homeostasis Flashcards
how is homeostasis defined by its latin roots
“similar condition” ( not identical condition!)
what do our bodies constantly monitor
the internal state and respond to any threat that might disturb the “similar internal conditions” of the body
why are our bodies constantly monitoring their internal state
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
what are examples of substantial changes in our internal environment that our bodies are not very tolerant of
temperature
pH of bodily fluids
concentration of hormones
what does failure to adequately correct imbalances in the internal environment of the body lead result in
illness and disease, or pathology (pathos - meaning suffering)
give an example of failure to maintain a relatively constant environment
diabetes mellitus occurs when the body can no longer maintain it’s optimal blood glucose concentration
what does homeostasis require
integration of organ systems
how is an increased energy demand an example of integration of organ systems
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.
why must all the systems integrate effectively
in order to maintain an optimum internal environment within the body for all cells to function and ultimately produce energy
what is homeostasis
maintaining an optimum internal environment within the body
what is the basic concept of homeostasis and how to maintain a constant level
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.
what types of changes result in loss of homeostasis
external and internal changes
what could happen when the organism attempts to compensate as a result of a loss of homeostasis
compensation fails - leading to illness or disease
compensation succeeds - leading to wellness
what are common everyday challenges to our internal environment
external temperature
access to nutrients
exersice
what can a loss of homeostasis impact
body fluid composition
energy stores
body temperature
what must act to counteract the potential threats to homeostasis
physiological mechanisms
how can external temperature be a threat to homeostasis
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!
how can access to nutrients be a threat to homeostasis
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
when does homeostasis become less effective
at extremes
what are the 3 main mechanisms in which homeostasis operates
negative feedback
feedforward
positive feedback
which of the 3 mechanisms in which homeostasis operates is the most important and the key mechanism by which homeostasis is maintained
negative feedback
what is negative feedback
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.
what is the size of the response proportional to
the size of the disturbance
what are the characteristics of negative feedback systems
there is oscillation around the set point.
restores the regulated condition after its initial disturbance, but cannot prevent it happening
what are feedforward systems
more sophisticated form of negative feedback, additional receptors permit system to anticipate change and therefore activate response earlier.
what are positive feedback mechanisms
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
what do these train of events that lead to an even greater disturbance lead to
instability, and are common in pathophysiology, rare in normal physiology. however they do occur e.g. in the nerve action potential
explain hoe positive feedback takes place in the action potential (message in the nervous system)
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
what is positive feedback associated with
while positive feedback is associated with some elements of normal function, it is more commonly associated with pathology
how does negative feedback maintain homeostasis of blood glucose
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
how does positive feedback lead to deregulation of homeostasis of blood glucose
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
what is the aim of diabetes treatments
to restore homeostatic balance of blood glucose
as a generalisation what does medicine aim to do
restore homeostatic control when this is disturbed by illness or disease
name a process that is homeostatically controlled and is critical for life
water balance
what percentage does water make up of the body
60% of the body weight
why is homeostatic maintenance of water crucial
because water affects the concentration of everything else in the body
which processes are regulated in order to maintain water balance
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
where is the water in our body
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
how does water move between the 3 compartments
it can move freely although movement is subject to forces such as osmosis
the body can survive as long as…
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
what is the ECF compartment subdivided into
plasma and interstitial fluid.
maternal moving between cels and ECF must cross the cell membrane
what is the capillary wall permeable to
everything except plasma protein and blood cells
what type of permeability do cell membranes have
selective permeability
how much water is in our body
42L
how is water distributed in the body
we have twice as much ICF as ECF so 1/3 in ECF and 2/3 in ICF
(ICF=24L , ECF=14L)
what percentage of ECF is interstitial fluid (ISF)
approximately 80%
what percentage of ECF is plasma
approximately 20%
what does hyper mean
greater than normal
what does hypo mean
less than normal
what does aemia/emia mean
in the blood
what does uria mean
in the urine
what does glyc mean
related to glucose
what does glycosuria mean
glucose in the urine
what does hypoglycaemia mean
low blood glucose levels
how does plasma continuously move
through blood vessels by the pumping action of the heart = dynamic component of the ECF
what does plasma exchange with the ISF
it freely exchanges nutrients (e.g. O2, glucose, ions) and waste (e.g. CO2 and urea)
how does exchange occur
as blood passes through the capillaries of the body
why does exchange not take place in arteries
as the walls of large vessels are too thick
what is the difference between the composition of plasma and ISF
they are virtually identical except for plasma proteins, which are too large and, therefore, restricted to the plasma
ECF is homogenous with the one exception of…
ISF being devoid of plasma protein
how is body fluid volumes measured
using the dilution principle
- c=m/v => v=m/c
- only plasma can be sampled so only compartments of which plasma is a component can be measured directly (plasma, ECF, TBW)
- 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
which compartments can be measured directly using the dilution principle
- 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.
- 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.
- 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
what is the method of practice of dilution principle
- Inject a substance that will stay in one compartment only (plasma, ECF, TBW)
- 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
why does the composition of the ICF differ from the ECF
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
why is there a large concentration gradient between ICF and ECF
as it is fundamental for nerve and muscle function
what happens if homeostasis does not operate effectively
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)
how does the proportion of body water vary with age and gender
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.
what can treating with lipid or H2O soluble drugs depend on and influence
the proportion of body water/fat will influence the rate at which the drug can be eliminated from the body
how are changes within the normal range restored
by physiological mechanisms which act to counteract change
what do pathophysiological disturbances require out with normal range
intervention to restore variables to normal range
what happens if extreme disturbance may fall off the homeostatic plateau
death
What is extracellular fluid (ECF) and where is it found
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
What is intracellular fluid (ICF) and where is it found
found within cells
Why do homeostatic mechanisms operate
to maintain constant optimal ECF despite the perturbation of life
