Fluid, Electrolyte ,And Acid –Base Homeostasis

1.Fluid compartments and fluid balance

The physiology of body fluids it is important and the

regulatory mechanisms are kidney and other organs

working to maintain homeostasis of the body fluids,

homeostasis can be known as: the state of balance in

our bodies and it is not constant ;means there is a

variation and changes in the water volume and dissolved

solutes in our bodies; it does not remain constant and

even if there are changes or variations the body has the

ability to overcome these changes to maintain a state of

balance ,so it is not a constant term it is the balance

that body can achieve in order to maintain a normal

function .

Also we know from our previous knowledge from

biology that our bodies are composed mainly from

water ,so it’s the main components of our bodies ,

Also water is the microenvironment and the media that

our cells are found in and is composed from , so let us

see the percentage of bodies components ,our body is

not a vessel )وعاء(it’s more complicated than that ,so

there are several compartments that contain water ;if

we look at body composition we will find that 55% by

mass ( من كتلة الجسم)is fluid and there is some variability;

45% solids and (55 or 60) % is fluid by mass ,if we look at

fluid part of it we will found the majority of our fluid lies

inside the cells and we called this compartment as(

intracellular compartment or intracellular fluid “the

fluid within the cells”) and it’s constitutes 40% of the

60% of the body fluid means the 2/3 of the 60% ; this is

the water that lies in the cells means that we collect all

cells in our body and count the volume of fluid in each

one and get its constituent 40% of 60% it’s intracellular

fluid .

The rest of 60% of the fluid in our body is 20% it is

Extracellular fluid (EC) “Outside the cells” ; anything

outside the cells is EC ; between the cells and between

the blood vessels or lymphatic vessels and anything

inside the vessels all of these are called Extracellular

fluid (EC) it is a wide definition .

The composition of the extracellular fluid (EC):

1) 80% of the 20%of(EC) is interstitial fluid ,which

occupies the microscopic space between tissue

cells .

2) 20% of the20%of ( EC )is plasma ,the liquid portion

of the blood .

**So if we asked : what is the major component or

compartment of fluid in our body ?

You may say that it is the blood or plasma ; but that is

not true ,SO the major compartment of fluid in our body is

the intracellular fluid inside the cells , NOT THE BLOOD .

Conclusion : 40% of the 60% is (IC)

fluid =60% by mass

20% of the 60% is(EC)

Intracellular (IC) = inside the cells (cytosol)

Extracellular(EC) =interstitial fluid +plasma (outside the

cells)

20% = ( 80% + 20% )

So Intracellular > extracellular

Interstitial fluid>plasma

Intracellular >interstitial >plasma

“All of these are taken by mass and we can also say by

volume because ( 1L 1Kg) so we can know the

volume of each compartment by know the weight of

each percent “.

Ex:

What is the approximate volume of blood plasma in a

lean 60 Kg female ?

60 L *20%(plasma )*20%(EC)*55%(body fluid)=3700 L

1L 1Kg *Lean body mean : athletic body

?? 60 Kg

volume 60 L

NOTE the percentage of fluids daffier by : the age,

Gender ,body mass index ,weight. As you can see in the

picture below

*Now *

How we maintain the balance between what we take

and what we eliminate of water ?

Do we take the same volume every day ? do we lose

the same volume of water every day ?actually No ;we

don’t do this the volume of what we take and eliminate

is changed according to specific conditions :

1)different temperature every day

2) variations in activities

All of these affect the metabolic activities in our body so

that’s affect what we take and what we lose but to

maintain homeostasis we need to have a balance by this

we will not have either less or more volume in each

compartment than needed so :gained water should

equal eliminated water in order to get balance .

If there is an increase in volume of our compartment

(we gain a lots of water ) and no eliminated this is too

dangerous and may cause problems .on the other hand

losing a lots of water leading to(dehydration ) which

cause serious problems one of them is death ;so that

we should maintain the balance between intake and

losing of water .

*Gain means how much we store in our bodies so the

use of ” intake” is better in this case .

The routs (or sources) of water intake

The body can gain water by:

1) ingestion

2) metabolic synthesis .

*these two main sources of body water are divided

into:

1)ingestion which include :

a)ingested liquids about (1600 ml)

b) ingested moist food (about 700 ml ) absorbed from

the gastrointestinal (GI) tract ,which total about 2300

ml/day .

2) 200 ml of metabolic water : it’s produced in the body

mainly by the reduction of O2 during aerobic cellular

respiration and to a smaller extant during dehydration

synthesis reaction so daily water gain from these two

sources totals about 2500 ml.

*Body fluid volume remains constant because water loss

equals water gain

# Water loss occur in four ways in each day:

1) kidney excrete about 1500 ml in urine

2) the skin evaporate about 600 ml including sweating

about 200 ml of the 600 ml

3) lungs exhale about 300 ml as water vapor

4)(GI)gastrointestinal tract about 100 ml in feces .

# Regulation of body water gain

How does our body interact with the change in

temperature ?and what are the main players involved

?

