fluid balance

Facts:

  • 50-60% of our body is made up of water

  • Fluid composition changes with age

    • The amount decreases as you get older

  • Males tend to have more fluid on their bodies than females

    • The more muscle mass a person has, the more fluid they have in their body.

    • Men tend to have more muscle 

  • The more adipose tissue a person has, the less fluid they have.

    • Women tend to have more fat.


Identify the body's fluid compartments:

  • Intracellular

    • The fluids inside the cell 

    • Found in the cytoplasm of cells

    • ⅔ of all fluids in our bodies is inside cells.

  • Extracellular

    • The fluids outside of cells

    • Can be divided into 

      • Interstitial fluid 

        • The majority of extracellular fluid is located here

      • Blood plasma


Discuss the composition of body fluids, and explain the osmotic power of each:

  • Water 

    • Solution that dissolves/ionizes electrolytes. Universal solvent 

  • Nonelectrolytes 

    • Do not dissociate = do not form ions 

    • Examples: organic molecules such as glucose/lipids 

  • Electrolytes 

    • Do dissociate = do form ions 

    • Examples: proteins, acids, bases, inorganic salts

    • Creates major osmotic potential 


Composition of electrolytes 

  • The electrolyte composition of our body varies by location 

    • It will be different in each fluid compartment 

  • Patterns seen:

    • The composition of both types of fluid is basically the same for all things expect proteins.

      • There are more proteins in the plasma than in the interstitial fluid because proteins are too large to get out of circulation.

    • If there is a high concentration of an electrolyte in the extracellular compartment, it's going to be low in the intracellular compartment. (and vice versa)

  • There is a higher concentration of sodium outside the cell than inside, and higher potassium inside the cell than outside. 


  • Buffers:

    • Bicarbonate is a buffer which works outside the cell

      • Blood plasma and interstitial fluid have a decent amount of this. 

    • Phosphate buffer system works inside the cell

    • Protein is mainly an intracellular buffer, but also can work in the blood/extracellular 


List the factors that control fluid exchanges, and differentiate the roles each plays in driving exchange between plasma and interstitial fluid; between interstitial and intracellular fluids. 


Fluid movement 

  • Forces that move fluids:

 1. Osmotic gradient 

2. Hydrostatic pressure


  • Directions of fluid movement: 

    • Between plasma and interstitial fluid 

      • This movement is primarily dependent on hydrostatic pressure. 

    • Between interstitial fluid and intracellular fluid 

      • Primarily dependent on osmotic gradient 


List the routes by which water enters and leaves the body. Do the same for electrolytes and acids and bases.


Water balance:


  • Water intake is:

    • Ingestion: most food is moist with some water, drinking (most water comes from this).

    • Metabolic water: what we produce (oxidative reactions liberate water)


  • Equal to water output:

    • Vaporization: insensible water loss (when we breathe/exhale)

    • Perspiration: sensible water loss (sweat) 

    • Elimination: defecation (feces)

    • Vomiting: only sometimes

    • Urination: micturition (voiding)


  • Balance is maintained homeostatically. If something happens to increase water output, we will get a feedback mechanism that makes us want to intake more. 


Describe the mechanisms that regulate water input and output


Regulation mechanisms 

  • Increase in plasma osmolality (due to fluid loss) or decrease in blood volume promotes thirst.

    • These are closely linked

    • Detected by the hypothalamus and promotes the thirst mechanism

  • Real life example:

    • First sign of diabetes is a thirst that won’t go away (not regulating glucose because of lack of insulin), glucose levels get really high and increase concentration of blood.


  • Decrease in extracellular fluid osmolality, the brain decreases production of ADH.

    • ADH is responsible for opening up the aquaporins in the kidney tubules to allow more water to be reabsorbed. 

    • If there is decreased production of ADH, there is less water reabsorbed 

  • Large decreases in blood pressure cause an increase in ADH production 

  • We want to increase the blood pressure by putting more water into the blood. 


Electrolyte (salt) balance 

  • Electrolyte intake is…

    • Ingestion 

    • Metabolic production (breakdown of nucleic acid)

  • Equal to electrolyte output 

    • Perspiration 

    • Elimination 

    • Vomiting 

    • Urination 


Describe the importance of ionic sodium in fluid and electrolyte balance in the body.


  • Sodium

    • Most sodium is found in extracellular compartments 

    • Sodium is one of the most important electrolytes in maintaining water balance in our body.

    • Directs the movement of water 


Describe mechanisms involved in regulating sodium (and therefore water) balance.

Sodium regulation

  • Sodium is linked to blood volume because it’s  linked to blood pressure 

  • 90% of sodium is reabsorbed naturally 

    • 65% was immediately reabsorbed in the proximal convoluted tubule. 

    • The remaining amount was reabsorbed hormonally.  


Aldosterone 

  • The most important mechanism of regulation 

  • Aldosterone increases sodium reabsorption from the ascending limb of the Loop of Henle. Can also affect the distal convoluted tubule and collecting duct.

