BIO 241 Fluid Dynamics

Name the three major buffer systems in body fluids.

carbonic acid - bicarbonate buffer system, phosphate buffer system, protein buffer system

What is the main component of all body fluids?

water

Identify the four general functions served by electrolytes.

1. Many are essential minerals 2. Because they are more numerous than non-electrolytes, they exert a far greater effect on water movement (osmosis) between body compartments. 3. They help maintain acid-base balance. 4. They carry electrical current, allowing the formation of action potentials in muscles and nerves, and the control of secretion of many hormones and neurotransmitters.

anti-diuretic hormone

ADH increases the permeability of the distal tubules and collecting ducts of the kidney to water, causing increased facultative reabsorption of water.

atrial natriuretic peptide

ANP decreased Na+ reabsorption, leading to decreased obligatory reabsorption and increased water loss in the urine (diuresis).

Aldosterone

Aldosterone increases Na+ reabsorption by the renal tubules, leading to increased obligatory reabsorption of water.

exhalation of CO2

Because of the relationship between CO2 and H+, alterations in respiratory rate affect changes in pH by changing the CO2 concentration of the body.

buffer systems

Buffer systems react quickly to bind H+ or OH- (hydroxide) ions to prevent drastic changes in pH.

The third major mechanism by which body fluid is maintained is through kidney excretion of hydrogen ions. How is this accomplished?

Distal tubules of the kidneys secrete hydrogen ions directly into the filtrate so that urine is acidified and the hydrogen ions are lost from the body. This is a normal process that occurs at some normal rate.

Describe the exchange of fluids between the interstitial and intracellular compartments. Why is there no net flow between the two compartments? What would result if there was a decreased sodium concentration in the body fluids?

Exchange of body fluids between the interstitial and intracellular compartments is accomplished primarily by osmosis. Under normal conditions the two compartments have the same osmotic pressures. Although the two compartments are drastically different in terms of their individual ionic compositions, the net flow of water between the two compartments is zero when the body is in fluid balance. Because Na+ is the principal extracellular cation and K+ the principal intracellular cation, these are the two ions most likely to effect changes in water balance between the two compartments. To understand this relationship, consider what happens when there is a decrease in [Na+] in the blood (excess sweating, vomiting, diarrhea, coupled with low Na+ intake or plain water intake.) The loss of Na+ causes the interstitial fluid osmotic pressure to decrease. As a result, the amount of water that normally leaves the intracellular compartment by osmosis in response to Na+ will be decreased. Since all of the other ions, and the effects they have on osmosis, remain the same, the net flow of water is now into the cell. This has two important ramifications for homeostasis. The net loss of water from the interstitium leads to decreased IFHP, leading to increased NFP, net loss of water from the plasma, decreased blood volume, decreased blood pressure, then circulatory shock. Net osmosis into the cell leads to overhydration. As a result, water intoxication occurs, producing neurological symptoms ranging from disorientation to convulsions to coma to death.

Describe the mechanism by which fluid intake is regulated by thirst.

Fluid intake is regulated by thirst. When water loss is greater than water gain, the resulting dehydration stimulates thirst in at least three ways. Dehydration causes decreased saliva production, resulting in dry mouth and throat. Tactile receptors in the mucosa relay nerve impulses to the thirst center of the hypothalamus, giving rise to a thirst sensation. Dehydration increases blood osmotic pressure, stimulating osmoreceptors in the hypothalamus, which in turn stimulate the hypothalamic thirst center. Dehydration decreases blood volume and thus blood pressure, causing the kidneys to activate the renin-angiotensin system. Angiotensin II directly stimulates the hypothalamic thirst center. As a result of these mechanisms the sensation of thirst, and the conscious desire to drink, are increased. One drinks, fluid increases, and normal fluid volume is restored. Initially, wetting the mouth and throat quenches thirst. But, the major inhibition of thirst results from stretch of the stomach and a decrease in the blood osmotic pressure.

kidney excretion of H+

H+ secretion from distal tubules of the nephrons directly into filtrate acidifies urine and removes the H+ form the body.

What is renal compensation?

If other mechanisms for acid - base balance fail and the rate of kidney excretion of hydrogen ions increases above normal, the process is called renal compensation. This is a much more permanent solution to hydrogen ion problems because the hydrogen ions are eliminated from the body.

What is a buffer system?

In a buffer system, a strong acid is converted to a weaker one, using a weak base, or a strong base, is converted to a weaker one using a weak acid. They function to prevent drastic changes in the pH of a body fluid.

Identify the various water avenues by which body water is lost.

Normally, daily water gain is equaled by loss. There are several avenues for water loss from the body: kidneys = 1,500 ml/day lungs = 300 ml/day skin = 500 ml/day feces = 200 ml/day Thus daily water loss equals about 2,500 ml

What separates the body fluid into compartments?

Selectively permeable cell membranes separate body fluids into the fluid compartments.

Identify the main differences between intracellular - extracellular

The intracellular fluid compartment differs greatly from the extracellular fluid compartments. In extracellular fluid, the most abundant cation is Na+. In the intracellular fluid, the most abundant cation is K+. In extracellular fluid the most abundant anion is Cl-, while in intracellular fluid the most abundant anions are proteins and phosphates (PO4--).

What is the main source of body water?

The main source of body water is from ingested liquids (1,600 ml/day) and foods (700 ml/ day) that have been absorbed by the gut from the diet. This water is called preformed water; it equals about 2,300 ml/day.

