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Fluid Distribution & Excretion.pptx
1.
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
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.