Physiology
Basic Cellular
A negative feedback mechanism comprises which of the following:
Answer:
A negative feedback system comprises three main components:- A detector (often neural receptor cells) to measure the variable in question and to provide input to the comparator
- A comparator (usually a neural assembly in the central nervous system) to receive input from the detector, to compare the variable against the variable set point and to determine the need for a response
- An effector (usually muscular or glandular tissue) that is activated by the comparator to enact the appropriate response to restore the variable to its set point
Homeostasis
Physiology / Basic Cellular / Homeostasis
Last Updated: 29th July 2024
Normal functioning of proteins is essential for life. Seemingly small changes in the external environment (particularly in temperature and pH) can irreversibly denature proteins that are essential for normal physiological function. As long as conditions are maintained within the normal physiological range within the internal environment, the cells of the body continue to live and function properly.
Definition
Homeostasis is defined as 'the property of a system in which variables are regulated so that internal conditions remain stable and relatively constant'. Homeostasis preserves protein functionality and maintains most physiological systems in the body.
Set Point
The 'set point' is a narrow range of values within which normal physiological function occurs. The set point can under certain circumstances be reset to meet physiological requirements e.g. acclimatisation at high altitude.
Negative Feedback
The most common type of regulation is by negative feedback e.g. control of body temperature, acid-base balance and blood pressure.
A negative feedback system comprises three components:
- A detector (often neural receptor cells) to measure the variable in question and to provide input to the comparator
- A comparator (usually a neural assembly in the central nervous system) to receive input from the detector, to compare the variable against the set point and to determine the need for a response
- An effector (usually muscular or glandular tissue) that is activated by the comparator to enact the appropriate response to restore the variable to its set point
The term 'negative feedback' refers to the fact that effectors always act to move the variable in the opposite direction to the change that was originally detected.
Due to the inherent time delay between detecting a change in a variable and effecting a response, negative feedback mechanisms cause oscillations in the variable they control. This delay means that feedback control always causes the variable to overshoot the set point slightly activating the opposite restorative mechanism to induce a smaller overshoot in that direction, until the oscillations fall within the range of values that are optimal for physiological function.
Negative Feedback Loop. (Image modified by FRCEM Success. Original by OpenStax [CC BY 4.0 , via Wikimedia Commons)
Positive Feedback
Some physiological systems use positive feedback mechanisms e.g. hormonal control of childbirth (where pressure on the cervix causes increased release of oxytocin increasing uterine contraction) or initiation of an action potential (where a sodium influx causes depolarisation which causes further sodium channel opening).
Positive feedback systems are less common in the body due to their inherent instability and risk of uncontrolled amplification. Positive feedback mechanisms require a mechanism to break the feedback loop ( such as by birth of the child in the first example above and by inactivation of sodium channels in the second).
Positive Feedback Loop. (Image by OpenStax [CC BY 4.0 , 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 |
