Physiology
Cardiovascular
Regarding the microcirculation, which of the following statements is INCORRECT:
Answer:
Across capillary walls, unlike proteins, most ions and small molecules diffuse easily and thus the crystalloid osmotic pressure they exert is roughly the same on either side of the capillary wall; for this reason, the osmotic force across the capillary wall is largely determined by protein concentration in the blood. Plasma protein concentration is normally much higher than interstitial protein concentration because very little protein is filtered; plasma colloid osmotic pressure is therefore higher than interstitial colloid osmotic pressure and tends to draw fluid intravascularly. Capillary hydrostatic pressure normally varies from about 35 mmHg at the arteriolar end to about 15 mmHg at the venous end, whereas the interstitial hydrostatic pressure is normally close to 0 mmHg (or is slightly negative). The greater hydrostatic pressure inside the capillary tends to drive filtration of water out of the capillary into the tissues. Normally overall the hydrostatic pressure along the length of the capillary is greater than plasma oncotic pressure and thus there is a small net filtration of fluid from the capillary into the interstitial space; of about 4000 L of plasma entering the capillaries daily as the blood recirculates, a net filtration of 8 L occurs. Although arteriolar constriction will reduce capillary hydrostatic pressure and therefore lead to the reabsorption of fluid, this will normally be transient due to the concentration of interstitial fluid, i.e. the increased interstitial oncotic pressure.Capillary Filtration
Physiology / Cardiovascular / Peripheral Vascular System
Last Updated: 22nd November 2022
The capillary wall is very permeable to water. Water tends to flow from a low to a high osmotic pressure, but from a high to a low hydrostatic pressure. The net flow of water across the capillary wall is therefore determined by the balance between the hydrostatic pressure which tends to drive water out of the capillaries and the oncotic pressure which tends to draw water into the capillaries from the interstitial space.
Microcirculation. (Image by Kes47 [Public domain], via Wikimedia Commons)
Starling's Equation
Starling's equation tells us that the net flow of water across the capillary wall is proportional to (Pc - Pi) - (πp - πi), where (Pc - Pi) is the difference in hydrostatic pressure between the capillary and interstitial space and (πp - πi) is the difference in osmotic pressure between plasma and interstitial fluid. A positive value means there is a net fluid movement out of the capillary (filtration), a negative value means there is a net fluid movement into the capillary (absorption).
Oncotic Pressure
Across capillary walls, unlike proteins, most ions and small molecules diffuse easily and thus the crystalloid osmotic pressure they exert is roughly the same on either side of the capillary wall; for this reason, the osmotic force across the capillary wall is largely determined by protein concentration in the blood. Plasma protein concentration is normally much higher than interstitial protein concentration because very little protein is filtered; plasma colloid osmotic pressure is therefore higher than interstitial colloid osmotic pressure and tends to draw fluid intravascularly.
Hydrostatic Pressure
Capillary hydrostatic pressure normally varies from about 35 mmHg at the arteriolar end to about 15 mmHg at the venous end, whereas the interstitial hydrostatic pressure is normally close to 0 mmHg (or is slightly negative). The greater hydrostatic pressure inside the capillary tends to drive filtration of water out of the capillary into the tissues.
Net Filtration
Normally overall the hydrostatic pressure along the length of the capillary is greater than plasma oncotic pressure and thus there is a small net filtration of fluid from the capillary into the interstitial space; of about 4000 L of plasma entering the capillaries daily as the blood recirculates, a net filtration of 8 L occurs. Although arteriolar constriction will reduce capillary hydrostatic pressure and therefore lead to the reabsorption of fluid, this will normally be transient due to the concentration of interstitial fluid, i.e. the increased interstitial oncotic pressure.
A reduction in plasma protein (e.g. starvation), or a loss of endothelium integrity with diffusion of protein into the interstitial space (e.g. inflammation, ischaemia), will reduce (πp - πi), leading to enhanced filtration and loss of fluid into the tissues. Increased filtration is also caused by high venous pressures.
Capillary Exchange. (Image by OpenStax College [CC BY 3.0 (https://creativecommons.org/licenses/by/3.0)])
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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 |
