Physiology - Carriage of blood gases Flashcards
What is the structure of hemaglobin ?
Haemoglobin is a tetramer, meaning it is made up of four subunits. Each subunit is formed of a globin polypeptide chain and an associated haem group (a porphyrin ring with a central iron atom). Each iron atom, and therefore each subunit, can reversibly associate with a single oxygen molecule.
Heamaglobin can bind 4 02 molecules
When oxygen is bound to haemoglobin what is it called?
Oxyhemoglobin
Explain the parts of the hemaglobin
Schematic diagram showing the basic structure of a single haemoglobin A molecule, including two α-globin chains (green), two β-globin chains (yellow), each containing a haem–iron complex (blue).
Explain the difference in binding affinity between deoxygenated oxygen and oxygenated haemaglobin
The transition from ‘tense’ to ‘relaxed’ haemoglobin. In its deoxygenated ‘tense’ form, the crevice containing the haem molecule is narrow, restricting the access of oxygen to its binding site. As each oxygen molecule binds, the position of the haem molecule changes which affects the interaction between adjacent globin chains, relaxing the molecule and so allowing easier access of subsequent oxygen molecules to their binding site.
What does this graph show ?
The relationship between percentage saturation of Hb with oxygen, and partial pressure of oxygen is a sigmoid curve. In the alveoli, the partial pressure of oxygen is about 104mmHg, which means that Hb is almost 100% saturated; it has a high AFFINITY for oxygen
In systemic veins, the partial pressure of oxygen is about 40mmHg, and Hb is around 77% saturated
This means that some of the oxygen has been released for use in aerobic respiration
In systemic veins Hb has a LOWER affinity for oxygen.
What would a shift to left suggest ?
increase affinity to oxygen and increase reluctant to release the oxygen
What would a shift to the right suggest ?
decreased affinity for oxygen, oxygen active unloads
what is the oxygen dissociation curve?
PH, TEMP, CO2, 23DPG
Explain the effect of PH on affinity of hemaglovin for oxygen?
Blue = high PH (7.6)
Black = normal pH (7.4)
Red = Low PH (7.2)
(BOAR effect)
When pH is reduced (high concentration of hydrogen ion), the affinity of Hb for oxygen reduces
This means that at any PO2 the SaO2 is lower – more oxygen has been released
The curve shifts to the right (Note curve down)
Anaerobic metabolism can lead to a shift in PH and can help in the offloading of O2 to the tissues to provide tissue oxygen during exercise! good!
Effect of PCO2 on affinity of haemoglobin for oxygen
Blue - Low PC02
Black - Normal PC02
Red - High PCO2
Blood also carries carbon dioxide
When blood PCO2 is high, the affinity of Hb for oxygen falls
The curve shifts to the right, and more oxygen is released
Effect of Temperature on haemoglobin O2 affinity
Increasing temperature reduces affinity and shifts the curve to the right
Decreasing temperature shifts the curve to the left
Example - Exercise high tempt - oxygen increase to be off laoded and deliver to oxygen needed. GOOD !
Effect of 2,3 DPG on affinity of haemoglobin
Blue - Decrease 2,3, DPG
Black - Normal
Red - Increase 2,3 DPG
2,3-Bisphosphoglyceric acid (2,3-BPG), is a three-carbon isomer of a glycolytic intermediate
Present in human red blood cells at @ 5 mmol/L
binds with greater affinity to deoxygenated haemoglobin
promotes the release of the remaining oxygen
2,3-BPG increases x5 within 1-2 hrs in patients with chronic anaemia
Decreases with dialysis and transfusion (RBC stress)
Explain Foetal Hb affinity for oxygen
Foetal Hb (α2γ2) predominates during most of gestation HbF (INSTEAD OF HBA IN ADULTS ) Foetal Hb has a higher affinity for oxygen than maternal HbA.
This is essential because it means that under conditions where maternal Hb is releasing some oxygen, foetal Hb can take it up.
This allows effective transfer of oxygen from maternal to foetal blood
Foetal can pull out oxygen wiht greater ease
Explain Transfer of oxygen to tissues
We have myoglobin which has a greater affinity for oxygen.
Myoglobin also has a greater affinity for O2 than haemoglobin (curve shifted left)
Accepts O2 from haemoglobin when PO2 in blood is low and releases O2 in muscle (e.g. during exercise).
The myoglobin binding curve lacks the sigmoidal shape of the haemoglobin binding curve because of the single O2 binding site in each molecule.
Explain role of CO2 in the blood.
A small amount of CO2 dissolves in plasma
Most carbon dioxide diffuses into red blood cells
Some carbon dioxide attaches to Hb, displacing oxygen (note O2 affinity)
Most carbon dioxide reacts with water catalysed by carbonic anhydrase to produce bicarbonate and hydrogen ion
Bicarbonate diffuses out of the red blood cell and is replaced by chloride ions (chloride shift)
In the lungs the process is reversed
What is the importance of the bicarbonate ion?
One of the most important buffer systems in the body
A reversible chemical reaction that can absorb or release hydrogen ions (H+).
Hydrogen ion concentration must be kept constant. Changes in pH affect:
The affinity of Hb for oxygen
The rate of enzyme reactions (optimal within a very narrow range)
The ionisation states of many substances, disrupting their structure e.g. DNA
Mechanisms involved in the bicarbonate balance
Carbon dioxide production from metabolism (Krebs cycle )
Exhalation of carbon dioxide by the lungs
Hydrogen ion excretion by the kidney
Bicarbonate reabsorption
What is the bicarbonate buffer solution equation ?
What are the metabolic and respiratory disorders that can change the bicarbonate?
Metabolic processes affect bicarbonate concentration, and respiratory processes affect PCO2.
Changes in ventilation compensate for metabolic disorders, and renal excretion of acid compensates for respiratory disorders.
In primary acid–base disorders, the underlying disorder will be evident from examination of the pH, PCO2, and serum bicarbonate.