First of all Metabolic water activity it can’t be controlled

because it’s differs according to the activity of the body

;may increase during exercise , and decrease during

sleeping its reflect the demand of ATP in body cells .it’s a

result of ongoing reactions .

For example :

a)During a hot day there will be an increase in sweating

from 100 ml to 1400ml this physiological increase is a

response in order to reduce body temperature .

b)On the other hand urine decrease from 1500 to 1200

trying to balance between the gain and loss of water.

*but if we look at the overall loss and intake of water

in this case they would be the same( 3500ml loss

=3500ml intake )

That’s can be explained by nervous system and other

hormonal factors ;an area in hypothalamus known as

thirst center that’s govern the urge to drink in this case

increase the intake of water to maintain homeostasis .

In the prolonged exercise :

There would be an increase in the loss of water ,

because of the increase in metabolic activity

through muscle more metabolic water

increase in the demand of O2 the rate of

respiration will be increase increase the

efficiency of gas exchange by this increase the loss

of water vapor by lungs .

Also massive increase in the sweating from 100 L to

5000 (increase the loss of water )

Urine reduced from 1500 L to 500 L the third (1/3)

here the kidney working realy hard to achieve a

source of balance to the massive loss of water by

sweating .

if we asked … what is the major organ that involve

in maintain water volume ?

*the kidney it’s the major regulator to maintain

fluid homeostasis .

It’s regulated by thirst center in the hypothalamus

in the brain .when we have a prolong exercise

activity or in a hot day .

The thirst center stimulated by different

pathways :

1. The first signal dryness due to decrease flow

saliva in mouth is detected by neurons in

mucosa in the mouth and they send a

message to the thirst centre and stimulate it

and give us the sense to drink water.

2. The volume of blood is decreased and it’s

directly related to the blood pressure so it will

decrease too ,and there is a receptors

(baroreceptos) that detect lowered blood

pressure in the heart and blood vessels ,as a

result the sensation of thirst increase which

usually leads to increase in fluid intake and

restoration of normal fluid volume .

3. the kidney receive about (25-30) % of blood to

be filtrated so it is a strategic place to sense

any reduction in the volume or increase in

osmolarity, osmosis :it is the movement of

water through a semi permeable membrane

from one compartment to another governed

by the concentration of effective osmoles or

solutes(Osmolarity ) to make water move from

the compartment with a low osmolarity to the

high osmolarity; to balance the concentration.

Now when solvent decrease the osmolarity

increase means there is a loss of water by this

it’s stimulates osmoreceptors in

hypothalamus and then stimulates the thirst

center in hypothalamus then increase thirst

and increase water intake so finally increases

body water to normal level and relieves

dehydration.

NOTE :osmorecepters are not connected with

kidney BUT it’s affected from its function

because the kidney make a correction by

increase the water reabsorption as we will

see later and it will reduce the increase in

osmolarity gradually and return to the

normal state so there will be no more firing

(stimulation ) in the nerve system ,all of these

pathways will be inhibited when the

physiological state return normal .

4. As we said previously that the decrease in

blood volume cause blood pressure to fall

down and this change stimulates the kidneys

to release nenin ( an enzyme) (endocrine

player :rennin-aniotensin system) ,which

promotes the formation of angiotensin II

(hormone that increase the reabsorption of

the water in kidney ) actually the function of

the kidney is to filtrate the received blood but

we don’t lose the whole fluids the majority of

them will be reabsorbed and the rest will be

eliminated .the kidney itself can know that it

must make further reabsorption but one of

the action of( angiotensin II) is reach to the

thirst center by the blood and make

stimulation to it so there is a link between

kidney and hypothalamus .as we said that

angiotensin II will increase the re absorption

of water and sodium (Na+) it’s actually doesn’t

increase water directly it’s increase the

reabsorption of sodium(Na+) to return to

blood circulation followed by water the rule:

“water followed solutes “ by this reducing

water loss and thus increase the blood

volume and by the way the blood pressure .

Conclusion :

increased nerve impulses from

osmoreceptors in the hypothalamus ,triggered

by increased blood osmolarity and increased

angiotensin II in the blood both stimulate the

thirst center in the hypothalamus then it will

cause an increase in water intake . so finally

increases body water to normal level and

relieves dehydration .

But sweating and metabolic water are not

controlled to maintain homeostasis so they

are not changeable SO they are not consider

as regulatory mechanisms .

*the major regulatory mechanism take place

by kidney and thirst center in the

hypothalamus .but the major is done by

kidney.

Quote

“ I’m not telling you it’s easy but I’m

telling you it’s going to be worth it “

*what is the main factor that maintain

homeostasis ?and who determine that we

have a high in osmolarity in our body so low

water content ?what is the main solute that

governs water homeostasis ?

It’s NaCl that: governs water movement so

it’s the main player that maintain fluid

homeostasis through the kidney and control

how much (Na+) we going to excrete or

eliminate or reabsorb here we determine how

much to keep more fluid or to lose .

The end

Done by : Noor Khanfar