Percentage Size of compartment
18% Bwgt is protein and related substances.
7% is mineral
15% is fat. The remaining 60% is water. The intracellular component of the body water accounts for about 40% and extracellular component about 20%., 25% of the extracellular component is in the vascular system (plasma = 5% bwgt) and 75% outside the blood vesselsinterstitial fluid = 15% of body weight).Total blood volume is about 8% of body weight.
INTRACELLULAR COMPARTMENT
The intracellular fluid – cytosol l(The cytosol is a mixture of substances dissolved in water.) or intracellular fluid (or cytoplasmic matrix) is the liquid found inside cells.
The concentrations of ions such as sodium and potassium are different in the cytosol than in the extracellular fluid; these differences in ion levels are important in processes such as osmoregulation and cell signaling.
The cytosol also contains large amounts of macromolecules, which can alter how molecules behave, through macromolecular crowding.
EXTRACELLULAR COMPARTMENT
Extracellular fluid (ECF) or extracellular fluid volume (ECFV) usually denotes all body fluid outside of cells, the extracellular fluid can be divided into two major sub compartments, interstitial fluid and blood plasma. The extracellular fluid also includes the transcellular fluid; making up only about 2.5% of the ECF
Interstitial compartment
Consists of all the bits of fluid which lie in the interstices of all body tissues. This is also a ‘virtual’ fluid (ie it exists in many separate small.
The ISF bathes all the cells in the body and is the link between the ICF and the intravascular compartment.
Oxygen, nutrients, wastes and chemicals -all pass through the ISF. ISF has the compositional characteristics of ECF
Lymph is considered as a part of the ISF. The lymphatic system returns protein and excess ISF to the circulation.
Blood plasma as a body fluid
Plasma is the only major fluid compartment that exists as a real fluid collection all in one location. It differs from ISF in its much higher protein content and its high bulk flow (transportfunction).
Blood contains suspended red and white cells so plasma has been called the ‘interstitial fluid of the blood’.
The fluid compartment called the blood volume is interesting in that it is a composite compartment containing ECF (plasma) and ICF (red cell water).
It contains dissolved proteins (major proteins are fibrinogens, globulins and albumins), glucose, clotting factors, mineral ions (Na+, Ca++, Mg++, HCO3-, and Cl- etc.), hormones, and carbon dioxide (plasma being the main medium for excretory product transportation)
The fluid of bone & dense connective tissue
The fluid of bone & dense connective tissue is significant because it contains about 15% of the total body water.
This fluid is mobilized only very slowly and this lessens its importance when considering the effects of acute fluid interventions.
Measurement of Body fluid volumes
The actual values shown are
calculated for a
70 kg man. TBW = 0.6 x
Body Weight
Relationship between the volumes of major fluid compartments
Example
Volume of distribution of injected material).
The volume of distribution is equal to the amount injected
(Minus any that has been removed from the body by metabolism or excretion during the time allowed for mixing) divided by the concentration of the substance in the sample.
150mg of sucrose injected into a 70 kg man plasma sucrose level after mixing is 0.01mg/ml.
10 mg has been excreted or metabolized during mixed period.
Volume distribution =
150mg – 10mg = 14,000 ml
0.01mg/ml
Inject fluids that will stay in only one compartment and then calculating the volume of fluid is which the test substance is distributed.
14,000ml is the space in which the sucrose was distributed – called sucrose space.
Markers for measurement must share following qualities: (ie dye or solution used to measure a compartment or fluid volume)
Ø They are measurable
Ø They remain in the compartment being measured
Ø They do not alter water distribution
Ø They are not toxic TBW is measured by triated water (tritium oxide)
Ø ECF volume is measured by inulin that is proportionally distributed between plasma volume and interstitial volume
Ø Plasma volume can be measured either by radioactive albumin or by Evans blue. These substances neither leave the vascular system nor penetrate the erythrocytes
Ø Interstitial volume cannot be measured directly, because no substance is distributed exclusively within this compartment.
Ø Therefore, the interstitial fluid volume is determined as the difference between ECF volume and plasma volume.
Ø ICF volume cannot be measured directly by dilution because no substance is confined exclusively to this compartment.
Ø The ICF volume is obtained by subtracting the ECF volume from the TBW
Water
is a major constituent of the human body, accounting for 60% of the body
weight. Body water is divided between two major compartments:
Water
moves freely and rapidly between the various body fluid compartments.
Two
forces determine this movement: hydrostatic pressure and osmotic pressure.
Intracellular
fluid (ICF) and extracellular fluid (ECF). Two thirds of the water is in the
ICF, and one third is in the ECF. Osmotic pressure gradients between ICF and
ECF drive water movement between these compartments.
The
tonicity of a solution is related to its effect on the volume of a cell.
Solutions that do not change the volume of a cell are said to be isotonic.
A
hypotonic solution causes a cell to swell, and a hypertonic solution causes a
cell to shrink.
The
intracellular fluid (ICF) compartment is the larger compartment and contains
approximately two thirds of the total body water.
The
remaining one third is contained in the extracellular fluid (ECF) compartment.
The
ECF is divided into a vascular compartment (plasma) and an interstitial fluid
compartment. Sodium is the major cation of ECF.
Potassium is the major cation of the ICF. This
asymmetric distribution of Na+ and K+ is maintained by the activity of the Na+,
K+-ATPase.
