Hormones involved in maintaining fluid balance
Hormone |
Net effect |
ADH (Antidiuretic hormone) |
Blood pressure increases (with fluid intake); blood volume increases (with fluid intake); blood osmolarity decreases |
Renin-angiotensin system |
Blood pressure increases |
Aldosterone |
Blood plasma Na+ maintained, blood plasma K+ decreases. Blood volume and blood pressure maintained (by decreasing urine output) |
Atrial natriuretic peptide (ANP) |
Promotes natriuresis, elevated urinary excretion of Na+ (and Cl-), accompanied by water. Increases loss of water in urine |
Water
Primary fluid in the body |
Most physiological processes require water |
Water delivers electrolytes & nutrients, and carried away waste. |
Helps regulate body temp. and helps maintain blood volume |
Water balance is affected by age, gender, muscle mass (more muscle = more water, more fat = less water) |
60% of an adults body weight is water - there is more water in a child and less in an eldery person, placing these populations at increased risk of fluid & electrolyte imbalances |
Daily intake requirements is 2000-3000mL (in ideal conditions, changes in situations such as fever, increased metabolism etc.) |
1L of water = 1kg weight so daily weighs can be a good indicator of whether a patient is maintaining good fluid & electrolyte balance |
Intake sources: Liquids (1500mL/day), Solid foods (800mL/day), Metabolism (300mL/day) |
Fluid loss sources: Kidney (1200-1500mL/day), Skin (500-600mL/day), Lungs (400mL/day), GI Tract (100-200mL/day) |
Measureable intake sources: Oral fluid, tube feedings, Parenteral fluid, Enemas, Retained irrigation fluid |
Measureable output sources: Urine, Emesis, Feces, Drainage from body cavities |
Not measureable intake sources: Solid foods, metabolism |
Not measurable output sources: Sweating, vaporisation through lungs |
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Regulation of fluid balance
Regulation of F&E movement
Osmosis |
Movement of water through a semi-permeable membrane from an area of low concentration to an area of high concentration |
Diffusion |
important in the transport of gases; sometimes requires the assisstance of a transport system (facilitiated diffusion - used in insulin & glucose); similar to filtration but more about movement of particles that movement of fluid (also includes gas transport & electrolytes) |
Flitration |
Differences in water volume, i.e. hydrostatic pressure. Happens at tissue capillary level from capillaries to interstitial fluid, i.e. edema; hydrostatic pressure pushes fluid through the membranes (cell & vessel walls) |
Active Transport |
Molecules have to move against a conc. gradient, requiring active energy and a transport system - such as a Na-K pump requiring ATP - which moves 2 substances at the same time in opposite directions, againt conc. gradients. |
Fluid Shifts
If intracellular fluid (ICF) becomes hypertonic relative to extracellular fluid (ECF), water moves from ICF to ECF via osmosis, causing cell death |
If ECF becomes hypotonic relative to ICF, water moves from ECF into cells, expanding the cell and potenially causing cell to burst |
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