Blood Gas Exchange (3)

Question 1

What is partial pressure?

Answer 1

The percentage of total atmospheric pressure ( 760mm Hg) made up by a specific gas.
= percentage composition of the atmosphere (0.x) x 760mm Hg

Question 2

What is Henry’s Law (the dissolved gas conc. equation)?

Answer 2

Dissolved gas conc. = Partial pressure of the gas x Solubility coefficient of the gas

• Gases dissolve into liquids until they reach equilibrium - dependent on the gases partial pressure & solubility
• Solubility coefficients in water: Oxygen = 0.024, Carbon dioxide = 0.57, Nitrogen = 0.04

Question 3

What is Nitrogen narcosis?

Answer 3

In environments where pressure increases (e.g. during diving) the PN2 in the lungs increases -> greater N diffusion across the respiratory membrane -> N changes the permeability of the cell membranes of nerve cells -> affects ion channels -> decreases nervous function (cells may not be able to fire action potentials).
Symptoms: Confusion, Disorientation, Colours appear brighter (‘rapture of the deep’).

Question 4

What is Decompression Sickness?

Answer 4

‘The bends’ = When a diver rapidly ascends to the surface -> the pressure rapidly changes from high to low -> the dissolved gases in the blood (especially N - after deep-diving there are high conc.) come out of solution & form bubbles.
The bubbles can occlude blood vessels & decrease blood supply to tissues -> Dizziness, Extreme fatigue, Tissue death, Paralysis, Unconsciousness.

Question 5

What are hyperbaric chambers used to treat?

Answer 5

Hyperbaric chamber = a chamber containing O2 at pressures of 3-4 atm

• Infections (e.g. Gangrene & Tetanus) caused by anaerobic bacteria: The chamber increases the patient’s PO2 to a level toxic to anaerobic bacteria.
• Carbon monoxide poisoning: haemoglobin has a greater affinity for CO than O2, so the O2 conc. has to be raised to a level to compete with the CO.
• Gas embolisms (e.g. Decompression sickness)
• Bone infections
• Smoke inhalation
• Near drowning
• Circulatory problems

Question 6

What is the diffusion pathway in the lungs?

through which layers do the gases pass through

Answer 6

Respiratory membrane (sits between the lumen of the alveoli & the pulmonary capillary)

• Alveolar fluid = lines the alveoli - stops them drying out, contains surfactants to prevent the alveolar walls sticking together
• Alveolar squamous epithelial cells
• Alveolar basement membrane
• Interstitial space = layer of connective tissue
• Capillary basement membrane
• Capillary endothelium

Question 7

What 4 factors affect the diffusion of gases across the respiratory membrane?

Answer 7

1. Thickness of the membrane: Normally about 0.5mm thick - fibrosis increases thickness.
2. The diffusion coefficient
3. The surface area
4. The partial pressure difference across the membrane

Question 8

What 2 factors change the matched relationship between blood flow & respiratory rate?

Answer 8

1. When ventilation exceeds the ability of the blood to take up the O2 (HR is too low)
   - Often due to heart failure (myocardial infarction, persistent hypertension)
2. When the blood supply to the lungs exceeds the ventilation rate (the blood is partially oxygenated = Shunted blood)
   - Often caused by decreased SA (pulmonary fibrosis)

Question 9

What is Physiological shunt, and what causes it?

Answer 9

= partially oxygenated blood that reenters the oxygenated circulatory system naturally

• Deoxygenated blood from the bronchial veins recombines with oxygenated blood from the pulmonary veins
• At rest the lungs aren’t fully inflated -> the PO2 in the uninflated alveoli is low so the blood passing through them isn’t as saturated with blood = 2% of the blood pumped by the heart

Question 10

What is the effect of activity level on the blood flow in the different regions of the lung?

Answer 10

At rest, a standing person will only ventilate the lower portion of the lungs -> PO2 decrease & PCO2 increase in the upper alveoli -> vasoconstriction of arterioles supplying those alveoli -> decreased blood flow to those regions.

Question 11

What is the oxygen diffusion gradient in the lungs?

Answer 11

Oxygen diffuses down its conc. gradient from the alveoli (PO2 = 105mm Hg) into the alveolar capillaries (PO2 = 40mm Hg)
*By the end of the pulmonary vein PO2 = 95mm Hg because deoxygenated blood from the bronchial veins has joined.

Question 12

What is the oxygen diffusion gradient in the tissues?

Answer 12

Oxygen diffuses down its conc. gradient from the blood, into the interstitial fluid (PO2 = 40mm Hg) and into the tissue cells (PO2 = 20mm Hg)

Question 13

What is the carbon dioxide diffusion gradient in the tissues?

Answer 13

Carbon dioxide diffuses down its conc. gradient from the tissue cells (PCO2 = 46mm Hg), to the interstitial fluid (PCO2 = 45mm Hg) and into the capillaries (PCO2 = 40mm Hg) where it is pumped to the lungs.

Question 14

What is the carbon dioxide diffusion gradient in the lungs?

Answer 14

Carbon dioxide diffuses down its conc. gradient from the pulmonary artery (PCO2 = 45mm Hg) into the alveoli (PCO2 = 40mm Hg).

