Homeostasis

• Homeostasis is the maintenance of equilibrium, or constant conditions, in a biological system by means of automatic mechanisms.

• In the 19th century, the French physiologist Claude BERNARD noted the constancy of the chemical composition and physical properties of blood and other body fluids.

• The term homeostasis was coined by the 20th-century American physiologist Walter B. Cannon, who refined and extended the concept of self-regulating mechanisms in living systems.

Homeostasis of Body Fluids• Fluid Compartments:-ICF: intracellular fluid = within the cell-ECF: extracellular fluid = outside the cells

- aka: interstitial fluidComposition of fluids change as substances

move between compartments: nutrients, oxygen, ions and wastes move in both directions

Neural and endocrine control

• Homeostasis is the process of maintaining a controlled condition

- Sensory receptors detect change in a monitored variable ( CO2 levels, glucose levels, water levels, temperature)

-nervous system and/or endocrine system responds

Feedback Loop• All homeostatic mechanisms use

negative feedback to maintain a constant value (called the set point).

• Negative feedback means that whenever a change occurs in a system, the change automatically causes a corrective mechanism to start, which reverses the original change and brings the system back to normal.

• In a system controlled by negative feedback, the level is never maintained perfectly, but constantly fluctuates about the set point.

• An efficient homeostatic system minimizes the size of the fluctuations.

• Negative feedback applies to electronic circuits and central heating systems as well as to biological systems.

                                                                                                                                                                   

Thermoregulation• One of the most important examples of

homeostasis is the regulation of body temperature.

• Animals that maintain a fairly constant body temperature (birds and mammals) are called homeotherms

• Homeotherms maintain their body temperatures at around 37°C. Why?

• Animals that have a variable body temperature (all others) are called poikilotherms.

-In humans, temperature homeostasis is controlled by the thermoregulatory centre in the hypothalamus.

-The thermoregulatory centre receives input from two sets of thermoreceptors:

a. receptors in the hypothalamus itself monitor the temperature of the blood as it passes through the brain (the core temperature)

b. receptors in the skin monitor the external temperature.

-The thermoregulatory centre sends impulses to several different effectors to adjust body temperature

Regulation of Blood Glucose

                                                                                                                                                                  

• The body requires glucose in order to create ATP. The amount of ATP required will fluctuate, and therefore the body regulates the availability of glucose.

• Two hormones are responsible for controlling the concentration of glucose in the blood: Insulin and Glucagon

Water RegulationOsmoregulation

• Osmoregulation is the regulation of water concentrations in the bloodstream, effectively controlling the amount of water available for cells to absorb.

                                                                                                                                                                       

Osmoregulation: water balance

Mechanism of Osmoregualtion

• Osmoreceptors that are capable of detecting water concentration are situated on the hypothalamus next to the circulatory system.

• The hypothalamus sends chemical messages to the pituitary gland next to it.

• The pituitary gland secretes anti-diuretic hormone (ADH), which targets the kidney that are responsible for maintaining water levels.

• When the hormone reaches its target tissue, it alters the tubules of the kidney to become more / less permeable to water

• If more water is required in the blood stream, high concentrations of ADH make the tubules more permeable.

• If less water is required in the blood stream, low concentrations of ADH make the tubules less permeable.