Physiology
Cardiovascular
Regarding capillary transport, which of the following statements is INCORRECT:
Answer:
Non-polar lipophilic substances e.g. CO2 and O2 can cross the endothelial lipid bilayer membrane easily. The membrane is however more impermeable to hydrophilic molecules such as glucose and polar molecules and ions. Such substances mainly cross the wall of continuous capillaries through the gaps between endothelial cells, slowed down by tight junctions between cells and by the glycocalyx so that diffusion is much slower than for lipophilic substances. Continuous capillaries with tight junctions are found at the blood-brain barrier. Discontinuous, not fenestrated capillaries, have large gaps between cells allow the transport of proteins and red blood cells.Transcapillary Exchange
Physiology / Cardiovascular / Peripheral Vascular System
Last Updated: 21st April 2019
Capillaries and the smallest venules are formed from a single layer of endothelial cells supported on the outside by a basal lamina containing collagen. The luminal surface is covered by the glycoprotein network called the glycocalyx.
Capillary Permeability
Capillaries throughout the body vary in their permeability based on the size of their pores. There are three basic types:
- Continuous capillaries, found in the skin, lungs, muscles and CNS, are the most selective with low permeability, as junctions between the endothelial cells are very tight, restricting the flow of molecules with MW > 10,000.
- Fenestrated capillaries, found in renal glomeruli, endocrine glands and intestinal villi, are more permeable with less tight junctions, and the endothelial cells are also punctured by pores which allow large amounts of fluids or metabolites to pass.
- Discontinuous capillaries, found in the reticuloendothelial system (bone marrow, liver and spleen), have large gaps between endothelial cells and are permeable to red blood cells.
Types of Capillary. (Image by OpenStax College [Public domain], via Wikimedia Commons)
Transcapillary Exchange
Water, gases and other substances cross the capillary wall mainly by diffusion down their concentration gradients.
Non-polar lipophilic substances e.g. CO2 and O2 can cross the endothelial lipid bilayer membrane easily. The membrane is however more impermeable to hydrophilic molecules such as glucose and polar molecules and ions. Such substances mainly cross the wall of continuous capillaries through the gaps between endothelial cells, slowed down by tight junctions between cells and by the glycocalyx so that diffusion is much slower than for lipophilic substances.
This small pore system also prevents the diffusion of substances greater than 10,000 Da such as plasma proteins. Plasma proteins can cross the capillary wall, but extremely slowly; this may involve large pores through endothelial cells, such as in fenestrated capillaries or large spaces between endothelial cells, such as in discontinuous capillaries.
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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 |
