Introduction to Medical Sciences & Homeostasis 1-2

Question 1

What is the general organisation of the body systems?

Answer 1

• Organelles (smallest) - which make up cells
• Cells
• Tissues (4 types: epithelial, connective, nervous and muscle) these all share the same characteristics or specialisations and perform a function
• Organs: collections of tissues (usually of different types) that synchronise to perform a more complex function
• Systems: many organs working in synchrony (e.g. Respiratory system)

Question 2

Outline the role of each major system in maintaining health and optimal balance of the equation of life?

Answer 2

Cardiovascular System – delivers O2
and nutrients to cells removes waste from cells
Respiratory System – acquires O2 disposes of CO2
Gastrointestinal System – acquires nutrients, disposes of waste
Renal System – disposes of waste
Musculoskeletal System – sources nutrients, removes us from
danger, protects from threat
Immune System – protects us from infection
Nervous System – coordinates other systems
Endocrine System – coordinates other systems, major role in
allowing cellular access to nutrients that have been absorbed
Reproductive System – ensures continuation of life

Question 3

Define the term homeostasis

Answer 3

Where systems are integrated effectively in order to maintain an
optimum internal environment within the body for all cells to function and ultimately produce energy

Question 4

Explain the importance of maintaining constancy of the internal environment

Answer 4

Common everyday challenges to our internal environment:
1. External temperature
2. Access to nutrients
3. Exercise
These factors impact on body fluid composition, energy stores and body temperature, and physiological mechanisms must act to counteract these potential threats to homeostasis.

Without homeostasis we would die. At a very simple level, being unable to maintain body temperature (a monitored variable) within a narrow range around 37 degrees c disrupts all the biochemical reactions taking place inside our cells, leaving us unable to generate energy to power cellular
function. Similarly, without homeostasis, eating a sugary donut would cause a dramatic rise in
blood glucose which over time leads to irreversible eye, kidney and nerve damage.

Question 5

Describe the principles behind negative feedback control systems

Answer 5

When a condition that is homeostatically regulated (e.g.
body temperature), is sensed to have shifted from the
normal range, a signal (usually nervous or endocrine), is
generated that produces a response (e.g. shivering or
sweating), that corrects the original disturbance and brings
the regulated condition back within the normal range.

Question 6

Explain what is meant by feed forward control.

Answer 6

In Feed-forward control, (more sophisticated form of
negative feedback), additional receptors permit system
to anticipate change and therefore activate response
earlier.

Question 7

Illustrate the concept of homeostasis by outlining daily water balance in man.

Answer 7

Person working outside on a hot, dry day, looses body water through evaportation, body fluids become more concentrated, internal receptors sense change in internal concertation, thirst pathways stimulated, person seeks out and drinks water, water added to body fluids decreases their concentration.

In the previous example, while negative feedback prompts the thirst response, at the same time the kidney detects the increased
body fluid concentration and pre-empts a state of dehydration. It responds by producing smaller
volumes of urine, and a more concentrated form of urine, thus conserving water.

Question 8

Identify the different body fluid compartments.

Answer 8

Split between 3 compartments;
1. Intracellular fluid
2. Interstitial fluid (fluid between cells) (extracellular)
3. Plasma (fluid component of blood) (extracellular)

Question 9

Explain the importance of the nature of the barriers which separate the body
compartments.

Answer 9

The body can survive only as long as the composition of the ECF is
maintained in a state compatible with the survival of its individual
cells i.e. composition of the ECF is very, VERY important.

Question 10

Define the dilution principle.

Answer 10

Is used to measure body volumes and is based off of 3 things
Concentration = mass / volume
1. c=m/v

ergo… v = m/c = dilution principle

2. ONLY plasma can be sampled, ∴only compartments of which plasma is a
   component can be measured directly (plasma, Extracellular volume (ECF), Total body water (TBW)).
3. The NATURE of BARRIERS which separate compartments is crucial in
   determining the test substance.

Question 11

Describe the use of the dilution principle in the measurement of body fluid compartments.

Answer 11

1. Inject a substance that will stay in one compartment only
   (plasma, ECF, TBW)
2. Then calculate the volume of distribution:
   = amount injected (minus any removed by excretion or metabolism),
   divided by the concentration in the sampled fluid.

Question 12

Example using sucrose which is restricted to ECF:
150mg of sucrose injected into plasma of 70kg man,
[sucrose] blood sample after distribution = 0.01mg/ml
10mg were excreted or metabolised.
What is the volume of ECF?

Answer 12

150mg of sucrose injected into plasma of 70kg man,
[sucrose] blood sample after distribution = 0.01mg/ml
10mg were excreted or metabolised.
What is the volume of ECF?
150-10mg=140mg distributed in ECF.
∴volume of distribution = 140mg/0.01mg/ml = 14000mls