Question 15

What is the difference between adult & embryonic/foetal haemoglobin?

Answer 15

Adult haemoglobin = two alpha chains, two beta chains (haem groups)
Embryonic/foetal haemoglobin = modified to have a greater affinity for oxygen

Question 16

What is:

a) Oxyhaemoglobin?
b) Deoxyhaemoglobin?

Answer 16

a) Oxyhaemoglobin = oxygen-rich haemoglobin

b) Deoxyhaemoglobin = oxygen defficient haemoglobin

Question 17

What is the life cycle of erythrocytes (RBCs)?

Answer 17

Erythropoiesis = production of RBC - occurs in bone marrow, takes 4 days, stimulated by erythropoietin (produced in the kidneys) -> low PO2 stimulates the kidneys to produce more erythropoietin
Life span: Men = approx 120 days. Women = approx 110 days
Death: approx 2.5 million (0.00001% of the total 25 trillion RBC) are destroyed every second

Question 18

What 4 factors affect gas exchange & transport?

Answer 18

1. The oxygen diffusion gradients
2. The carbon dioxide diffusion gradients
3. Haemoglobin’s oxygen dissociation relationship
4. Carbon dioxide transport mechanisms

Question 19

What is haemoglobin’s oxygen dissociation relationship?

Answer 19

As the PO2 decreases haemoglobin more readily offloads oxygen = oxygen delivery system where the offload of O2 is matched to the tissue requirements of O2.

• Oxygenated blood in systemic arteries: PO2 = 100mm Hg -> 100% haemoglobin saturation of O
• Deoxygenated blood in systemic veins: PO2 = 40mm Hg -> 75% saturation
• Deoxygenated blood in capillaries in contracting skeletal muscle: PO2 = 20mm Hg -> 35% saturation

Question 20

What is the effect of PCO2 on haemoglobin’s affinity for oxygen?

Answer 20

• In tissues: once haemoglobin has offloaded one O2 molecule it is easier for a CO2 molecule to bind -> haemoglobin undergoes a conformational change so it has a lower affinity for O2 -> offloads more O2 (useful in respiring tissues that require more oxygen)
• In the lungs: CO2 concentrations decrease -> CO2 dissociates from the haemoglobin -> haemoglobin undergoes a conformational change so it has a greater affinity for O2.
• At PO2 = 15mm Hg (during heavy exercise):
• Low PCO2 -> 30% O2 saturation
• High PCO2 -> 10% O2 saturation
• Linked to pH: as PCO2 increases -> pH decreases -> haemoglobin has a lower affinity for O2 -> the curve is shifted to the right.

Question 21

What is the effect of temperature on haemoglobin’s affinity for oxygen?

Answer 21

• At high temperatures = 43 C (in exercising tissues): haemoglobin undergoes a conformational change -> decreases O2 affinity
• At low temperatures = 20 C (in the lungs during forced ventilation): haemoglobin undergoes another conformational change -> increased O2 affinity

Question 22

What is BPG?

Answer 22

= 2,3-bisphosphoglycerate = decreases haemoglobin’s affinity for O2 - produced by erythrocytes

• BPG levels in stored blood decrease over time - important for donated blood
• People living at high altitudes have increased conc. of BPG -> thought to increase O2 delivery at high altitudes

Question 23

How is CO2 transported in the blood?

Answer 23

1. Dissolved in plasma = 8%
2. Combined with blood proteins = 20%
   - Carbamino compounds = blood proteins that bind to CO2
   - Carbaminohaemoglobin = CO2 bound to haemoglobin
3. In the form of bicarbonate ions = 72%

Question 24

What is the Haldane effect?

Answer 24

The affinity of haemoglobin for CO2 is greater if that haemoglobin molecule has just given up oxygen.

Question 25

How is CO2 transported in the tissues?

Answer 25

• Aerobic respiration uses up O2 & produces CO2 -> CO2 diffuses into the blood from the tissues
• Some CO2 is bound to the haemoglobin (4x per molecule)
• The rest of the CO2 associates with water -> carbonic acid (mediated by carbonic anhydrase) which dissociates -> H+ & bicarbonate ions
• Some of the H+ bind to the haemoglobin -> haemoglobin undergoes a conformational change so its affinity for O2 decreases
• The greater the conc. of bicarbonate ions creates a conc. gradient -> bicarbonate diffuses out of the RBC -> creates a positive charge in the cell -> Chloride shift (Cl- diffuse into the cell)

Question 26

How is CO2 transported in the lungs?

Answer 26

• CO2 conc. deacreases -> CO2 dissociates from the haemoglobin -> conformational change -> haemoglobins affinity for O2 increases
• O2 binding to haemoglobin -> conformational change -> decreased affinity for H+ -> H+ dissociate from the haemoglobin
• H+ reassociates with bicarbonate ions -> carbonic acid -> CO2 (mediated by carbonic anhydrase)
• Bicarbonate conc. decreases -> bicarbonate diffuses into RBC -> cell becomes negatively charged -> Reversed Chloride Shift -> Cl- diffuses out of the RBC