Physiology
Respiratory
The functional residual capacity (FRC) will be increased in which of the following:
Answer:
Factors increasing FRC:- Emphysema
- Air trapping in asthma
- Ageing (due to loss of elastic properties)
- Increasing height of patient
Functional Residual Capacity
Physiology / Respiratory / Lung Volumes and Pressures
Last Updated: 21st April 2019
Definition
The volume left in the lungs at the end of a normal breath is called the functional residual capacity (FRC). At FRC, the respiratory muscles are relaxed and its volume is determined by the elastic properties of the chest wall (tending to spring outwards) and the lungs (tending to collapse).
If the elastic recoil of either the lungs or chest wall is abnormally large or small, the FRC will be abnormal. In fibrosis, the lungs are stiff and have decreased compliance, and so the FRC is reduced. In emphysema, there is loss of alveolar tissue, and with it elastic recoil of the lungs and hence the FRC is increased. As such, patients with emphysema often have noticeably broader chests due to the relatively unopposed outward recoil of the chest wall.
Factors Affecting FRC
Factors increasing FRC:
- Emphysema
- Air trapping in asthma
- Ageing (due to loss of elastic properties)
- Increasing height of patient
Factors decreasing FRC:
- Restrictive ventilatory defects e.g. pulmonary fibrosis
- Posture - lying supine
- Increased intra-abdominal pressure (e.g. obesity, pregnancy, ascites)
- Reduced muscle tone of diaphragm e.g. muscle relaxants in anaesthesia, neuromuscular disease
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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 |