Identify the main differences between the interstitial fluid - plasma

The plasma contains many proteins anions (plasma proteins) while interstitial fluid has virtually none. Since normal capillary membranes are impermeable to proteins, plasma proteins do not leave the blood to enter the interstitial fluid. Plasma also contains slightly more Na+ and slightly less Cl- than interstitial fluid, but in all other aspects, the two fluid compartments are identical.

How is this separation of body fluid accomplished? What cells are involved?

The plasma membranes of: 1. individual body cells separate intracellular fluid from interstitial fluid, and 2. endothelial cells separate interstitial fluid from plasma. Although fluids are in constant motion between the three compartments, the volume of fluid in each compartment at any given time remains fairly constant. How can this be?

What is metabolic water?

The second source of body water is metabolic water, the water produced by the various chemical reactions of the body. It amounts to about 200 ml/day. Therefore, preformed water + metabolic water = 2, 500 ml/ day.

What is body fluid?

The term body fluid refers to the body water and all the dissolved substances within.

How is body fluid distributed in the body?

Two-thirds of body fluid is located within the cells and is termed the intracellular fluid. The remaining one-third of body-water, the extracellular fluid (ECF), includes all other body fluids. About 80% of ECF is interstitial fluid and about 20% of ECF is plasma.

respiratory compensation

Use of respiratory system to correct body fluid pH

Name the three compartments of the body.

Using these designations, body fluid is discussed in terms of its location within one of the three main compartments of the body: intracellular, interstitial, and plasma.

How much of total body weight is water? What determines the exact percentage?

Water makes up 45 - 75% of total body weight. Water content is dependent upon age, body size, and gender. Infants have the highest percentage. This decreases with age and is lowest with geriatrics. Lean people have a greater percentage than overweight people, since fat is virtually water-free. Males (60% body water) have more body water that females (55%) do, since females have a greater amount of subcutaneous fat.

What does the term fluid balance mean?

When you say fluid balance, you mean that the required amount of water is present and proportioned among the compartments appropriately.

Are there other ECFs?

Yes, other extracellular fluids include lymph, CSF, synovial fluid, humors of the eye, endolymph, and perilymph, serous fluids, and glomerular filtrate.

Name the three hormones that adjust fluid output

anti-diuretic hormone aldosterone atrial natriuretic peptide

Electrolytes

are those compounds with at least one ionic bond that dissociate freely when dissolved in water (NaCl, KCl, CaPO4). Electrolytes have a greater effect on osmosis (water movement between com-partments) than do nonelectrolytes because water moves to an area with a greater total number of particles in solution. Consider the following: C6H12O6 in water = C6H12O6 NaCl in water = Na+ and Cl- CaCl2 in water = Ca++ and Cl- and Cl- Which of these three compounds will exert the greater effect on osmosis? Why? CaCl2 in water = Ca++ and Cl- and Cl-. CaCl2 dissociates into three ionic components and therefore has 3 times the effect of glucose and half again the effect of NaCl. Just as important, once the electrolyte dissociates, its ions attract other ions of the opposite charge by creating an electrochemical gradient. For example: a Ca++ will attract 2 Cl- or a PO4--; a Na+ will attract a Cl- How would these ions attracted by the electrochemical gradient influence water movement? The attracted ions add to the osmotic gradient, resulting in increased water movement into the compartment with the greater ion concentration.

Nonelectrolytes

are those compounds with covalent bonds that therefore do not dissociate when dissolved in water (glucose, urea, creatinine).

Normally body fluid pH is 7.35 - 7.45. Name the three major mechanisms that maintain this range.

buffer systems, exhalation of CO2 kidney excretion of H+

Why does fluid balance mean water balance

but imply electrolyte (or solute) balance as well?, Osmosis is the means by which water moves from one compartment to another. As a result, the concentration of the solutes (mainly electrolytes) in the compartments is a major determinant of fluid balance. The two are inseparable.

Negative feedback situation of body fluid pH

controlled condition -- Homeostasis is disrupted by a decrease in body fluid pH (increased H+ concentration. receptors -- Chemoreceptors in the medulla detect the increased H+ concentration and generate nervous input into the respiratory center of the medulla. control center -- The inspiratory area of the respiratory center integrates the input and increases the rate of nerve impulse output to the inspiratory muscles. effectors -- In response to nerve impulses the diaphragm and external intercostal muscles contract more forcefully and more often (hyperventilation). return to homeostasis -- With an increased respiratory rate there is a loss of carbon dioxide and therefore H+, leading to an increase in body fluid pH and a return to homeostasis.

The exchange of body fluids and solutes between plasma and interstitial fluid occurs across capillary membranes in three ways. Name them.

diffusion, vesicular transport, and bulk flow (most important) Bulk flow is dependent on four pressures that determine the net filtration pressure (NFP). How is NFP determined at the capillary? NFP = (BHP + IFOP) - (BCOP + IFHP) = +10 mm Hg (net filtration on the arterial end) = -9 mm Hg (net reabsorption on the venous end)

Describe the role of the respiratory system in control of body fluid pH? What is respiratory compensation? Breathing plays a most important role in the control of acid-base balance. Remember the reaction?CO2 + H2O < -- > H2CO3 < -- > H+ + HCO3- In the tissues where carbon dioxide is abundant

the reaction is shifted to the right: CO2+ H2O H2CO3 H+ + HCO3- In the lungs where hydrogen ions are liberated from hemoglobin, the reaction is shifted to the left: CO2+ H2O H2CO3 H+ + HCO3- From an acid- base balance standpoint, increased respirations tend to cause a decrease in hydrogen ion concentration and therefore an increase in body fluid pH. The opposite is true for decreased respiration.