Body Fluids

Objectives• Outline the functions of water in the body.• State how water content varies with age and sex.• Differentiate between intracellular and extra-cellular

fluid.• Explain how water moves between fluid

compartments.• List the major sources of body water gains and losses.• State the function and stimuli of thirst.• Briefly explain how ADH regulates urine concentration.• Briefly describe the role of aldosterone in fluid

balance.

Functions of water

Water is the solvent used in the body:

to transport nutrients to cells

to remove waste products from cells

to act as a transport vehicle for blood gases O2

Functions of water

Water is also necessary ..

* for body structure (e.g. skin turgor)

* in temperature regulation

* as a medium for chemical reactions

* as a lubricant

* as a cushion and shock absorber

Body water and age and sexThe volume of total body water in a healthy adult male is

approximately 45 L (i.e.60% of their body is H2O)

Variations in water content with ageInfant Adult Older Adult

% H2O 80% % H2O 55%

Infants also ingest and excrete a greater volume of water in relation to their size ? why

Fluid Compartments and Fluid Exchange

2/3 body H2O 1/3 body H2O

Inside cells Outside cells

Intra-Cellular Fluid Extra-Cellular Fluid

(ICF) = 30L (ECF) = 15LInterstitial fluid 12LPlasma 3L

Also … Lymph

Cerebrospinal fluid

Body space fluid e.g.

Intra-ocular fluid

About 65% of Body Weight

Adult has 45 litres 45 litres of water in the body:of water in the body:

Total Body WaterTotal Body Water

Intracellular30 Litres

Extracellular15 litres

Tissue Fluid 12L Plasma 3L

Water gains and lossesGains Losses

Water 1300 ml urine 1700 ml

Food 1000 ml insensible losses ..

Metabolism 400 ml faeces 200 ml

lungs 300 ml

skin 500 ml

Total 2700 ml 2700 ml

Alterations in homeostasis

• If the water gained = water lost each day the body will be in water equilibrium

• If the water gained is less than water lost DEHYDRATION

What interventions may be necessary treatment of dehydration?

[Application to clinical practice: Dehydration]

What patients may be at risk of dehydration?

Have you ever been dehydrated and how did you feel?

[Application to clinical practice: Dehydration]

What are the signs and symptoms of dehydration?

What is minimum urine output/hr to maintain kidney function?

Whose role is it to ensure that patients are adequately hydrated?

Regulation of Intake

• The sensation of thirst ensures that water intake is adequate

• What patients may not be able to respond to the thirst sensation?

• The thirst centre is in the hypothalamus where there are nerve cells called osmoreceptors

Regulation of intake

Saliva Blood osmolarity

Dehydration

Mouth / throat dry Thirst centre stimulated

Sensation of thirst and desire to drink

Mucosa of mouth/ pharynx wetted

Distension of intestineOsmotic pressure

Initial quenching of thirst Major inhibition of thirst

Regulation of Fluid Volume

Increased Osmotic Decreased Pressure (blood) Flow of Saliva

THIRSTTHIRST

INCREASED INTAKE

INCREASED BLOOD VOLUME

DEHYDRATION

HypothalamusDry Mouth &

Throat

Regulation of Output

Fluid output adjusted by... • ADH • Aldosterone

(both regulate urine production)

• Anti Diuretic hormone (ADH)

• Increases re-absorption of water by the kidneys

Anti Diuretic Hormone

The hypothalamus synthesizes ADH which is transported to the posterior pituitary and stored there

Alcohol Inhibits release of ADH

Control of ADH secretion

• Changes in extra-cellular osmolality

• Detected by Osmoreceptors in hypothalamus

• ADH production and release will be increased or decreased

• What will the effect be of increased ADH?

• What will the effect be of decreased ADH?

Aldosterone

• Aldosterone is a hormone (a mineralocorticoid) produced by the adrenal cortex

Aldosterone

Aldosterone is a Mineralocorticoid produced by the Zona glomerulosa of the adrenal cortex

Function of aldosterone is to:• Stimulate kidneys to reabsorb Na+ and

secrete K+ sodium and water reabsorption

Aldosterone

Tissue fluid production

Arteriole end Venule end

Lymph vessel

Tissue Fluid

CapillaryHP = 35mmHg HP = 15mmHg

OP = 25mmHg

Hydrostatic pressure?

Osmotic pressure?

Structure of the capillary?