Internal environment - body temperature, concentration of blood (eg water, glucose), levels of oxygen and carbon dioxide etc.

The point of homeostasis is that the levels fluctuate but they are maintained within a narrow range. This is because of the way negative feedback works.

For negative feedback to work there has to be a movement away from the normal (and within the body, this is a small movement). Negative feedback works in a way that returns the factor back to the normal level.

Receptors - such as those that detect the blood concentration of glucose, carbon dioxide, core body temperature, oxygen for example.

Effectors - these are usually glands or muscles. They make changes such as secrete hormones or constrict blood vessels to return the factor to it's normal level.

Feedback systems - usually involve a control or co-ordination centre, such as the hypothalamus, that receives input from receptors and sends signals (hormones or nerve impulses) to effectors.

nerve impulses travel quickly so the nervous system is usually involved in the most rapid feedback mechanisms.

As the endocrine releases hormones into the blood and transported to the effectors (target organs) the response can be slower than those mechanisms involving the nervous system.

It is important to remember that the nervous system and endocrine system work together in many homeostatic mechanisms.

The brain is the organ with the highest consumption of glucose and the second highest consumption of oxygen. This explains the need for regulation of blood glucose concentration , breathing rate and heart rate.

If you observe erythrocytes under a microscope and add pure water, you will see the cells swell and burst as water enters by osmosis. Similarly if you add a drop of salt solution, you will see the cells shrink and become crinkled as water leaves by osmosis.

To function properly our body cells need to be bathed in fluid (tissue fluid) that maintain the constant concentration of water. This explains the need for osmoregulation.

Bacteria that live in hot springs can survive perfectly well at temperatures close to the boiling of water. Their enzymes are remarkably heat stable when compares to our own (that start to denature at about 40 degrees Celsius). This is because human enzymes have evolved alongside the development of a body temperature around 37 degrees. Any significant variation above or below this temperature will affect the rate of enzyme controlled reactions. Too low and the reactions are too slow; too high and the enzymes lose activity and start to denature.

This explains the need for thermoregulation.

The regulation of pH of the blood is also an important homeostatic mechanism as enzymes need a constant pH to function effectively.

Carbon dioxide is an acidic gas. The concentration of carbon dioxide in the blood is regulated via gas exchange in the lungs and regulation of heart rate.

Blood pressure (BP) in children is lower than in adults.

BP becomes a normal adult range of 120/180 mmHg by the age of 17-18 years.

BP tends to increase with age as artery walls become less elastic.

In children there are slight differences between boys and girls - boys have slightly higher BP.

Height of the child more than actual age may determine BP.

Normal resting heart rate (HR) for children is higher than adults. Normal resting heart rate for babies up to 3 months old is 100-180 beats per minute (bpm).

Resting HR in adults remains fairly constant as we age. Other factors affect the HR rather than age,

Respiratory rate (RR) decreases with age, as does lung function. The decrease in lung function is partly due to muscle mass loss as we age.

Core body temperature decreases with age. Blood vessels supplying the skin are less able to constrict and therefore the body is not able to restrict as much loss of heat. Shivering reflex declines as muscle mass declines.

Weight

Excess weight (a higher BMI - Body MAss Index) can increase blood pressure above the typical range for a person's age. This can also mean an increased HR as the heart has to work harder against the higher pressure.

Fat distribution is important. Subcutaneous fat lies in the lower level of the dermis. Visceral fat surrounds the internal organs such as heart and liver is a bigger health risk and can lead to increased BP which in turn leads to increased risk of cardiovascular disease and type 2 diabetes.

Being underweight can reduce BP.

Exercise

If you do regular weight training your muscles become bigger and stronger.

If you regularly undertake cardiovascular (cardio) exercise such as running or cycling then the heart will become larger and stronger as it needs to provide the body with oxygen and nutrients when doing strenuous exercise.

This means the heart will pump more blood with each stroke (stroke volume increases). It will not need to beat as fast when at rest. Therefore the resting heart rate of a well-trained athlete will ne lower than an average adult (possibly down to 40 bpm).

Sex

It is only recently that sex-based differences in normal physiology have been fully recognised.

There are differences between body mass and body composition in males and females.

Sex hormones have an effect on development of males and females but there are less obvious differences also.

Males have larger lungs than females (even when adjusted for height differences). Therefore maximum exercise capacity in females may be limited by lung capacity, especially with aging.

Males have larger left ventricles which makes the stroke volume greater in males.

During pregnancy the functional residual capacity (FRC) of the lungs can fall.

FRC is the amount of air left in the lungs after a normal passive exhalation. This is due to the increased pressure on the diaphragm by the increase uterus size. The FRC reduced as the pregnancy advances. Other changes in the respiratory system happen during pregnancy, partly in response to the increased level of progesterone. The efficiency of the gas exchange system is maintained to ensure the foetus and mother are receiving sufficient oxygen.

Pre-menopausal women tend to have a lower resting BP and higher resting HR compared to males of the same age.

After the menopause BP of women rises to the equivalent of males of the same age.

Overall health

Overall state of health of an individual will contribute to whether their physiological measurements lie within or outside of normal parameters.