Topic objective
Identify units of measuring solute
concentration Measuring physiologically important
substances In considering the effects of various
physiologically important substances and the interactions between them, the
number of molecules, electrical charges, or particles of a substance per unit
volume of a particular body fluid, are often more meaningful than simply the
weight of the substance per unit volume. Concentrations
are expressed in Ø Moles Ø Equivalents Ø Osmoles Mole A mole is the gram-molecular weight of a substance, i.e, the
molecular weight of the substance in grams 1mole
= 6 x 10 23 molecules. Millimole
(mmol)is 1/1000 of a mole. Micromole
(umol) is 1/1000, 000 of a mole. Molarity v Molarity is the amount of a substance relative to its molecular
weight. For example, glucose has a molecular weight of 180 g/mole. v For uncharged molecules, such as
glucose and urea, concentrations in the body fluids are usually expressed in
terms of molarity. v Because many of the substances of
biologic interest are present at very low concentrations, units are more
frequently expressed in the millimolar range (mmol/L or mM). Equivalents v The concept of electrical
equivalence is important in physiology because many of the important solutes in
the body are in the form of charged particles. v The concentration of solutes, which
normally dissociate into more than one particle when dissolved in solution
(e.g., NaCl), is usually expressed in terms of equivalence. v One equivalent (eq) is 1mole of an
ionized substance divided by its valence. v One equivalent (eq) is 1mole of an
ionized substance divided by its valence. v e.g.
1eq Na+ = 23g/1 = 23g but the Eq of CA2+ = 40g/2= 20g v Millequivalent (meq) is 1/1000
of 1 eq Osmoles v When dealing with the osmotically
active particles, the amounts of these particles are usually expressed in
osmoles. v One (1) osmole equals the molecular
weight of the substance in grams divided by the number of freely moving
particles each molecule liberates in solution. v The milio osmole (mosm) is 1/1000
of 1 osmole Using millequivalents(MEQ) to
measure distribution of electrolytes in the various body fluids compartment
Shows the following – MEQ /LITER
OF WATER Blood plasma: •
Non electrolyte •
Sodium (Na+) 152 •
HCO3 – 27 •
Protein – 16 •
CL – 113 Interstitial fluid: •
Non-electrolyte •
Na+ - 143 •
Cl
- 117 •
HCO3 - 27 •
Protein - 2 Intracellular fluid •
Potassium K+-
157 •
PO43 - 113 •
HCO3 - 10 •
Na+
- 14 •
Protein - 74 •
Mg2+
- 26 •
Non electrolytes.
Topic objective
1 Explain the terms: Hydrostatic pressure; Oncotic pressure and Colloid Osmotic pressure, as they relate to movement of substances.
2 Describe mechanisms of transport
Hydrostatic pressure.
• Hydrostatic pressure is generated by the systolic force of the heart. It pushes water out of the capillaries. The water potential is created due to the ability of small solutes to pass through the walls of capillaries. This buildup of solutes induces osmosis.
• The water passes from a high concentration (of water) outside of the vessels to a low concentration inside of the vessels, in an attempt to reach an equilibrium.
• The osmotic pressure drives water back into the vessels. Because the blood in the capillaries is constantly flowing, equilibrium is never reached.
• The balance between the two forces differs at different points on the capillaries.
• At the arterial end of a vessel, the hydrostatic pressure is greater than the osmotic pressure, so the net movement favors water and other solutes being passed into the tissue fluid.
• At the venous end, the osmotic pressure is greater, so the net movement favors substances being passed back into the capillary.
• This difference is created by the direction of the flow of blood and the imbalance in solutes created by the net movement of water favoring the tissue fluid.
Prevention of fluid build up
• To prevent a buildup of tissue fluid surrounding the cells in the tissue, the lymphatic plays a part in the transport of tissue fluid.
• Tissue fluid can pass into the surrounding lymph vessels, and eventually ends up rejoining the blood.
Sometimes the removal of tissue fluid does not function correctly, and there is a build-up. This can cause swelling, often around the feet and ankles, which is generally known as edema. The position of swelling is due to the effects of gravity.Osmotic pressure
• The movement of water across cell membranes occurs by the process of osmosis. The driving force for this movement is the osmotic pressure difference across the cell membrane.
• Osmotic pressure is determined solely by the number of solute particles in the solution. It is not dependent upon such factors as the size of the solute particles, their mass, or chemical nature.
• To exert an osmotic pressure across a membrane, a solute must not permeate that membrane.
• Osmotic pressure differences between ECF and ICF are responsible for fluid movement between these compartments. Because the plasma membrane of cells contains water channels
Oncotic pressure
• Oncotic pressure is the osmotic pressure generated by large molecules (especially proteins) in solution.
• Capillary wall is the barrier between the plasma and the interstitial fluid.
• The apertures in the wall (junctions between the endothelial cells) are too small to permit plasma proteins and other colloids to pass through in significant quantities.
• The degree to which oncotic pressure influences capillary fluid movement depends on the permeability of the capillary wall to the protein molecules. The capillary wall therefore behaves like a membrane impermeable to colloids
• This exerts an osmotic pressure of about 25 mm Hg due to plasma Colloids called the oncotic pressure.(colloidal osmotic pressure )
• Filtration across the capillary membrane due to hydrostatic pressure in the vascular system is opposed by the oncotic pressure
• The protein that leaks across the capillary wall into the interstitium exerts an oncotic pressure and promotes the movement of fluid out of the capillary lumen.
Transport mechanisms
