Introduction to Principles of Physiology - Lecture Notes (University of Surrey BMS1032)

HOMEOSTASIS
Created Jun 1, 2019 414 PM

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Updated Jun 1, 2019 438 PM

HOMEOSTASIS

~ What is homeostasis?

it is the maintenance of a stable internal environment, to maintain normal
healthy function of the body's cells

the body is in homeostasis when its needs are met and are functioning
properly

it does not involve keeping conditions static

it is keeping conditions within a tightly regulated physiological range

every organ system constributes to homeostasis

but endocrine and nervous systems are particularly important

a loss of homeostasis is reflected in ill health

~ Feedback Loop

homeostasis is often maintained through the principle of feedback
mechanism

is a control system that acts to maintain the level of some variable
within a given range following a disturbance

components of feedback loop:

sensor

afferent pathway

control centre

controller

sets the level of a variable (physiological range)

upper brain centres



HOMEOSTASIS 1

, comparator

compares the set level of a variable with the signal from receptor

output usually an opposite influence to the signal

except for some variables, eg. giving birth and micturition

effector (target organs)

efferent pathway

there is always continuous real-time communication between receptor,
control centre and effector

how the feedback loop works:

when the body is in homeostasis:

the set levels of all variables are within physiological range

there is a dynamic balance (no difference) between set level and
input at the comparator

when there is a different between the set level and input at comparator:

output to target organs will modulate the variable

there are two types of feedback loops

Negative feedback

the output (change) is in the opposite direction as the original
stimulus

reduces the output of a system

decrease in function in response to stimulus

variables are restored to the steady state

Positive feedback

the output (change) is in the same direction as the original stimulus

increases in function in response to stimulus

enhances the stimulus

~ Negative Feedback Loop

 an external factor causes the input variable to decrease/increase




HOMEOSTASIS 2

,  the sensor registers a decreased/increase signal and sends the information
to comparator

 comparator output acts to increase/decrease the value of the variable to
the target organs

Example:

low blood pressure

arterial blood pressure is reduced

baroreceptors register the reduced signal

the sinus nerve sends signals to comparator in a reduced manner

the medullary cardiovascular centres send signals to increase blood
pressure

arterioles

SA node and myocardium (heart)

water balance

drinking 2 liters of water in 30 minutes

plasma osmolality is reduced

osmoreceptors register decreased plasma osmolality

hypothalamus receives reduced signals and send signals to kidney

decrease water reabsorption in collecting duct of kidneys

micturition of less concentrated urine




HOMEOSTASIS 3

, ELECTROLYTE AND FLUID
BALANCE
Created Jun 1, 2019 439 PM

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Updated Jun 1, 2019 528 PM

ELECTROLYTE AND FLUID BALANCE

~ What is the human body composed of?

solid matter

tissue and bones

makes up 40% of total body weight

intracellular fluid

makes up 40% of total body weight

extracellular fluid

plasma, lymph, cerebrospinal fluid, fluid in GI tract, synovial fluid

makes up 20% of total body weight

~ Body Fluid Compartments

The total body water content is about 42 L 60% of total body weight)

Intracellular Fluid ICF - 2/3 Total Body Weight

the fluid within cells, also known as cytoplasm

28 L in a 70 kg young male

67% of total body water

Extracellular Fluid ECF - 1/3 Total Body Weight

the fluid found outside cells

14 L in a 70 kg young male

made up of 3 different fluid groups:

Plasma


ELECTROLYTE AND FLUID BALANCE 1