Physiology
Cardiovascular
Question 110 of 180
Autoregulation is particularly important in the following tissues:
Answer:
Autoregulation is the ability to maintain a constant blood flow despite variations in blood pressure (between 50 - 170 mmHg). It is particularly important in the brain, kidney and heart. There are two main methods contributing to autoregulation.- The myogenic mechanism involves arterial constriction in response to stretching of the vessel wall, probably due to activation of smooth muscle stretch-activated Ca2+ channels and Ca2+ entry. A reduction in pressure and stretch closes these channels, causing vasodilation.
- The second mechanism of autoregulation is due to locally produced vasodilating factors; an increase in blood flow dilutes these factors causing vasoconstriction, whereas decreased blood flow has the opposite effect.
Local Control of Blood Flow
Physiology / Cardiovascular / Peripheral Vascular System
Last Updated: 21st April 2019
In addition to central control of blood pressure, tissues can regulate their own blood flow to match their requirements via autoregulation, metabolic factors and local hormones (autocoids).
Autoregulation
Autoregulation is the ability to maintain a constant blood flow despite variations in blood pressure (between 50 - 170 mmHg). It is particularly important in the brain, kidney and heart. There are two main methods contributing to autoregulation.
- The myogenic mechanism involves arterial constriction in response to stretching of the vessel wall, probably due to activation of smooth muscle stretch-activated Ca2+ channels and Ca2+ entry. A reduction in pressure and stretch closes these channels, causing vasodilatation.
- The second mechanism of autoregulation is due to locally produced vasodilating factors; an increase in blood flow dilutes these factors causing vasoconstriction, whereas decreased blood flow has the opposite effect.
Metabolic Factors
Many factors may contribute to metabolic hyperaemia (increased blood flow), with the most important being K+, CO2 and adenosine, and in some cases hypoxia itself.
- K+, released from active tissues and in ischaemia, causes vasodilation partly by stimulating the Na+ pump, thus increasing Ca2+ removal from smooth muscle cells and hyperpolarising the cell.
- CO2 and acidosis cause vasodilation largely through increased nitric oxide production and inhibition of smooth muscle Ca2+ entry.
- Adenosine, released from the heart, skeletal muscle and brain during increased metabolism and hypoxia, causes vasodilation by stimulating the production of cAMP in smooth muscle.
- Hypoxia may reduce ATP sufficiently for K+ channels to activate causing hyperpolarisation.
Autocoids
Autocoids are mostly important under certain circumstances, for example:
- In inflammation, local inflammatory mediators such as histamine and bradykinin cause vasodilation and increased permeability of exchange vessels, leading to swelling but allowing access by immune cells to damaged tissues.
- In clotting, serotonin and thromboxane A2 released from activated platelets cause vasoconstriction to help reduce bleeding.
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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 |
