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Physiology

Cardiovascular

Question 62 of 180

Continuous capillaries are typically found where in the body:

Answer:

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.

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.

Via Wikimedia Commons.

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. COand 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

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