1.0 Introduction to Physiology and Homeostasis

Question 1

Define the term homeostasis.

Answer 1

An organism’s ability to keep its internal environment relatively stable. All cells are in contact with this watery internal environment which makes life-sustaining exchanges possible.

Variables that are under homeostatic control are:
= osmolarity, temperature and pH
nutrients, water, sodium, calcium (and other inorganic ions), oxygen
hormones, other cell communication molecules

Question 2

Describe the body fluid compartments

Answer 2

Intracellular fluid (ICF) - the fluid collectively contained within all of the cells of the body

Extracellular fluid (ECF) - the fluid outside of the cells in which all cells are bathed in and is the internal environment

The ICF and ECF are separated by the semi-permeable plasma membrane

The external environment is outside the body.

Question 3

Define the general concepts of feedback regulation, set-point and steady state.

Answer 3

Feedback regulation: A process by which the product of a metabolic pathway influences its own production by controlling the amount and/or activity of one or more enzymes involved in the pathway.

Set point: The set point in homeostasis is the balanced physiological value for a variable.

Steady state: steady state refers to the maintenance of constant internal concentrations of molecules and ions in the cells and organs of living systems.

Question 4

Explain water and osmotic equilibrium, osmolarity, osmolality and tonicitiy

Answer 4

Osmolarity:
- total number of dissolved osmotically active particles (ions or intact molecules) in a solution. For example, since NaCl dissociates in solution into two ions, Na+ and Cl- ions, a solution of 150 mmol/L of NaCl will have twice the osmolarity of a solution of 150 mmol/L of glucose.
- expressed in osmoles per litre (osmol/L).
- the normal osmolarity of the human body is 300 mosmol/L.

Osmolality:
- concentration expressed as osmoles of solute per kilogram of solvent (osmol/kg).
- because biological systems are quite dilute, this term in used interchangeably with osmolarity.

Water and osmotic equilibrium
isosmotic - 2 solutions that contain the same number of solute particles

hyposmotic - fewer solute particles in a solution than another solution

hyperosmotic - more solute particles in solution than another solution

Tonicity:
Tonicity predicts the movement of water into and out of cells by osmosis. It describes a solution and how that solution would affect cell volume if the cell was placed in the solution and allowed to come to equilibrium.

There are 3 types of tonic solutions
1. if a cell is placed in the solution and gains water at equilibrium and swells, the solution is hypotonic to the cell

2. if the cell loses water and shrinks at equilibrium, the solution is said to be hypertonic
3. if the cell is in the solution and does not change size at equilibrium, the solution is isotonic

Question 5

Explain chemical gradients of polar molecules

Answer 5

Gradients are responsible for the movement of various substances throughout the body, including the movement of blood through the circulatory system, air into lungs, and ions through membranes of cells

Question 6

Explain what is meant by osmotic pressure

Answer 6

Osmotic pressure is the hydrostatic pressure required to raise water activity sufficiently to prevent net water movement across a semipermeable membrane (osmosis). Thus the magnitude of the osmotic pressure is equal to the opposing hydrostatic pressure necessary to completely stop osmosis.

Question 7

Discuss how plasma membrane maintains discrete extracellular fluid and intracellular fluid composition and cell volume

Answer 7

Dynamic steady state is not the same as equilibrium where the composition of each compartment is kept identical.

ICF and ECF have different compositions of ions, for example (see figure below). These differences in concentrations between the two fluid compartments mean that ECF and ICF are in a state of disequilibrium. This is achieved by selectively permeable barriers and/or expenditure of energy. An example of this is osmosis where the hydrostatic pressure difference to just balance the osmotic water flow exemplifies two opposing forces balancing out and resulting in no net movement (i.e. achieving homeostasis).

Question 8

3 - step simple homeostatic control mechanism

Answer 8

To maintain homeostasis, the simplest control systems have

1. an input signal that detects deviations from normal via a sensor in any given regulated variable (e.g. blood glucose or blood pressure) to within pre-determined narrow limits.
2. an integrating centre which integrates the incoming information with any other relevant information, and making adjustments to initiate an appropriate response.
3. an output signal that adjusts the response of the contributing body components, restoring the variable to the optimal (normal) value, i.e. its set point.

Question 9

Describe negative feedback loop

Answer 9

• Involves a pathway in which the response opposes, or removes, the signal
• stabilises a function and maintains it within the normal range
• cannot prevent the initial disturbance
• seeks to remove the variable change by moving the factor in the opposite direction to the initial change
• ceases in the absence of input, i.e. loss of a stimulus for the pathway shuts off the response loop
• is the main operating principle underlying homeostatic control mechanisms

Question 10

Describe positive feedback loop

Answer 10

• the response is continually enhanced or amplified so that the control value is continually moved in the direction of the initial change
• the response reinforces the stimulus (as opposed to decreasing or removing it)
• requires an intervention outside the loop to stop it
• does not occur very often in the body

Question 11

Define bulk flow

Answer 11

Both liquids and gases are considered fluids in physiology because both flow.

Bulk flow
- is the most general form of fluid transport
- fluids (with both their dissolved and suspended materials) are moved via a pressure gradient
- fluids flow from regions of higher pressure to regions of lower pressure
- movement of a substance across the membrane depends on the permeability of the membrane

Question 12

6 step negative feedback loop of temperature control

Answer 12

1. Fall in body temperature below set point
2. temperature monitoring nerve cells
3. stimulates temperature control centre
   4.stimulates skeletal muscles (and other effectors)
4. increase heat production through shivering and other means
5. increase body temperature to set point

Question 13

Mindmap drawing of temperature decrease

Answer 13

see orientation folder or OFC

Question 14

Mindmap drawing of temperature increase

Answer 14

see orientation folder or OFC