decreases Hb: O2 affinity

  • shifts curve to the right
  • promotes release of O2 in warm, metabolising tissues

Decreased temp increases Hb: O2 affinity
-shifts curve to the left

30
Q

The Bohr effect: H+/pH and Co2

A

-H+ ions bind to Hb and make it harder for O2 to bond
-Co2 contributes to Bohr effect by forming H+ when it combines with H2O
Co2 + H2O
⬆ H+ (⬇pH) decreases Hb: O2 affinity
-shift curve right
-promotes release of O2 in acidic metabolising tissue (lots of Co2)
-⬇ H+ (⬆pH) increases Hb: O2 affinity
-shifts curve to the left
-promotes binding of O2 in lungs but releases less O2 to tissues

31
Q

Tell me how exercising influences temp, curve, acidic

A

Exercising muscle is:

  1. Hot
  2. Hypercarbic (increased CO2)
  3. Acidic (decreased pH)

All shifts the curve to the right and deliver more O2 to the muscle cells

32
Q

How does 2,3-DIphosphoglycerate

A
  • 2,3-DPG formed in RBC by glycolysis (anaerobic metabolism)
  • 2,3-DPG production inhibited by oxyhemoglobin (O2-rich conditions)
  • 2,3 DPG increases in hypotonic conditions lasting > few hours

Increased 2,3-DPG decreases Hb: O2 affinity (right shift)
-promotes release of O2 in low O2 conditions
-important for adaptation to hypoxia (eg anaemia, high altitude, lung disease)
Decreased 2,3-DPG increases Hb: O2 affinity (left shift)

33
Q

Hb: O2 dissociation curve -putting it all together.

What happens when it shifts right?

A

Right shift:

  • O2 unloading in tissues
  • caused by ⬆ temp
  • ⬆P(Co2)/⬇pH & ⬆ 2,3 DPG
  • occurs in metabolically active tissues, hypoventilation, hyperthermia and chronic hypoxia.

Left shift:

  • ⬇ O2 unloading in tissues but ⬆ O2 loading in lungs
  • caused by ⬇ temp,
  • ⬇P(CO2)/⬆pH and ⬇ 2,3 DPG
  • occurs in hypoventilation, hypothermia and disease states
34
Q

Pulse oximetry

A
  • measures the saturation of Hb with O2 (as a %) in arterial blood
  • light emitted from probe is absorbed by Hb to varying degrees, depending on O2 saturation
  • SpO2 (saturation of peripheral O2) represents the proportion of Hb O2
  • SpO2 in arterial blood usually 97% to 99%, <90% under general anaesthesia is cause for concern
35
Q

Transport of carbon dioxide. How?

A
  • at rest, respiring tissues produce about 200 ml/min CO2
  • In the tissues Co2 diffuses down a pressure gradient (cells ➡ blood)
  • Co2 transported in blood 3 ways:
    1. Dissolved directly in blood (5-6%)
    2. Bound to Hb in RBC as carbaminohemoglobin (5-8%)
    3. As HCO3- in blood (86-90%)
36
Q

Carbon dioxide- in Tissues

A
  • if HCO3- and H+ was permitted to build up in erythrocytes, the equation would reach equilibrium and Co2 transport would become very inefficient.
  • HCO3- is exchanged for Cl- at the red cell membrane (chloride shift)
  • H+ ions are buffered by binding to Hb
  • All reactions reverse in the lungs
37
Q

CO2 unloading in alveoli

A
  • CO2 diffuses down pressure gradient from blood to alveoli
  • decreases in Co2 pushes equation to the left
  • HCO3 moves into the cells through a reversed “chloride shift”
  • decreased Co2 in red blood cells encourages Carbaminohemoglobin to release CO2
38
Q

Control of respiration. How?

A
  • Aim of respiratory control is to match O2 delivery and CO2 removal to matabolic demands
  • to maintain partial pressures, body regulates alveolar ventilation (frequency and volume of breaths)
39
Q

How are the respiratory muscle controlled?

A
  • Control of alveolar ventilation requires control of respiratory muscle contraction
  • respiratory muscles are skeletal muscles
  • skeletal muscles require stimulation from somatic motor neurons to contract