Water Balance

Body FluidsA. Body water content

1. Water is the largest single component of the bodyA) Early embryo = 97% waterB) Newborn infant = 77% waterC) Adult male = 60% waterD) Adult female = 54% waterE) Elderly adult = 45% water

Water Balance

B. Fluid compartments1. Intracellular fluid compartment (ICF) = 25L2. Extracellular fluid compartment (ECF)

=15L A) divided into two sub-compartments:

1) Plasma – fluid portion of blood = 3L2) Interstitial fluid – the fluid in the spaces between cells = 12L

3. Other = lymph, cerebrospinal fluid, humors of the eye, synovial fluid, serous fluids, and secretions of the gastrointestinal tracts

Water Balance

C. Major Components of Body Fluids1. Water – 79-85%2. Proteins – 10-20%3. Lipids – 2% (adipose tissue itself

has ~95%)4. Carbohydrates – 1%5. Electrolytes – substances that

dissociate into ions when dissolved in water

Water Balance

A) Major Cations1) Na+, K+, Ca++

B) Major Anions1) Cl-, HCO3

-, HPO4-

D. Comparison of ECF & ICF

ECF ICF

pH 7.35-7.45 7.35-7.45

Nutrients

glucose higher used up immediately

fatty acids higher used up immediately

amino acids higher used up immediately

Gases

oxygen higher lower

carbon dioxide lower higher

Ions

cations Na+ & Ca++ K+

anions Cl- & HCO3- HPO4

-

Water Balance

Water BalanceA. To maintain proper water volume,

the body must balance water losses (both obligatory and nonobligatory) with water gains over the course of the day1. To remain properly hydrated, water intake must be equal to water output (~2500ml/day)

Water Balance

B. Water Output1. Water output (loss) can be divided

into 2 categories:A) Nonobligatory

1) water loss beyond that created by normal homeostatic events; not necessary to maintain homeostasis, and may or may not be avoidable

Water Balance

2) includes excess perspiration due to exercise, strenuous work, etc.; also includes vomiting and diarrheal illnesses

3) vary from person to person, hard to measure, and are generally not calculated into a person’s “normal” daily water loss

Water Balance

B) Obligatory 1) water loss created by normal

homeostatic events; necessary to maintain homeostasis, and are unavoidable and necessary to maintain life

2) fall into 1 of 2 categories:a) Insensible losses

i) Skin (16%)ii) Lungs (12%)

Water Balance

b) Sensible lossesi) Urine (60%)ii) Sweat (perspiration) (8%)iii) Feces (4%)

Water Balance

3) The kidneys are responsible for our largest obligatory water loss each daya) They must produce at least a small

amount of urine each day because:i) They must remove unnecessary

blood solutes to maintain normal blood homeostasis

ii) They must then flush those solutes out of the body in water (as opposed to in solid form)

Water Balance

b) Failure to do so would result in improper blood composition which, in turn, could lead to imbalances/disease/death of other tissues in the body

c) Beyond the homeostatic minimum, the solute concentration and volume of urine excreted depend on fluid intake, diet, and water loss via other mechanisms

Water Balance

C. Water Intake1. Varies greatly from person to

person, and is often dependent on diet, lifestyle, activity level, etc.

2. Major sources of water intake:A) Liquids (60%)B) Solid foods (30%)C) Metabolic water (10%)

Water Balance

3. Regulation of Water Intake: thirst mechanismA) Increased plasma osmolarity (high

solutes) or decreased plasma volume triggers the thirst mechanism, which is mediated by hypothalamic osmoreceptors

B) When osmoreceptors lose water by osmosis to a hypertonic ECF, the hypothalamic thirst center is stimulated motivating the individual to drink

Water Balance

C) Thirst is inhibited by distention of the GI tract by ingested water and then by osmotic signals1) May be dampened before the body

needs for water have been met

Water BalanceHomeostatic Imbalances

A. Dehydration – water loss exceeds water intake over a period of time1. May result from hemorrhage, severe

burns, vomiting, diarrhea, profuse sweating, water deprivation, or diuretic abuse

B. Hypotonic Hydration – excessive water build up in the cells causing them to swell1. Particularly damaging to neurons

Water Balance

2. May result from excessive water intake in a short period of time or renal insufficiency

C. Edema – accumulation of fluid in the interstitial spaces, leading to tissue swelling1. May result from increased blood

pressure and capillary permeability, hormones, blockage of the lymphatic vessels, or low plasma proteins as a result of glomerulonephritis, malnutrition, or liver disease

Acid-Base Balance

Acid-Base BalanceA. Recall the definitions of acids and

bases:1. Acids (pH 1-6.9) – release H+ when in solution; often called hydrogen donors

