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PHGY 216 Final Study Set Exam
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PHGY 216 Final Study Set ExamWater Balance - Answer - Every cell in the body requires a well-regulated environment
in order to survive and function
- Body water plays a large role in creating this environment, which is comprised of
intracellular and extracellular fluid
- When we talk about the major body compartments for water (fluid) there are three key
pools: ICF, plasma, and interstitial fluid, which remain distinct due to the presence of
"barriers" between them
Intracellular Fluid (ICF) - Answer This is the fluid within the cells and comprises about
two thirds of total body fluid
Extracellular Fluid (ECF) - Answer - This is the fluid surrounding the cells which includes
the plasma, the interstitial fluid, lymph, and transcellular fluid (such as cerebrospinal
fluid) and comprises about one third of total body fluid
- The plasma is about one fifth of the ECF, the interstitial fluid is about four fifths of the
ECF, and the lymph and the transcellular fluid are considered negligible
Transcellular Fluid - Answer The portion of total body water contained within epithelial
lined spaces
Barriers Between Body-Fluid Compartments - Answer - Several barriers separate the
body-fluid compartments
- This limits the movement of water and solutes between the various compartments to
differing degrees
Barriers Between the Plasma and Interstitial Fluid - Answer - The plasma and the
interstitial fluid are separated by the blood vessel walls
- At the level of the capillaries, water and everything else in the plasma (except proteins)
can freely exchange with the interstitial fluid
- Because of this, the composition of the plasma and the interstitial fluid are essentially
identical, except for the plasma protein
- Consequently, any change in one of these compartments is quickly reflected in the
other compartments
Barriers Between the Intracellular Fluid and the ECF - Answer - This barrier is the
plasma membrane that surrounds each cell in the body
- The ICF contains proteins that do not exchange with the ECF
- There is an unequal distribution of ions across this barrier because the barrier does
,not allow the passive movement of either ICF or ECF constituents across the plasma
membrane, preventing them from equilibrating through diffusion
ECF Volume and Osmolarity - Answer - All exchanges of water and other constituents
between the ICF and the external world are dependent upon the ECF
- Even though cells tightly regulate their own ICF, it can be said that overall control of
fluid balance is dependent upon regulating the ECF
- In order to maintain fluid balance in the body the ECF volume and osmolarity are
regulated
ECF Volume - Answer - This is closely regulated to maintain blood pressure
- The maintenance of salt balance is important in the long-term regulation of ECF volume
ECF Osmolarity - Answer - This is closely regulated to prevent the swelling or shrinkage
of cells
Control of ECF Volume - Answer - ECF volume directly influences blood pressure by
changing plasma volume
- Increasing ECF volume will increase plasma volume ,and thus increase arterial blood
pressure
- Consequently there are mechanisms in place to adjust blood pressure until the ECF
volume is returned to normal values
- There are short term control factor including the baroreceptor reflex and fluid shifts
- There are long term control factors including fluid input and output
The Baroreceptor Reflex - Answer - Baroreceptors are mechanoreceptor that are
located in the carotid artery and the aortic arch (areas within major arteries of the
body), and they detect changes in arterial blood pressure
- Through the effects of the autonomic nervous system on the heart and blood vessels,
teh baroreceptor reflex regulates blood pressure
- When pressure falls too low, cardiac output and total peripheral resistance will
increase to raise blood pressure
- When blood pressure rises above normal, both decrease to reduce blood pressure
Total Peripheral Resistance - Answer - The resistance to blood flow due to the
constriction of blood vessels
- Higher total peripheral resistance leads to increased blood pressure
Cardiac Output - Answer The amount of blood pumped by the heart per minute
,Fluid Shifts - Answer - A decrease in plasma volume can temporarily be compensated
for by a shift of the fluids out of the interstitial compartments to the plasma
- The opposite is also true, an increase in plasma volume can cause fluid to shift to the
interstitial compartment
Fluid Input/Output - Answer - The short term control mechanisms are only temporary
and can only compensate for fairly minor changes in ECF fluid volumes
- Long-term regulation of blood pressure is a primary function of the kidneys and the
thirst mechanism, controlling fluid output and input, respectively
- Control of urine output by the kidneys is critical for long-term regulation of blood
pressure
Control of Salt - Answer - Sodium, and the anions that are associated with it (mainly
chloride), account for more than 90% of the ECF's solutes
- Whenever salt is transported across a membrane, water follows due to osmosis
- Therefore, by controlling salt levels, ECF volume is being controlled
- To maintain salt balance, salt input must equal salt output
Salt Input - Answer - Regulation of salt input is poor as it is dependent upon dietary salt
- It is necessary on a daily basis to replace the salt lost in the feces and sweat, which is
about 0.5g/day for normal activity levels (higher activity levels will increase the amount
lost in sweat)
- The average input for Canadians is around 3.5 g/day
Salt Output - Answer - The excess salt must be eliminated, which occurs in the kidneys
- The three pathways to eliminate salt are in the feces, sweat, and the kidneys
- The kidneys have the greatest role in salt elimination
Control of ECF Osmolarity - Answer - Water moves down its concentration gradient until
the osmotic pressure across a membrane is equalized
- Because of this, regulating osmolarity is very important in preventing changes in cell
volume
- When there is a decrease in ECF water the osmolarity increases and the ECF is
hypertonic
- Since there is normally a balance in the osmolarity of the ICF and ECF, hypertonicity of
the ECF causes water to move out of the cells into the ECF until the osmotic pressure is
equalized
, - This loss of water from cells can cause them to shrink
- When there is an increase in ECF water, the osmolarity would decrease and the ECF
would become hypotonic
- This would result in water moving into the cells until the osmotic pressures were
equalized and this movement would cause the cells to expand
- If the ECF was very hypotonic, the amount of water moving into the cells would cause
them to burst
Osmolarity - Answer - Defined as a measure of the concentration of a particular solute in
solution
- A high osmolarity means that there is more solute, and therefore less water, in solution
Hypertonic - Answer A hypertonic solution is one in which the concentration of solutes
within that solution is greater than that of another solution that is separated by a
membrane
Hypotonic - Answer A solution that has a lower osmotic pressure than the surrounding
cells
Hypotonicity - Answer - Hypotonicity of the ECF is usually associated with
overhydration, or excess free water, and has three major causes: renal failure, rapid
water ingestion, and over secretion of vasopressin
- The osmolarity of the ECF must be regulated to prevent these undesirable shifts of
water into or out of the cell
Renal Failure - Hypotonicity - Answer These individuals are not able to produce a
concentrated urine
Rapid Water Ingestion - Hypotonicity - Answer This can occur in healthy individuals if
they drink volumes of water in excess of what the kidneys can deal with in a timely
manner
Over Secretion of Vasopressin - Hypotonicity - Answer Vasopressin promotes water
retention
Hypertonicity - Answer - Hypertonicity of the ECF, the excessive concentration of ECF
solutes, is usually associated with dehydration and has three major causes:
- Insufficient water intake, or not drinking enough
- Diabetes insipidus, which involves a deficiency in vasopressin
- Excessive water loss due to heavy sweating during extreme exercise, prolonged bouts
of vomiting, or diarrhoea
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