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foundations of medicine 1 > homeostasis > Flashcards

homeostasis Flashcards

1
Q

how is homeostasis defined by its latin roots

A

“similar condition” ( not identical condition!)

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2
Q

what do our bodies constantly monitor

A

the internal state and respond to any threat that might disturb the “similar internal conditions” of the body

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3
Q

why are our bodies constantly monitoring their internal state

A

in an effort to prevent disturbance and maintain optimum “similar conditions” or, in other words, a relatively constant internal environment in which all processes work optimally

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4
Q

what are examples of substantial changes in our internal environment that our bodies are not very tolerant of

A

temperature
pH of bodily fluids
concentration of hormones

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5
Q

what does failure to adequately correct imbalances in the internal environment of the body lead result in

A

illness and disease, or pathology (pathos - meaning suffering)

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6
Q

give an example of failure to maintain a relatively constant environment

A

diabetes mellitus occurs when the body can no longer maintain it’s optimal blood glucose concentration

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7
Q

what does homeostasis require

A

integration of organ systems

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8
Q

how is an increased energy demand an example of integration of organ systems

A

Nervous, endocrine and Musculoskeletal system to seek, supply and access nutrients, Respiratory system to supply O2 , Alimentary system to break down food into useable forms and absorb across gut wall into bloodstream, Cardiovascular system to transport O2 and nutrients via the bloodstream to the cells and transport CO2 and waste from the cells to the Respiratory, Alimentary and Renal systems for disposal.
Nervous and Endocrine systems co-ordinate and control all these systems.

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9
Q

why must all the systems integrate effectively

A

in order to maintain an optimum internal environment within the body for all cells to function and ultimately produce energy

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10
Q

what is homeostasis

A

maintaining an optimum internal environment within the body

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11
Q

what is the basic concept of homeostasis and how to maintain a constant level

A

the body usually has a range within which it can tolerate change and so the output must be equal to the input. what we gain, we must lose. what we lose, we must replace.

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12
Q

what types of changes result in loss of homeostasis

A

external and internal changes

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13
Q

what could happen when the organism attempts to compensate as a result of a loss of homeostasis

A

compensation fails - leading to illness or disease

compensation succeeds - leading to wellness

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14
Q

what are common everyday challenges to our internal environment

A

external temperature
access to nutrients
exersice

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15
Q

what can a loss of homeostasis impact

A

body fluid composition
energy stores
body temperature

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16
Q

what must act to counteract the potential threats to homeostasis

A

physiological mechanisms

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17
Q

how can external temperature be a threat to homeostasis

A

In Abu Dhabi the temperature can reach 40oC…but our bodies don’t. Heat loss is triggered by sweating and vasodilation and core body temperature remains close to the optimal 37oC.
Experiments have shown naked humans can maintain core body temperature pretty close to 37oC in external temperatures ranging from 10oC to 55oC!

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18
Q

how can access to nutrients be a threat to homeostasis

A

eat a sugary donut, glucose absorbed across intestinal tract, blood glucose rockets skywards and all sorts of problems ensue…..only they don’t, because the hormone insulin comes along and effectively removes glucose from the blood almost as fast as it enters

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19
Q

when does homeostasis become less effective

A

at extremes

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20
Q

what are the 3 main mechanisms in which homeostasis operates

A

negative feedback
feedforward
positive feedback

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21
Q

which of the 3 mechanisms in which homeostasis operates is the most important and the key mechanism by which homeostasis is maintained

A

negative feedback

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22
Q

what is negative feedback

A

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.
“Negative” feedback because the condition that triggered the homeostatic response becomes switched off/removed by that response.

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23
Q

what is the size of the response proportional to

A

the size of the disturbance

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24
Q

what are the characteristics of negative feedback systems

A

there is oscillation around the set point.

restores the regulated condition after its initial disturbance, but cannot prevent it happening

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25
Q

what are feedforward systems

A

more sophisticated form of negative feedback, additional receptors permit system to anticipate change and therefore activate response earlier.

