Physiology
Respiratory
Regarding the foetal oxygen-haemoglobin dissociation curve, which of the following statements is CORRECT:
Answer:
Foetal haemoglobin (HbF) has a higher affinity for oxygen than adult haemoglobin (HbA) because it's gamma chains bind 2,3-DPG less avidly than beta chains of HbA. The HbF dissociation curve lies to the left of that for HbA. In the placenta PCO2 moves from the foetal to the maternal circulation, shifting the maternal curve further right and the foetal curve further left (the double Bohr effect). The higher affinity of HbF relative to HbA helps transfer oxygen from mother to foetus. Therefore even through blood returning from the placenta to the foetus in the umbilical vein has a PO2 of only about 4 kPa, its saturation is 70%. Oxygen transport in the foetus is also helped by a high Hb of about 170 - 180 g/L.Oxygen Transport
Physiology / Respiratory / Gas Transport in Circulation
Last Updated: 26th July 2024
The solubility of oxygen in blood plasma is low and only a very small percentage of the body's requirement can be carried in the dissolved form (< 10 mL), therefore most oxygen is carried bound to haemoglobin in red blood cells.
Haemoglobin
Each gram of haemoglobin binds with up to 1.34 mL oxygen, so with a haemoglobin concentration of 150 g/L, blood contains a maximum of 200 mL/L oxygen bound to haemoglobin; this is the oxygen capacity, which varies with [Hb]. The actual amount of oxygen bound to haemoglobin depends on the PO2. Low PO2 in tissue capillaries promotes oxygen release from haemoglobin, whereas the high PO2 in pulmonary capillaries promotes oxygen binding.
Each molecule of haemoglobin can bind up to four molecules of oxygen, at which point it is said to be saturated. Haemoglobin binds oxygen in a cooperative fashion; this means as each oxygen molecule binds, there is a conformational change in its protein structure and its affinity for oxygen increases, making it easier to bind the next oxygen molecule.
Oxygen-Haemoglobin Dissociation Curve
The oxygen dissociation curve is a graph that plots the proportion of haemoglobin in its oxygen-laden saturated form on the vertical axis against the partial pressure of oxygen on the horizontal axis.
Cooperative binding is responsible for the steepness of the oxygen-haemoglobin dissociation curve in the middle. The curve flattens again at partial pressures above about 8 kPa because there are few unfilled haemoglobin binding sites. Thus for a normal arterial PO2 (about 13 kPa) and [Hb], the blood is about 97% saturated and any increase in PO2 will have little effect on the blood oxygen content. On the steep part of the curve however (< 8 kPa), small changes in PO2 will have large effects on the blood oxygen content. The PO2 at which the haemoglobin is 50% saturated is known as the P50 (the P50 is higher for a right-shifted curve and lower for a left-shifted curve).
![Modified by FRCEM Success. Original image by Peter Southwood (Own work) [CC0], via Wikimedia Commons](https://mrcemsuccess.com/wp-content/uploads/2017/05/Oxyhaem-curve.png)
Oxygen-Haemoglobin Dissociation Curve. (Image modified by FRCEM Success. Original by Peter Southwood (Own work) [CC0], via Wikimedia Commons)
Factors Affecting Oxygen-Haemoglobin Curve
The affinity of haemoglobin for oxygen, and hence the position of the dissociation curve, varies with local conditions.
A decreased affinity of haemoglobin for oxygen (and hence increased ease of dissociation), shown by a right shift in the oxygen dissociation curve, is caused by a fall in pH, a rise in PCO2 (the Bohr effect) and an increase in temperature. These changes occur in metabolically active tissues such as in exercise, and encourage oxygen release. The metabolic by-product 2,3-diphosphoglycerate (2,3 -DPG) also causes a right shift; 2, 3 -DPG may also be raised in chronic anaemia, chronic lung disease, or at high altitude.
Conversely, an increased affinity of haemoglobin for oxygen, shown by a left shift in the oxygen dissociation curve, is caused in the lungs by a rise in pH, a fall in PCO2 and a decrease in temperature.
Anaemia
In anaemia, at any given PO2, the oxygen capacity is reduced because of the reduced concentration of haemoglobin binding sites. The dissociation curve would not be altered if it was drawn as Saturation (y-axis) versus PO2 (x-axis) but if drawn as Oxygen content (y-axis) versus PO2(x-axis), the oxygen content value at each PO2 would be reduced. In chronic anaemia, red cell 2,3 -DPG levels rise and the curve will be right shifted.
Carbon Monoxide
Carbon monoxide (CO) binds 240 times more strongly than O2 to haemoglobin and by occupying O2-binding sites, reduces oxygen capacity. CO also increases oxygen affinity, shifting the oxygen haemoglobin curve to the left and making O2 release to tissues more difficult.
Foetal Haemoglobin
Foetal haemoglobin (HbF) binds 2, 3 -DPG less strongly than does adult haemoglobin (HbA), and so the HbF dissociation curve lies to the left of that for HbA, reflecting its higher oxygen affinity. This helps transfer oxygen from mother to foetus.
Adult vs Foetal Oxygen-Haemoglobin Dissociation Curve. (Image by OpenStax College [CC BY 3.0 , via Wikimedia Commons)
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- Biochemistry
- Blood Gases
- Haematology
| Biochemistry | Normal Value |
|---|---|
| Sodium | 135 – 145 mmol/l |
| Potassium | 3.0 – 4.5 mmol/l |
| Urea | 2.5 – 7.5 mmol/l |
| Glucose | 3.5 – 5.0 mmol/l |
| Creatinine | 35 – 135 μmol/l |
| Alanine Aminotransferase (ALT) | 5 – 35 U/l |
| Gamma-glutamyl Transferase (GGT) | < 65 U/l |
| Alkaline Phosphatase (ALP) | 30 – 135 U/l |
| Aspartate Aminotransferase (AST) | < 40 U/l |
| Total Protein | 60 – 80 g/l |
| Albumin | 35 – 50 g/l |
| Globulin | 2.4 – 3.5 g/dl |
| Amylase | < 70 U/l |
| Total Bilirubin | 3 – 17 μmol/l |
| Calcium | 2.1 – 2.5 mmol/l |
| Chloride | 95 – 105 mmol/l |
| Phosphate | 0.8 – 1.4 mmol/l |
| Haematology | Normal Value |
|---|---|
| Haemoglobin | 11.5 – 16.6 g/dl |
| White Blood Cells | 4.0 – 11.0 x 109/l |
| Platelets | 150 – 450 x 109/l |
| MCV | 80 – 96 fl |
| MCHC | 32 – 36 g/dl |
| Neutrophils | 2.0 – 7.5 x 109/l |
| Lymphocytes | 1.5 – 4.0 x 109/l |
| Monocytes | 0.3 – 1.0 x 109/l |
| Eosinophils | 0.1 – 0.5 x 109/l |
| Basophils | < 0.2 x 109/l |
| Reticulocytes | < 2% |
| Haematocrit | 0.35 – 0.49 |
| Red Cell Distribution Width | 11 – 15% |
| Blood Gases | Normal Value |
|---|---|
| pH | 7.35 – 7.45 |
| pO2 | 11 – 14 kPa |
| pCO2 | 4.5 – 6.0 kPa |
| Base Excess | -2 – +2 mmol/l |
| Bicarbonate | 24 – 30 mmol/l |
| Lactate | < 2 mmol/l |
