Physiology
Basic Cellular
Which of the following does NOT affect the rate of flow of a liquid through a tube:
Answer:
Flow through a tube is dependent on the pressure differences across the ends of the tube (P1 - P2) and the resistance to flow provided by the tube (R). Resistance is due to frictional forces and is determined by the length of the tube (L), the radius of the tube (r) and the viscosity of the fluid flowing down that tube (V).Tube Flow
Physiology / Basic Cellular / Vessel Fluid Dynamics
Last Updated: 29th July 2024
Flow through a tube is dependent on the pressure differences across the ends of the tube (P1 - P2) and the resistance to flow provided by the tube (R).
Darcy's law states that: Flow = (P1 - P2)/R.
Resistance is due to frictional forces and is determined by the length of the tube (L), the radius of the tube (r) and the viscosity of the fluid flowing down that tube (V).
Poiseuille's law states: R = (8VL)/(πr4).
Combining these equations shows us that flow ∝ (radius)4. Therefore the radius of the tube has the largest effect on resistance and therefore flow; the constriction of an artery by 20% will decrease the blood flow by ~ 60%. This explains why smaller gauge cannulas (with larger diameters) have a faster rate of flow.
Fluids with higher viscosity also have a slower rate of flow. Plasma has a similar viscosity to water, but blood contains cells which effectively increase the viscosity by three- to four-fold. Changes in cell number e.g. polycythemia, therefore affect blood flow.
Turbulent vs Laminar Flow
Frictional forces at the sides of a vessel cause a drag force on the fluid touching them, creating a velocity gradient where the flow is greatest at the centre. This is termed laminar flow which for the most part is the normal physiological flow. A consequence of the velocity gradient is that blood cells tend to move away from the sides of the vessel and accumulate towards the centre, aligning themselves to the flow, which effectively reduces blood viscosity and minimises resistance.
At high velocities, especially in large arteries or where the velocity increases sharply at points of sudden narrowing in the vessels, or across valves, laminar blood flow may become disrupted and flow may become turbulent.
Turbulent blood flow is multidirectional and travels at different velocities leading to increased resistance and additional shear stress on the vessel wall. This may result in damage to endothelium or existing plaques resulting in an increased tendency to thrombus formation. Clinically turbulence may be heard as a murmur or a bruit. Turbulent blood flow may sometimes occur due to elevated cardiac output, even across anatomically normal cardiac valves, resulting in physiological murmurs e.g. in pregnancy.
Turbulent vs Laminar Blood Flow. (Image by Lucho w2ed (Own work) [CC BY-SA 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 |
