Physiology
Respiratory
Regarding pressures and airflow during the normal breathing cycle, which of the following statements is CORRECT:
Answer:
The lung volume at rest is the functional residual capacity (FRC). During inspiration, the inspiratory muscles contract, the chest wall is expanded and intrapleural pressure falls (making it more negative). In expiration, both alveolar pressure and intrapleural pressure rise. In quiet breathing, intrapleural pressure remains negative for the whole of the respiratory cycle, whereas alveolar pressure is negative in inspiration and positive during expiration. Alveolar pressure is always greater than intrapleural pressure because of the recoil of the lung.Alveolar Pressure
Physiology / Respiratory / Lung Volumes and Pressures
Last Updated: 21st April 2019
At Rest
At rest (before inspiration begins), alveolar pressure equals atmospheric pressure. Because lung pressures are expressed relative to atmospheric pressure, alveolar pressure is said to be zero.
The lung volume at rest is the functional residual capacity (FRC). At FRC, the opposing forces of the lungs trying to collapse and the chest wall trying to expand create a negative pressure in the intrapleural space between them.
Inspiration
During inspiration, the inspiratory muscles contract, the chest wall is expanded and intrapleural pressure falls (making it more negative). This increases the pressure gradient between the intrapleural space and alveoli, stretching the lungs. The alveoli expand and alveolar pressure falls, creating a pressure gradient between the mouth and alveoli and causing air to flow into the lungs.
Expiration
During expiration alveolar pressure becomes greater because alveolar gas is compressed by the elastic forces of the lung. Alveolar pressure is now higher than atmospheric pressure, the pressure-gradient is reversed and air flows out of the lungs. Intrapleural pressure also rises, returning to its resting value during a normal expiration.
In quiet breathing, intrapleural pressure remains negative for the whole of the respiratory cycle, whereas alveolar pressure is negative in inspiration and positive during expiration. If ventilation is increased, the changes of intrapleural and alveolar pressure are greater. In forced expiration (e.g. coughing or sneezing), intrapleural pressure becomes positive.
Dynamic Compression
The lungs never fully empty. Physiologically this is important as a completely deflated lung with collapsed alveoli requires significantly more energy to inflate. Even with a maximum expiration, there is still a residual volume of air in the lungs. This occurs because as the expiratory muscles contract during forced expiration, all the structures within the lungs, including the airways, are compressed by the positive intrapleural pressure (dynamic compression). Consequently the smaller airways collapse before the alveoli empty completely and some air remains within the lungs.
In patients with COPD, dynamic compression limits expiratory flow even in tidal breathing - this is because there is a loss of radial traction due to destruction of lung architecture meaning the airways are more readily compressed, and there is increased lung compliance, leading to lower alveolar pressure and less force driving air out of the lungs.
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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 |
