Homeostasis (Edexcel International A Level Biology)

• In order to function properly and efficiently organisms have different control systems that ensure their internal conditions are kept relatively constant
• Physiological control systems maintain the internal environment within restricted limits through a process known as homeostasis
• This keeps the internal environment of the body fluctuating around a specific normal level
  • This is known as a state of dynamic equilibrium
• Sensory cells known as receptors can detect information about the conditions inside and outside the body

The importance of homeostasis

Temperature

• Homeostasis is critically important for organisms as it ensures the maintenance of optimal conditions for enzyme action and cell function
  • For example, an increase in body temperature above 40 °C would cause enzymes to denature
    • This is due to an increase in kinetic energy which would result in the breakage of hydrogen bonds holding the enzyme in a specific 3D shape
    • The active site will change shape and will no longer be complementary to the substrate molecule
    • An enzyme-substrate complex cannot form and the enzyme cannot catalyse that reaction anymore, leading to less efficient metabolic reactions

Blood glucose

• Cells also need a constant supply of energy in the form of ATP to work efficiently
• Glucose is respired to supply this ATP, meaning that the body needs to carefully monitor and control blood glucose concentrations
  • Cells in the pancreas monitor blood glucose concentrations

Water

• Water is another essential requirement for cells to function optimally; it makes up the cell cytoplasm and it takes part in metabolic reactions
• It is therefore crucial for the amount of water in the blood to remain constant
  • Water is lost during excretion of waste products, e.g. urine, and in sweat
  • The kidneys are responsible for regulating the amount of water in the blood

Control mechanisms for maintaining body temperature

• Maintenance of a constant internal body temperature is known as thermoregulation
• This process involves both cooling and warming mechanisms depending on whether there is an increase or decrease in body temperature

Cooling mechanisms

• Vasodilation of the blood vessels that supply skin capillaries
  • Heat exchange during both warming and cooling occurs at the body's surface as this is where the blood comes into close proximity to the environment
    • The warmer the environment, the less heat is lost from the blood at the body's surface
  • One way to increase heat loss is to supply the capillaries in the skin with a greater volume of blood, which then loses heat to the environment via radiation
    • Arterioles have muscles in their walls that can relax or contract to allow more or less blood to flow through them
    • During vasodilation these muscles relax, causing the arterioles near the skin to dilate and allowing more blood to flow through skin capillaries
    • This is why pale-skinned people go red when they are hot
• Sweating
  • Sweat is secreted by sweat glands
  • This cools the skin by evaporation; heat energy from the body converts liquid water into water vapour
  • Sweating is less effective as a cooling mechanism in humid environments; sweat evaporates more slowly due to a reduced concentration gradient between the sweat and the surrounding air
• Flattening of hairs
  • The hair erector pili muscles in the skin relax, causing hairs to lie flat
    • These muscles can be described as effectors, as they respond to a change in body temperature
  • This stops them from forming an insulating layer of trapped air and allows air to circulate over skin; heat can therefore leave by radiation

Changes in the skin help to increase heat loss when body temperature rises

Warming mechanisms

• Vasoconstriction of blood vessels that supply skin capillaries
  • One way to decrease heat loss is to supply the capillaries in the skin with a smaller volume of blood, minimising the loss of heat to the environment by radiation
  • During vasoconstriction the muscles in the arteriole walls contract, causing the arterioles near the skin to constrict and allowing less blood to flow through skin capillaries
  • Instead, the blood is diverted through shunt vessels, which are deeper in the skin and therefore do not lose heat to the environment
  • Vasoconstriction is not, strictly speaking, a 'warming' mechanism as it does not raise the temperature of the blood but instead reduces heat loss from the blood as it flows through the skin
• Boosting metabolic rate
  • Most of the metabolic reactions in the body are exothermic and this provides warmth to the body
  • In cold environments the hormone thyroxine, released from the thyroid gland, increases the basal metabolic rate (BMR), increasing heat production in the body
  • Adrenaline may also be released to speed up the metabolic rate and release more heat
• Shivering
  • This is a reflex action in response to a decrease in core body temperature
    • This means it is a nervous mechanism, not a hormonal one
  • In this case muscles are the effectors and they contract in a rapid and regular manner
  • The metabolic reactions required to power this shivering generate sufficient heat to warm the blood and raise the core body temperature
• Erection of hairs
  • The hair erector pili muscles in the skin contract, causing hairs to stand on end
  • This forms an insulating layer over the skin's surface by trapping air between the hairs and stops heat from being lost by radiation
  • Note that, like vasoconstriction, this is a heat retention mechanism rather than a warming mechanism
• Less sweating
  • The sweat glands will secrete less sweat when it is cold
  • This will reduce the amount of heat lost through the evaporation of sweat
  • This is a heat retention mechanism rather than a warming mechanism

Changes in the skin reduce heat loss when the body cools

The role of the hypothalamus in thermoregulation

• The hypothalamus is an area of the brain that is responsible for controlling many functions in the body, including
  • Hormones
  • Sleep
  • Growth
  • Body temperature
  • Blood pressure
• Mammals detect external temperatures via thermoreceptors found in the skin and mucous membranes
  • There are receptors for both heat and cold
  • These communicate with the hypothalamus along sensory neurons
  • The hypothalamus will send impulses along motor neurons to effectors to bring about a physiological response to changing external temperatures
• The hypothalamus also helps to regulate body temperature by monitoring the temperature of the blood flowing through it and initiating homeostatic responses when it gets too high or too low

The regulation of body temperature involves communication between thermoreceptors, the hypothalamus and effectors to respond to change