Homeostasis 1 Objectives.

Define and apply these definitions to determine the roll of a process in a negative feedback system.

Controlled variable- Physiological parameter being controlled (ie. blood pressure). Sensor- Receptor that detects changes in controlled variable (ie. baroreceptors). Comparator- Integration center of feedback response (ie. cardiovascular control center of brain), which analyzes data & compares it to a set point. Sends out signal if different from set point. Set point- Influenced by inheritance and adaptations to environment. If data is different from set point, comparator sends out error signal. Effectors- Nerve pathways and cells/tissues that carry out response to restore homeostasis (ie. sympathetic nervous system causes constriction of blood vessels in response to lowered blood pressure).

Define equilibrium and steady state.

Homeostasis is not about reaching equilibrium; it's about maintaining a steady state. Steady state- Maintenance of a state that doesn't change with time. Amount/concentration of substance in compartment is constant, and there is no net gain or loss of a substance in a compartment. (ie. [Na+] much higher extracellularly → moves down concentration & electrical gradients into cell. But to maintain steady state, Na+ is pumped out of the cell.) Energy expenditure may be necessary to maintain this state Equilibrium- Also no change in state over time, but it is energetically favorable (low energy). Opposing forces are balanced and there is no net transfer of substance and energy between compartments.

List the components of, and the relationship between, the three major body fluid compartments.

Major constituent of body fluid is within cells (intracellular). Substances in plasma must pass through interstitial fluid before entering cell. The compartments must interact to maintain homeostasis. Osmolarity is equal in all compartments, plasma & interstitial fluid have similar ion concentrations but there is NO protein in interstitial fluid. There is less Na+ & Cl-, but more K+ intracellularly.

Define and explain redundancy and hierarchy with respect to homeostatic control systems.

More vital parameters require more homeostatic mechanisms to control them (incase one should fail →backup system). This is a redundancy of control systems. A complex hierarchy among various control systems is set up. The set points of some variables are altered in order to maintain others. ie. During maximal exercise, requirement to maintain BP overwhelms that to maintain body temperature.

Explain the concept of negative feedback.

Negative feedback is a mechanism to bring something back to normal when there is a perturbation from the set point (whether it has increased OR decreased). It rarely overcompensates and goes beyond the set point. Example: ↑ blood glucose → β cell stimulated → insulin secreted → glucose (in blood) taken into cell → ↓ blood glucose level until back to normal

Discuss factors that may produce a deviation in set points.

Set points can vary from person to person OR within an individual. They can deviate: Over a 24 hour cycle (circadian rhythm)- ie. You ingest more during the day → K+ excretion is greater during the day. Due to environmental changes- Acclimatization (ie. Set point for PO2 levels changes when living at high altitudes.) Due to pathological changes- ie. Plaques form & raise blood pressure, which resets the BP set point. Due to protective response- ie. Infections cause a decrease in plasma iron concentration to deprive bacteria of iron required to replicate. Set point is decreased until bacteria are no longer present.

Give examples of processes controlled by homeostatic mechanisms in the body.

Temperature, pH, O2 & CO2 levels, and blood pressure.

Explain the concept of feed-forward regulation.

The body anticipates a change of a controlled variable (ie. body temp) before it happens and prepares the body for it (ie. when moving from hot to cold, body stimulates heat production and conservation to prevent internal body temp. from falling even though it hasn't yet begun to fall). Another example- Ring bell for dog to eat & it starts to salivate.

Explain the concept of positive feedback.

The comparator potentiates the error signal and the controlled variable moves further away, rather than bring it back to normal like in negative feedback. ie. childbirth & blood clotting

Discuss the relationship between external and internal environments.

The internal environment contains extracellular fluid which surrounds cells and fluid in the blood. The external environment cannot be controlled by us, and it surrounds the body. In our tissues, the intracellular fluid (majority of fluid in body) communicates with extracellular fluid that is bathing the cells.

What is Homeostasis?

maintenance of steady state within the body environment The cell's ideal functions are: 1. Maintain functional integrity. 2. Act with other cells of its type to perform function of associated organ. 3. Act to contribute to survival of whole organism.