Outline of Fluid Balance in the HUMAN BODY

2. Intro
 Water constitutes to 60% of the total
body mass
 A balance of water in and out of the
body is required for homeostasis
(2300mL/day)

3. Water Loss & Gain
Water Gain (2300mL/day)
 Food and Drinks: 2100mL/day
 Carbon Metabolism: 200mL/day
Water Loss (2300mL/day)
 Insensible Pathways
 Lungs: 350mL/day
 Skin: 350mL/day
 Sweat: 100mL/day
 Faeces: 100mL/day
 Urine: 1500mL/day

4. Thirst Sensation
 Osmoregulatory mechanism to increase
water input
 Activation of the thirst mechanism occurs
when there is a change in: (i) water volume
in the blood (ii) blood osmolality
 Blood osmolarity: ratio between
concentrations of certain components (i.e.
Na+) to water present in the blood (normal
290mOsm). Primarily driven by Na+

5. Steps:
1. Receptor proteins or osmoreceptors in the kidneys,
hypothalamus and adrenal glands detect decreased fluid volume or
increased osmolarity in the blood.
a. Kidneys detect blood volume
 Detect change using pressure or stretch in blood
vessels of the kidneys
 Secrete renin which splits a protein (angiotensinogen)
into an active hormone (angiotensin)
 Angiotensin narrows blood vessels (vasoconstriction),
stimulates thirst in the hypothalamus, stimulates
secretion of aldosterone by adrenal cortex
b. Hypothalamus detects blood osmolarity
 Osmoreceptors detect high Na+ levels in the blood
c. Adrenal glands detect blood osmolarity
 Cells detect low Na+ levels and high K+ levels in the
blood
 Aldosterone secreted to stimulate Na+ reabsorption (in
turn water reabsorption)

6. 2. Hormonal and neural messages are sent to the
hypothalamus.
a. Hormonal Response (Vasopressin):
 Posterior pituitary gland releases Antidiuretic
Hormone (ADH)
(Note: diuretic- to urinate; antidiuretic- stop urination)
 ADH is sent to the kidneys via the bloodstream;
acting specifically on the collecting ducts of the
nephrons by the release of aquaporins (proteins that
enable water reabsorption into the blood).
b. Neural/Behavioural Response (Thirst):
 Neural signals are sent to the cortex, stimulating the
conscious thought to drink water.
 Fluid is consumed
 Receptors in the mouth detect mechanical
movements involved in fluid ingestion
 Neural signals are relayed to the brain, switching off
the “thirst mechanism”.
3. Homeostasis restored

7. Fluid Compartments
 Intracellular Fluid (ICF)
 Extracellular Fluid (ECF)
 Blood Plasma
 Interstitial Fluid
 *Transcellular Fluid

8. Distribution of Fluid
 Blood Plasma/Intravascular Fluid:
 Constitutes to about 4%-7% of the
total body mass
 90% water
 10% ions, proteins (i.e. antibody
proteins, coagulation factors, albumin,
fibrinogen), dissolved gases, nutrient
molecules and waste
 Net negative charge

9.  Interstitial Fluid
 Constitutes to 15% of the total body
mass
 Components are similar to that of
plasma, yet do not contain protein
 Receive constituents via the capillary
membrane, from blood plasma
 Capillary membranes are highly
permeable to all solutes, except protein
 High levels of sodium ions (Na+ )and
chloride ions (Cl-)
 Due to the negative charge in the blood
plasma repelling anions, they move into
the interstitial fluid

10.  Intracellular Fluid
 Constitutes to about 40% of the total
body mass
 Components include water, dissolved
ions, and large, water-soluble molecules
(i.e. protein)
 High levels of potassium ions (K+) and
phosphate ions (PO4
3-)
 Receive constituents via the
phospholipid bilayer (cell membrane)
 Cell membranes are selectively
permeable: highly permeable to water,
yet relatively impermeable to most
electrolytes.

11. NB: Variations in the average fluid present in the compartments are
dependent on age, sex, and percentage of body fat.

12. Movement of Fluid Across
Compartments
 Membrane Permeability
 Osmotic Forces: aids movement of
interstitial fluid into the capillaries (blood
plasma)
 Hydrostatic Pressure: aids movement of
fluid from blood plasma into the interstitial
space
 ATPase-dependent Na+/iv. Pump: group of
enzymes which act as an active transport
mechanism to help movement of ions (i.e.
Na+/K+ ) through the phospholipid bilayer.

13. Principle Abnormalities of Water
Balance
 Dehydration: water loss is greater than water intake
 Possible causes: severe burns, prolonged vomiting
or diarrhea, profuse sweating, water deprivation,
diuretic abuse and haemorrage
 Hypotonic hydration: ECF becomes dilute due to
excess amounts of water
 Possible causes: renal insufficiency or intake of
abnormal amounts of water
 Oedema: accumulation of fluid in interstitial space
 Possible causes: any mechanism that increases flow
of fluid out of the bloodstream compared to the
inflow.

14. THE END.