BALANCE

•     Water occupies two main fluid compartments

–    Extracellular fluid (ECF)

•   Consists of two major subdivisions

–   Plasma – the fluid portion of the blood

–   Interstitial fluid (IF) – fluid in spaces between cells

–   Other ECF – lymph, cerebrospinal fluid, eye humors, synovial fluid, serous fluid, and gastrointestinal secretions

–    Intracellular fluid (ICF)

•   About two thirds by volume, contained in cells

•     Barriers

–    Plasma membrane

–    Blood vessel walls

•     Fluid balance

•     Electrolyte balance

•     Acid-base balance

Extracellular and Intracellular Fluids

•     Extracellular fluids

–   Sodium is the chief cation

–   Chloride is the major anion

•     Intracellular fluids have low sodium and chloride

–   Potassium is the chief cation

–   Phosphate is the chief anion

•     Sodium and potassium concentrations in extra- and intracellular fluids are nearly opposites

•     This reflects the activity of cellular ATP-dependent sodium-potassium pumps

Water Balance and ECF Osmolality

•     Homeostasis monitors ECF

•     Receptors monitor fluid/electrolytes indirectly

•     Active transport cannot move water

–    Water moves according to osmotic gradients

•     To remain properly hydrated, water intake must equal water output

–    Losses should equal gains

•     Water intake sources

–    Ingested fluid (60%) and solid food (30%)

–    Metabolic water or water of oxidation (10%)

•     Water output

–    Urine (60%) and feces (4%)

–    Insensible losses (28%), sweat (8%)

Regulation of Water and Solute Loss

•    NaCl loss

–  Hyponatremia

•  Renal insufficiency or an extraordinary amount of water ingested quickly can lead to cellular overhydration, or water intoxication

•  The resulting hyponatremia promotes net osmosis into tissue cells, causing swelling

•    Hypernatremia

–  Dehydration

•  Thirst center

Hormonal Control

•    Angiotensin II

•    Aldosterone

•    Atrial natriuretic peptide (ANP)

•    Antidiuretic hormone (ADH)

–  Osmoreceptors

Regulation of Water Intake: Thirst Mechanism

Electrolyte Balance

•     Electrolytes are salts, acids, and bases, but electrolyte balance usually refers only to salt balance

•     Electrolyte functions

•     Salts enter the body by ingestion and are lost via perspiration, feces, and urine

•     Individual electrolyte concentrations

–   Expressed in milliequivalents per liter (mEq/L), a measure of the number of electrical charges in one liter of solution

Acids and Bases

•     Acid – increases the [H⁺] of a solution

•     Base – decreases the [H⁺] of a solution

–   May form OH^- ion or absorb H⁺ ions

•     Strong acid/base

•     Weak acid/base

•     Each 1 change in pH = 10x change in [H⁺]

pH Control

•     Normal range: 7.35 – 7.45

–    Min/max = 6.8/7.7

^•         Control loss/gain of H⁺

•     Most hydrogen ions originate from cellular metabolism

–    Anaerobic respiration of glucose produces lactic acid

–    Transporting carbon dioxide as bicarbonate releases hydrogen ions

•     Ions must travel through the body

–    Buffer systems

–    Concentration of hydrogen ions is regulated sequentially by:

•   Chemical buffer systems – act within seconds

•   The respiratory center in the brain stem – acts within 1-3 minutes

•   Renal mechanisms – require hours to days to effect pH changes

Chemical Buffer Systems

•    Three major chemical buffer systems

–  Bicarbonate buffer system

–  Phosphate buffer system

–  Protein buffer system

•    Any drifts in pH are resisted by the entire chemical buffering system

Buffer Systems

•    Protein buffer system

–  ECF & ICF

Buffer Systems

•    Carbonic acid-bicarbonate

–  ECF only

–  Bicarbonate reserve

Buffer Systems

•    Phosphate buffer system

–  Mainly in ICF

^–      H₂PO₄^-

^–      HPO₄^2-

–  Phosphate reserve

Buffer Systems

•    Short-term solution

–  H⁺ has to be eliminated

•    Respiratory compensation

–  Volatile acid

•    Renal compensation

–  Nonvolatile acids

–  pH 4.0 – 4.5

Acid-Base Disorders

•     Acidemia/acidosis

–   Arterial pH drops below 7.35

•     Alkalemia/alkalosis

–   Arterial blood pH rises above 7.45

•     Compensation

–   Complete vs. partial

–   Respiratory

–   Renal

Acid-Base Disorders

•     Respiratory acidosis

–   The most common cause of acid-base imbalance

–   Occurs when a person breathes shallowly, or gas exchange is hampered by diseases such as pneumonia, cystic fibrosis, or emphysema

–   Hypercapnia

–   Acute

–   Chronic

•     Respiratory alkalosis

–   Hypocapnia

•   A common result of hyperventilation

Acid-Base Disorders

•    Metabolic acidosis

–  The second most common cause of acid-base imbalance

–  Lactic acidosis

–  Ketoacidosis

–  Kidney damage

–  Bicarbonate loss

Acid-Base Disorders

•    Metabolic alkalosis

–  Typical causes are:

•  Vomiting of the acid contents of the stomach

•  Intake of excess base (e.g., from antacids)

–  Alkaline tide