Physiology
Respiratory
Regarding chemical control of respiration, which of the following statements is INCORRECT:
Answer:
CSF is separated from the blood by the blood-brain barrier. This barrier is impermeable to polar molecules such as H+ and HCO3- but CO2 can diffuse across it easily. The pH of CSF is therefore determined by the arterial PCO2 and the CSF HCO3- and is not affected by blood pH.Chemoreceptors
Physiology / Respiratory / Control of Respiration
Last Updated: 11th April 2019
Chemical control of ventilation is mediated via central and peripheral chemoreceptors which detect arterial PCO2 and pH (central and peripheral) and PO2 (peripheral only), and modulate ventilation via a distributed network of neurones in the brainstem.
Factors Affecting Ventilation
PCO2 is the most important factor. An increase in PACO2 causes ventilation to rise in an almost linear fashion.
A metabolic acidosis (e.g. lactic acidosis in strenuous exercise) causes the relationship between PCO2 and ventilation to shift to the left (where ventilation increases more for any given rise in PACO2) and a metabolic alkalosis causes a shift to the right.
PO2 normally stimulates ventilation only when it falls below around 8 kPa. However, when a fall in PO2 is accompanied by an increase in PCO2, the resultant increase in ventilation is far greater than would be expected from the effects of either alone; there is thus a synergistic relationship between PO2 and PCO2.
Central Chemoreceptors
The central chemoreceptor consists of a diffuse collection of neurones located near the ventrolateral surface of the medulla. These are sensitive to the pH of the surrounding cerebrospinal fluid (CSF), thus indirectly respond to blood PCO2 (but do not respond to changes in PO2).
CSF is separated from the blood by the blood-brain barrier. This barrier is impermeable to polar molecules such as H+ and HCO3- but CO2 can diffuse across it easily. The pH of CSF is therefore determined by the arterial PCO2 and the CSF HCO3- and is not affected by blood pH. Stimulation of the central chemoreceptor by a fall in CSF pH (with a rise in blood PCO2) causes an increase in ventilation to blow off CO2; the response is delayed because CO2 has to diffuse across the blood-brain barrier.
The central chemoreceptor is responsible for about 80% of the ventilatory response to changes in PCO2 in humans.
Peripheral Chemoreceptors
The peripheral chemoreceptors are found within the carotid body, innervated by the glossopharyngeal nerve and located at the bifurcation of the common carotid arteries, and the aortic bodies, innervated by the vagus nerve and located on the aortic arch. The aortic bodies are functionally less important. Carotid bodies respond to increased PCO2 and decreased blood pH in addition to reduced PO2. They are responsible for about 20% of the ventilatory response to increased PCO2.
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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 |