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26
Q

what are positive feedback mechanisms

A

positive feedback has the opposite effect of negative feedback. Where negative feedback aims to restore disturbed conditions to optimum, positive feedback sets off a train of events that lead to an even greater disturbance.
Positive feedback systems are rare in normality but frequently occur in pathophysiology

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27
Q

what do these train of events that lead to an even greater disturbance lead to

A

instability, and are common in pathophysiology, rare in normal physiology. however they do occur e.g. in the nerve action potential

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28
Q

explain hoe positive feedback takes place in the action potential (message in the nervous system)

A

initial triggers allows positively charged Na+ ions to enter a nerve cell.
increase in Na+ influx to nerve cell.
charge inside cell become more positive (depolarisation).
increase Na+ permeability across membrane

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29
Q

what is positive feedback associated with

A

while positive feedback is associated with some elements of normal function, it is more commonly associated with pathology

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30
Q

how does negative feedback maintain homeostasis of blood glucose

A

when you eat a meal, your BG levels increase resulting in the release of insulin. glucose is then taken up by cells leading to a decrease in BG levels

31
Q

how does positive feedback lead to deregulation of homeostasis of blood glucose

A

when you eat a mean, your BG levels increase.

the lack of glucose uptake by cells leads the body to perceive starvation so the liver produces even more glucose

32
Q

what is the aim of diabetes treatments

A

to restore homeostatic balance of blood glucose

33
Q

as a generalisation what does medicine aim to do

A

restore homeostatic control when this is disturbed by illness or disease

34
Q

name a process that is homeostatically controlled and is critical for life

A

water balance

35
Q

what percentage does water make up of the body

A

60% of the body weight

36
Q

why is homeostatic maintenance of water crucial

A

because water affects the concentration of everything else in the body

37
Q

which processes are regulated in order to maintain water balance

A

input is regulated by the thirst mechanism
output by regulation of kidney function (urinary losses)
other processes that alter water balance are also regulated, but their control is not aimed at maintaining water balance e.g. sweating is controlled as part of temperature regulation, so possible conflict between water and temperature regulation

38
Q

where is the water in our body

A 
it is split between 3 compartments
1. intaccellular fluid
2. interstitual fluid (fluid between cells)
3. plasma (fluid component of blood)
2 and 3 are extracellular fluids
39
Q

how does water move between the 3 compartments

A

it can move freely although movement is subject to forces such as osmosis

40
Q

the body can survive as long as…

A

the composition of the extracellular fluid (ECF) is maintained in a state compatible with the survival of its individual cells i.e. composition of the ECF is very very important

41
Q

what is the ECF compartment subdivided into

A

plasma and interstitial fluid.

maternal moving between cels and ECF must cross the cell membrane

42
Q

what is the capillary wall permeable to

A

everything except plasma protein and blood cells

43
Q

what type of permeability do cell membranes have

A

selective permeability

44
Q

how much water is in our body

A

42L

45
Q

how is water distributed in the body

A

we have twice as much ICF as ECF so 1/3 in ECF and 2/3 in ICF
(ICF=24L , ECF=14L)

46
Q

what percentage of ECF is interstitial fluid (ISF)

A

approximately 80%

47
Q

what percentage of ECF is plasma

A

approximately 20%

48
Q

what does hyper mean

A

greater than normal

49
Q

what does hypo mean

A

less than normal

50
Q

what does aemia/emia mean

A

in the blood

51
Q

what does uria mean

A

in the urine

52
Q

what does glyc mean

A

related to glucose

53
Q

what does glycosuria mean

A

glucose in the urine

54
Q

what does hypoglycaemia mean

A

low blood glucose levels

55
Q

how does plasma continuously move

A

through blood vessels by the pumping action of the heart = dynamic component of the ECF

56
Q

what does plasma exchange with the ISF

A

it freely exchanges nutrients (e.g. O2, glucose, ions) and waste (e.g. CO2 and urea)