  • Produced when: 

    • Low blood pressure activates production of renin by JG cells located in the afferent arteriole. Renin activates angiotensinogen to angiotensin, which acts on the adrenal cortex. The adrenal cortex releases aldosterone which causes increased reabsorption and reduces urinary output 


Cardiovascular baroreceptors 

  • Receptors that monitor blood pressure around the heart

  • If pressure of blood going to the heart increases, receptors send a signal to the brain 

  • The brain then sends a signal which decreases sympathetic stimulation to the kidneys. This causes the afferent arteriole to dilate, which increases filtration rate in the glomerulus.

  • Sodium is moved out of the bloodstream and into the tubule/filtrate, which then causes water to stay in the tubule and be eliminated as urine.

  • Blood pressure goes down - the baroreceptors are no longer stimulated and the cycle is shut off. 



ANP 

  • When blood volume or blood pressure is high, it causes stretch on the walls of the atria of the heart. This stretch stimulates the production of ANP which causes vasodilation in the kidneys. This leads to an increase in filtration and water loss through urination.

  • ANP also causes a reduction in ADH and renin, so subsequently reduces aldosterone



Estrogen 

  • Causes an increase in sodium reabsorption

  • Water follows sodium and is retained 



Progesterone 

  • Causes an decrease in sodium reabsorption 



Glucocorticoids 

  • Causes an increase in sodium reabsorption

  • Linked to stress and causes increased blood pressure 

Acid-base balance 

  • pH is closely regulated in the body

    • This is important because enzymes have an optimal pH under which they function, and enzymes are the biochemical catalysts that keep us alive.

    • Most enzymes function at relatively neutral pH

      • There are exceptions, such as pepsin, which requires a very acidic pH to be active.

  • Blood is close to neural but slightly on the basic side. 

    • Ranges between 7.35-7.45

  • The pH of the intracellular fluid compartment is about 7 

    • Different due to the amount of carbon dioxide and other cellular metabolites present inside of cells. 

  • pH is a measurement of hydrogen ion concentration in a solution.


Sources of H+

  • Dietary 

    • Most of the food we eat and liquids we consume are generally on the acidic side of the pH scale

  • Metabolic 

    • Produced through break down processes such as dehydration synthesis reactions. 



Distinguish between acidosis and alkalosis, both respiratory and metabolic 


Abnormalities 

  • Acidosis: <7

  • Alkalosis: >7 


  • There are two types of acidosis and alkalosis:

    • Respiratory involves: shift of CO2 

    • Metabolic involves: shift of any acid other than CO2 (usually bicarbonate) 


  • Respiratory acidosis:

    • Caused by: changes in breathing (slow rate of breathing → CO2 accumulates in blood)


  • Metabolic acidosis:

    • Caused by: too much acid other than CO2 


  • Respiratory alkalosis:

    • Caused by: changes in rate of breathing (increase rate of breathing → too much CO2 leaves)


  • Metabolic alkalosis:

    • Too little acid other than CO2 


Regulation 

  • Chemical buffer systems 

    • Buffers function as proton acceptors or proton donors 

      • Bind to or release hydrogen ions 

      • Changes pH temporarily 

  • Physiological buffer systems 

    • Changes in breathing rate 

    • Changes pH permanently 

    • Slower than chemical buffer systems, but have twice the buffering power. 

      • Physiological buggers are twice as effective as all chemical buggers combined.

  • Renal mechanisms

    • Works on the acids other than carbon dioxide 

    • Changes pH permanently 



List the three major chemical buffer systems and describe how they resist pH change. 

Chemical buffer systems (temporary fix)

  • Bicarbonate:

    • Happens in the bloodstream (extracellular)

    • HCO3 can accept or donate hydrogen to offset changes in pH 

  • Phosphate:

    • Intracellular 

    • If phosphate get in solution → phosphoric acid = dissociates = phosphate ion 

    • Accepts or liberate hydrogens 

    • ATP is produced inside cells 

  • Proteins:

    • Most important buffer 

    • Amphoteric (can function as acid or base)

    • In both intracellular and extracellular fluid compartments 

    • Can donate/accept H+ ions = acts as acids/bases 

    • Examples: albumin, hemoglobin, myoglobin (contractile proteins), intracellular proteins 



Describe the influence of the respiratory system an acid-base balance 

Physiological buffer systems (permanent fix)

  • Respiratory mechanisms

    • If we have an increase in blood pH, this promotes a decrease in respiration. 

      • This leads to: respiratory acidosis 

    • If we have a decrease in blood pH, we increase respiration 

      • This leads to: respiratory alkalosis 



Describe how the kidneys regulate hydrogen and bicarbonate ion concentrations of the blood. 

Renal mechanisms (permanent fix)

  • Bicarbonate reabsorption (increase)

    • Moves bicarbonate back into the system, which affects buffering 

  • Bicarbonate synthesis (increase)

    • The kidneys can make new bicarbonate which is then reabsorbed in the blood.

    • This increases the buffering capacity of the blood to change pH.

  • Bicarbonate excretion 

    • If we have too much, we get rid of it