2. Bases (pH 7.1-14) – release OH- when in solution; often called hydrogen acceptors

B. The homeostatic pH range of arterial blood is 7.35 to 7.451. Higher pH = alkalosis 2. Lower pH = acidosis

Acid-Base BalanceC. Abnormalities of Acid-Base Balance

1. Respiratory acidosis is the result of an increase in CO2 in the blood

A) may be caused by hypoventilation (for any reason), when there is airway obstruction (ex. asthma), or due to alveolar dysfunction (ex. pulmonary edema)

B) Increased CO2 = increased H+ = decreased pH

Acid-Base Balance

2. Respiratory alkalosis is the result of a decrease in CO2 in the blood

A) May be caused by hyperventilation (for any reason) or mechanical ventilation

B) Decreased CO2 = decreased H+ = increased pH

Acid-Base Balance

3. Metabolic acidosis is due to a decrease in HCO3

- which lowers pH

A) May be caused by excessive alcohol consumption, prolonged diarrhea, renal dysfunction, hyperkalemia

B) Decreased HCO3- = decreased pH

Acid-Base Balance

4. Metabolic alkalosis is due to an increase in HCO3

- which increases pH

A) May be caused by excessive vomiting, hypokalemia, or excessive NaHCO3 (sodium bicarbonate; ex baking soda & some antacids) consumption

B) Increased HCO3- = increased pH

Acid-Base Balance

D. Chemical Buffering Systems 1. Work by replacing a strong acid with

a weak one or a strong base with a weak one to minimize the pH change in the body fluid

2. Responsible for immediate changes to pH

3. 3 examples

Acid-Base Balance

A) Bicarbonate buffer system – ECF; utilizes NaHCO3 (sodium bicarbonate) and H2CO3 (carbonic acid)

1) NaHCO3 functions as a weak base

2) H2CO3 functions as a weak acid

Acid-Base Balance

3) when a strong acid is added to the solution, NaHCO3 dissociates to form HCO3 and Na+

a) HCO3 binds with excess H+ create H2CO3 (weak acid) eliminating large amounts of H+ from the solution and preventing a drastic drop in pH

Acid-Base Balance

4) when a strong base is added to the solution, H2CO3 dissociates to form HCO3

- and H+

a) HCO3 binds with the Na+ to form NaHCO3 (weak base) and preventing a drastic rise in pH

b) H+ binds with excess OH- to create H2O eliminating large amounts of OH- from the solution and preventing a drastic rise in pH

Acid-Base Balance

B) Phosphate buffer system – urine & ICF; utilizes Na2HPO4 (disodium monohydrogen phosphate) and NaH2PO4 (sodium dihydrogen phosphate)1) Na2HPO4 functions as a weak base

2) NaH2PO4 functions as a weak acid

Acid-Base Balance

3) when a strong acid is added to the solution, Na2HPO4 dissociates into NaHPO4 and Na+

a) NaHPO4 binds with excess H+ to create NaH2PO4 (weak acid)

4) when a strong base is added to the solution, NaH2PO4 dissociates into NaHPO4 and H+

a) NaHPO4 binds with the Na+ to form Na2HPO4 (weak base)

b) H+ binds with excess OH- to create H2O

Acid-Base Balance

C) Protein buffer system – plasma & ICF; utilizes carboxyl and amine side groups on amino acids1) Most abundant chemical buffering

system in the body

2) Utilizes the carboxyl group or amine group from an amino acid

Acid-Base Balance

a) alkalosis – rising pH (decreasing H+) results in the release of H+ from -COOHi) causes H+ levels to rise = decreased

pHb) amine – dropping pH (increasing H+)

causes excess H+ to bind to NH2 NH3

i) causes H+ levels to drop = increased pH

Acid-Base Balance

E. Physiological Buffering Systems1. Respiratory Control

A) responsible for minute-to-minute changes in pH

B) Utilizes bicarbonate reaction1) recall CO2 + H2O H2CO3 HCO3

- + H+

C) Driven by CO2 levels

Acid-Base Balance

1) Decreased pH (acidosis) causes increased ventilation; pushes the reaction to the left (decreased CO2 = decreased H+ = increased pH)

2) Increased pH (alkalosis) causes decreased ventilation; pushes the reaction to the right (increased CO2 = increased H+ = decreased pH)

Acid-Base Balance

2. Renal ControlA) The kidneys provide the major long-

term mechanism for controlling acid-base balance

B) In addition, metabolic acids (phosphoric, uric, lactic, and keto) can only be eliminated by the kidneys

C) Works by creating/reabsorbing or secreting (excreting) HCO3

-

Acid-Base Balance

D) Utilizes bicarbonate reaction1) Tubule cells are impermeable to HCO3

on their tubule borders but not on their vascular borders

a) Therefore, HCO3- is continually lost

in urineb) Blood HCO3

- levels are controlled by the bicarbonate reaction within the tubule cells

Acid-Base Balance

i) To counteract acidosis, HCO3- is

produced and reabsorbed resulting in more H+ secretion

ii) To counteract alkalosis, HCO3- is

produced and secreted resulting in more H+ reabsorption