The process of keeping the internal environment stable.

Homeostatic mechanisms are used to keep the internal environment stable.

Homeostasis is a dynamic equilibrium.

Homeostasis is regulated using control systems. These control systems need a method of communication between components. The main methods of communication in homeostasis is through the nervous system and the endocrine system, which involves hormones.

  • Paracrine control : when hormones act locally and don’t travel through the blood to act
  • Autocrine control: A variety of chemicals released by a cell which acts on the releasing cell (i.e. on itself)
  • Endocrine control: when hormones act on different areas from the releasing cell, here they need to travel through the blood to reach the cells they target.

Things that are controlled through homeostasis include:

  • Supply of nutrients
  • Supply of Oxygen
  • Blood Flow
  • Body temperature
  • Removal of Waste
  • Removal of CO2
  • pH

Homeostasis examples at different levels:

Intracellular – the control of [Ca2+]

  • Normally maintained at 10^-7 M
  • An increase in Ca2+ is actually an activation signal
  • Actions – Muscle contraction, release of neurotransmitters, phosphorylation of enzymes
  • Action: local control of blood flow autoregulation (for example skeletal muscle). Whole body: thermoregulation

Characteristics of control systems:

  1. Communication: this can be through the nervous system, endocrine hormones, paracrine hormones and autocrine hormones
  2. Control Centre : determines a set point. It analyses an input and determines a response
  3. Receptor : stimuli – sensor – afferent pathway signal – control centre
  4. Effector : control centre – effector efferent pathway


When the temperature is above 37.2C:

  • Temperature receptors in the skin and hypothalamus detect the change
  • This sends a signal through the afferent neurones
  • This triggers the thermoregulatory centre – hypothalamus
  • Signal is sent through the afferent nerves
  • Causes the dilation of blood vessels in the skin, cauisng increased sweating
  • As a result this reduces core body temperature

When the temperature is below 36.7C:

  • Temperature receptors in the skin and hypothalamus detect a change
  • Sends a signal through afferent neurons
  • Thermoregulatory centre is triggered – this is the hypothalamus
  • Efferent nerves carry the signal for a response
  • Constriction of blood vessels cause decreased sweating, increased metabolism and shivering.
  • This raises core temperature

Biological Rhythms:

The body sets biological rhythms to follow. These rhythms have different set points, such as for example the circadian rhythm and the diurnal rhythm.

An example of a hormone that is controlled by a biological clock is cortisol, which peaks at 7am and reaches its lowest at 7pm. Other substances controlled by the biological clock include the menstrual cycle and melatonin, which is a hormone involved in the light/dark cycle.

The biological clock is controlled by the suprachiasmatic nucleus of the hypothalamus

Positive Feedback:

This is when the stimulus produces a response that increases the effect.

In rare cases the system goes out of control leading to a catastrophic change in state. The changes are very fast however rare.

Examples: Blood Clotting and Ovulation.

Negative Feedback:

This is where the effector opposes the stimulus. This occurs in most homeostatic control systems. An example of a hormone which is controlled by negative feedback is thermoregulation.

Clinical applications of homeostasis:

  1. Pyrexia

This is a normal response to infection. The core temperature of the body is set to a higher point as prostaglandin acts on the thermoregulatory centre.

Antipyretic drugs such as paracetamol inhibits cyclooxygenase 2 (COX-2) that is in charge of producing prostaglandin PGE2.

  1. Hyperthermia

This is when the core temperature of the body is set to above 38C.

Sometimes some people develop a condition also known as malignant hyperthermia, which comes as a rare reaction to some anaesthetic agents

Artificially induced hyperthermia is also common, which is a therapeutic action used to treat some forms of cancers.

  1. Hypothermia

This is when the core temperature of the body is set to below 35C

Artificially induced hypothermia is used as a treatment for stroke patients as well as preterm infants and patients with severe head injury.

Cooling is sometimes used in brain and cardiac surgery to reduce tissue damage.

Body water homeostasis

Usually men have 50-60% of water in their bodies and women have 40-50% of water in their bodies.

Average lean male 70kg, who would have 42L of water in their bodies.


  • Intracellular fluid content = 28L
  • Extracellular fluid content = 9.4L
  • Blood plasma = 4.6L

Fluid content is maintained through osmolality and Na+ concentration. This is monitored by osmoreceptors of the hypothalamus

When osmolality increases:

ADH is released from the posterior pituitary gland.

This causes water reabsorption in the collecting ducts of the kidneys.

As a result this decreases osmolality.

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