Blood Gas Transport Flashcards
Why is haemoglobin critical to O2 transport?
Oxygen has low solubility in plasma (0.225mL/L/kPa)
In order to dissolve the amount of O2 needed to supply tissues, an impossibly high alveolar PO2 would be required
The presence of haemoglobin overcomes this problem- it enables O2 to be concentrated within blood (↑ carrying capacity) at gas exchange surfaces and then released at respiring tissues
The vast majority of O2 transported by the blood is bound to haemoglobin (>98%)
How is oxygen content of blood measured/defined?
1) O2 partial pressure (PaO2), expressed as kPa
≈ “the partial pressure of O2 within a gas phase (at a gas-liquid interface) and that would yield this much O2 in the plasma at equilibrium”
2) Total O2 content (CaO2), expressed as mL of O2 per L of blood (ml/L)
≈ “the volume of O2 carried in each litre of blood, including O2 dissolved in the plasma and O2 bound to Hb”
3) O2 saturation (SaO2= measured directly in arterial blood, SpO2= estimated by pulse oximetry), expressed as %
≈ “the % of total available haemoglobin binding sites that are occupied by oxygen”
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How does Hb O2 affinity change depending on the local environment?
Hb O2 affinity changes depending on the local environment, enabling O2 delivery to be coupled to demand
1) Lungs = ↑PO2, ↓PCO2, ↑pH ∴ ↑O2 saturation
2) Resting tissue = ↓PO2 ∴ ↓O2 saturation
O2 moves from Hb to tissue
3) Working tissue = ↓↓PO2
However anaerobic respiration & hypoxia also produces lactic acid (H+), CO2, and 2,3-DPG ∴ ↑ O2 demand = ↑ CO2, ↓pH, & 2,3-DPG
= ↓ Hb-O2 affinity + binding ∴ ↓↓ O2 saturation
↑↑ O2 moves from Hb to tissue
What are some clinical aspects of Hb and O2 transport?
Oxyhaemoglobin (Hb-O2) appears red where as deoxyhaemoglobin (Hb) appears blue
The relative concentrations determines the colour of blood
Cyanosis- purple discolouration of the skin and tissue that occurs when the [deoxyhaemoglobin] becomes excessive
Q: Why is cyanosis often less obvious in patients with low RBC density?
Central cyanosis:
Bluish discolouration of core, mucous membranes and extremities
Inadequate oxygenation of blood
E.g. hypoventilation, V/Q
Peripheral cyanosis:
Bluish coloration confined to extremities (e.g. fingers)
Inadequate O2 supply to extremities
What happens if you have insufficient haemoglobin?
Insufficient haemoglobin (anaemia)
Hypoxia can occur despite adequate ventilation and perfusion, if the blood is not able to carry sufficient oxygen to meet tissue demands
Causes of anaemia (insufficient RBCs or haemoglobin):
Iron deficiency (↓ production)
Haemorrhage (↑ loss)
What happens with carbon monoxide poisoning?
Hb has >200x affinity for carbon monoxide (CO) than O2 and competes for the same binding site \↑CO-Hb= ↓O2 capacity Carboxyhaemoglobin has cherry red pigmentation, hence hypoxia occurs in the absence of cyanosis Signs include: Headaches Nausea Dizziness Breathlessness Collapse Loss of consciousness
How and why does transport of CO2 differ to O2?
1) CO2 has a higher H2O solubility than O2 does- therefore a greater % of CO2 is transported simply dissolved in plasma (CO2= 7%, O2= 1%)
Concentration= partial pressure x solubility
2) CO2 binds to Hb at different sites than O2 (R-NH2 residues at the end of peptide chains, forming carbamino-Hb, R-NHCOOH) and with decreased affinity
Thus, a lower % of CO2 is transported in this manner (=23%)
3) Co2 reacts with water to form carbonic acid, which accounts for the majority (=70%) of CO2 transported
CO2 + H2O ->
What happens to CO2 respired from the tissues?
- CO2 is produced by respiring cells and dissolves in the plasma + enters RBCs.
- Conversion of CO2 + H2O to H2CO3 within RBCs (catalysed by carbonic anhydrase)
- The effective removal of CO2 by (2) enables further CO2 to diffuse into the RBC (& more can then enter the plasma).
- H2CO3 ionises to HCO3- + H+.
The RBC cell membrane is impermeable to H+, therefore H+ cannot leave - Accumulation of H+ within cell, and ∴ cessation of (2), is prevented by deoxy-Hb acting as a buffer and binding H+.
Movement of O2 into tissues from RBCs ∴ ↑[deoxy-Hb] and enables more CO2 to be transported. - The increased [HCO3-] creates a diffusion gradient for HCO3- to leave the cell.
It is exchanged for Cl- to maintain electrical neutrality.
What happens to CO2 in the lungs?
- Low PACO2, creates a diffusion gradient for CO2 to diffuse out of the blood into the airspace
- Increased PAO2 leads to O2-Hb binding. O2-Hb binds less H+ than deoxy-Hb, increasing free [H+]
- Increased free [H+] leads to increased H2CO3 and ultimately CO2 which contributes to CO2 plasma saturation.
- The changing equilibrium of carbonic acid reaction, also leads to decreased [HCO3-], as it binds the free H+. This creates a diffusion gradient that allows HCO3- ions to entry the RBC in exchange for Cl-.
What is the net result from the previous two cards?
The net result of these effects is transport of O2 and CO2 interact:
Deoxygenated blood carries more CO2
Oxygenation of blood causes CO2 to leave (both points = “the Haldane effect”).
What is the difference between the Haldane effect and the Bohr effect?
The Haldane effect describes the impact of O2 on CO transport, whereas the Bohr effect describes the impact of CO2 on O2 transport
A) Binding of O2 to Hb induces a structural change to Hb, reducing Hb affinity for CO2 and H+
Therefore deoxygenated blood carries more CO2 at any given PCO2 (the Haldane effect)
B) Binding of CO2 (or H+) to Hb induces a (different) structural change to Hb, reducing Hb affinity for O2
Therefore Hb releases more O2 at any given PO2 when CO2 and/or H+ levels rise (the Bohr effect
What is the relationship between PaCO2 and [H2CO3]?
The relationship between PCO2 and [H2CO3] means that Co2 transport is important in acid-base balance
This means that increased CO2 = increased H+ (acidity, lower pH)
As the lungs play a role in regulating CO2 levels, they also therefore contribute to acid-base balance
Furthermore, signs of respiratory and metabolic distress can be diagnosed and interpreted from analysis of ABG and pH
