Body Fluid Compartments Flashcards
Total fluid compartments
Total Body: 42L: ICF: 28L
ECF: 14L
The various fluid quantities in the ECF
Plasma(IVF): 3L
Tissue fluid( ISF): 11L
Transcellular fluid: 1-2L
What is meant by the term Steady State
The body fluid constancy, in terms of composition and volume, is maintained by way of a dynamic equilibrium
or so-called “steady state”, which implies a continuous turnover (exchange) of solutes and fluid. This is the
opposite of a static equilibrium where there is no turnover once equilibrium has been reached.
Fluid intake(high variable) and output are balanced
Ensures homeostasis by preventing Increase/Decrease body fluid volumes
Kidney plays an important role.
Discuss Fluid Intake
By:
- Ingestion of liquids or water in food (2100 ml/day)
- Synthesized by oxidation of carbohydrates (200 ml/day)
Total water intake = 2300 ml/day
Fluid Output/Loss
Insensible (consciously unaware) Water Loss:
• Evaporation from the respiratory tract and diffusion through the skin (700 ml/day)
Fluid Loss in Sweat
- Varies - dependent on climate and physical activity
- Normal conditions (100 ml/day)
- Hot weather/heavy exercise (increases to 1 to 2 L/hour)
Water Loss in Feces
• Small amount of water lost in feces (100 ml/day) normally
• Severe diarrhea – Life-threatening if not corrected (Several L lost)
Water Loss by the Kidneys
▪ Urine excreted by the kidneys
The most important means by which the body maintains a balance between water and
electrolyte intake and output, is by controlling the rates at which the kidneys excrete
these substances
Urine volume can be as low as 0.5 L/day in a dehydrated person or as high as 20 L/day
in a person who has been drinking lots of water
Obligatory Urine Volume
Minimal volume of urine that must be excreted by the kidneys
A normal 70 kg human must excrete about 600 milliOsmoles of solute each day.
If maximal urine concentrating ability is 1200 mOsm/L, the minimal volume of urine that must be excreted
Obligatory Urine Volume Calculation
600 mOsmol/day / 1200 mOsmol/L
= 0.5 L/day
Human Waste Products a day
600 mOsmol/day
Clinical abnormalities of fluid volume regulation
HYPONATREMIA:
-Excess water in the ECF or loss of sodium from the
ECF
HYPERNATREMIA (↑plasma sodium concentration):
-Loss of water from the ECF (concentrates sodium ions) orexcess sodium in the ECF
Hyponatremia
Excess water in the ECF or loss of sodium from the
ECF
➢ The loss of sodium results in hyponatremia dehydrationassociated with decreased ECF volume:
▪ Overuse of diuretics - inhibits the ability of the kidneys
to conserve sodium
▪ Addison’s disease - ↓ secretion of aldosterone, impairs the ability of the kidneys to reabsorb sodium
➢ Excess water retention results in hyponatremia
overhydration:
▪ Excessive secretion of ADH, causes the kidney tubules to reabsorb more water
▪ Brain swelling (skull is rigid – herniation)
Hypernatremia
(↑plasma sodium concentration)
Loss of water from the ECF (concentrates sodium ions) or excess sodium in the ECF
➢Loss of water from the ECF:
▪ Occurs from an inability to secrete ADH - large amounts of dilute urine excreted (“central” diabetes insipidus)
▪ More common cause is simple dehydration, promotes intense thirst and stimulates secretion of ADH
➢Excess sodium in the ECF:
▪ Hypernatremia—overhydration – associated with extra water retention by the kidneys
Rapid fluid replacement could lead to cerebral edema (ICF and ECF loads increase causing increased pressure within the brain space)
What is the Gibbs-Donnan Effect phenomenon
The phenomenon of predictable and unequal distribution of permeant charged ions on either side of a semipermeable membrane, in the presence of impermeant charged ions
Osmotic pressure
π(pi)
π = MRT
M is the molar concentration of dissolved
species (units of mol/L).