57
Q

how does exchange occur

A

as blood passes through the capillaries of the body

58
Q

why does exchange not take place in arteries

A

as the walls of large vessels are too thick

59
Q

what is the difference between the composition of plasma and ISF

A

they are virtually identical except for plasma proteins, which are too large and, therefore, restricted to the plasma

60
Q

ECF is homogenous with the one exception of…

A

ISF being devoid of plasma protein

61
Q

how is body fluid volumes measured

A

using the dilution principle

  1. c=m/v => v=m/c
  2. only plasma can be sampled so only compartments of which plasma is a component can be measured directly (plasma, ECF, TBW)
  3. the nature of barriers which separate compartments is crucial in determining the test substance
e.g. m=10m glucose
c=1mg/ml glucose
v=m/c=10/1=10
mg/(mg/ml) = ml
10ml
62
Q

which compartments can be measured directly using the dilution principle

A
  1. Plasma Volume (PV): Since plasma proteins cannot cross the capillary walls, can use dyes or radioactive labels that attach to plasma proteins, e.g. Evans blue or I125albumin.
  2. Extracellular Volume (ECF): Need something that freely crosses capillary walls, but cannot cross cell membranes, e.g. inulin, sucrose, mannitol, which are all too large to cross cell membrane or 24Na+ , 36Cl-, which are actively extruded from cells.
  3. Total Body Water (TBW): There is no barrier to water in the body, so can use a loading dose of heavy water/ deuterated water (D2O).
    Other compartments (where plasma is not a component) cannot be directly sampled, therefore calculate volume indirectly;
    ISF = ECF-PV, ICF = TBW-ECF
63
Q

what is the method of practice of dilution principle

A
  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.

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?
150-10mg=140mg distributed in ECF.
therefore volume of distribution = 140mg/0.01mg/ml = 14000mls
ECF volume = 14,000ml or 14L

64
Q

why does the composition of the ICF differ from the ECF

A

ECF bathing cells must be maintained at a constant composition, however the composition of the ICF differs markedly from the ECF, particularly for ions because cell membrane is a selective barrier

65
Q

why is there a large concentration gradient between ICF and ECF

A

as it is fundamental for nerve and muscle function

66
Q

what happens if homeostasis does not operate effectively

A

Consider an example where extracellular potassium concentration ([K+]ECF ) was allowed to increase beyond the normal range:
Loss of concentration gradient between ECF and ICF
This disrupts nerve and muscle function, including cardiac muscle  ventricular fibrillation and death.
ESSENTIAL to regulate ECF K+ (Kidney normally sorts it all out).

Disease states and illness are associated with perturbation, and even breakdown, of homeostatic control mechanisms.
Eg. in diabetes, breakdown of the normal regulation of blood glucose  Hyperglycaemia (i.e. excess glucose in the blood)

67
Q

how does the proportion of body water vary with age and gender

A

Females are “less wet”, because they have a higher proportion of body fat, which has less water content than muscle.
Muscle contains ~70% water by weight, fat only 10% water.

For the same reason, older people have lower water content.

68
Q

what can treating with lipid or H2O soluble drugs depend on and influence

A

the proportion of body water/fat will influence the rate at which the drug can be eliminated from the body

69
Q

how are changes within the normal range restored

A

by physiological mechanisms which act to counteract change

70
Q

what do pathophysiological disturbances require out with normal range

A

intervention to restore variables to normal range

71
Q

what happens if extreme disturbance may fall off the homeostatic plateau

A

death

72
Q

What is extracellular fluid (ECF) and where is it found

A

surrounds all cells and is composed of

  • plasma, the dynamic component of ECF that is contained within blood vessels ( it surrounds blood cells)
  • internal fluid (ISF), the fluid outside blood vessels that surrounds cells other than blood cells
73
Q

What is intracellular fluid (ICF) and where is it found

A

found within cells

74
Q

Why do homeostatic mechanisms operate

A

to maintain constant optimal ECF despite the perturbation of life

foundations of medicine 1 (7 decks)

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