R is the ideal gas constant (0.08206 L atm
mol-1 K-1)
T is the temperature on the Kelvin scale.
Osmotic pressure caused by proteins are critical for keeping fluid inside capillaries
Transcapillary Fluid Movement
Arterial blood pressure (ABP) exerts an outward
hydrostatic force on the fluid inside blood vessels, while the plasma proteins are responsible for an inward osmotic force (COP).
The two forces, causing filtration and reabsorption, balance each other almost exactly, except that the outward force is a bit larger than the inward force.
Thus slightly more fluid moves out of the
capillaries than comes back.
This small volume of fluid is returned to the blood by the lymphatic system.
Lymphatic Capillaries
1/10 of the fluid that leaks out is transported by the
lymphatic system back into the blood
Total estimated lymph flow of about 120 ml/hr or 2-
3L/day
Crucial for transport of proteins
Allows for passage of large molecule (proteins)
Edema
Accumulation of excess fluid in the interstitial spaces
Develops as a result of tissue losing its negative
pressure
It is an abnormal and a harmful condition
Disruption of one of the four Starling Forces
What is Edema
Refers to excess fluid in the interstitial spaces. It is an
abnormal, harmful condition that, inter alia, is unfavourable to exchange between the intravascular andintracellular compartments.
Causes of Intracellular Edema
- Hyponatremia
- Depression of the metabolic systems of the tissues; and
- Lack of adequate nutrition to the cells
- Inflamed tissues - Inflammation increases cell membrane permeability, allowing sodium and other ions to diffuse into the interior of the cell resulting in osmosis of water into the cells
Causes of Extracellular Edema
- Lymphedema - failure of the lymphatics to return fluid from the interstitium back into the blood
- Excessive capillary fluid filtration
- Increased capillary pressure
- Decreased plasma proteins
- Increassed capillary permeability
- Blockage of lymph return
Four major factors that cause increased capillary filtration of fluid and protein into the interstitium
- Increased capillary hydrostatic pressure
- Decreased plasma colloid osmotic pressure
- Increased capillary permeability
- Increased interstitial fluid colloid osmotic pressure
What is Lymphedema
- FAILURE OF THE LYMPH VESSELS TO RETURN FLUID AND
PROTEIN TO THE BLOOD
Plasma proteins that accumulate in the interstitium have no other way to be removed
↑ Colloid osmotic pressure of the interstitial fluid, which removes more fluid out of the capillaries
• Caused by infection, cancer or after surgery in which lymph vessels are removed or obstructed
Edema caused by Heart Failure
Systemic/Peripheral Edema:
• Heart is too weak to pump blood around the body
effectively
• Raises venous pressure and capillary pressure,
causing increased capillary filtration
Pulmonary Edema:
• Left-sided heart failure but without significant failure
of the right side of the heart
• Blood is pumped into the lungs normally by the right
side of the heart but cannot enter from the pulmonary
veins to the left side of the heart
• Pulmonary vascular and capillary pressures ↑
Kidney Disease
Decreased urinary excretion of salt and water
• Large amounts of sodium chloride and water are added to the ECF
• Most of the salt and water leaks from the blood into the interstitial
spaces
The main effects of this are:
1. Widespread increases in interstitial fluid volume (extracellular edema) and
- Hypertension due to the increase in blood volume
Safety factors that prevent Edema
Low tissue compliance in the negative pressure
range (- 3 mm Hg):
▪ The interstitial fluid hydrostatic pressure is in the
negative pressure range, small increases in
interstitial fluid volume cause relatively large
increases in interstitial fluid hydrostatic pressure,
opposing further filtration of fluid into the tissues
Increased lymph flow (7 mm Hg):
▪ Lymph flow can increase 10- to 50-fold when
fluid begins to accumulate in the tissues
Wash-down of proteins (7 mm Hg):
▪ Proteins are removed by lymph flow, decreasing
the interstitial fluid proteins, lowers the net
filtration force across the capillaries and prevents
further accumulation of fluid
