Buffers and Fluid Compartments in Physiology

Buffers

• Buffers resist changes in pH by converting strong acids or bases into weak ones
• Two categories: chemical buffers and physiological buffers
• Chemical buffers include: bicarbonate buffer system, phosphate buffer system, and protein buffer system
• Physiological buffers use the respiratory and urinary systems to alter the output of acids, bases, or CO2.
• Not all buffer systems begin working at the same time. The order is: chemical buffers, respiratory system, then renal system.
• Maintaining the body's pH range of 7.35 to 7.45 depends on a precise ratio of bicarbonate ions to carbonic acid
• An excess of carbonic acid causes acidosis
• An excess of bicarbonate causes alkalosis

Fluid Compartments

• Intercellular fluid (ICF) - Most of the body's water (about 65%) resides inside the cells
• Extracellular fluid (ECF) - 35% of body's water is outside the cells
• Interstitial Fluid - Extracellular fluid includes the fluid between the cells inside tissue
• ICF and ECF continually mingle
• If the concentration of electrolytes of tissue fluid rises, water moves out of the cells and into the tissue
• If the osmolarity of tissue fluid falls, water moves out of the tissue and into the cells

Balancing Water Gain and Loss

• Normally the amount of water gained and lost by the body through the course of the day is equal
• An adult gains and loses about 2500 mL of fluid each day
• Most fluid intake occurs through eating and drinking
• Cells produce water as a byproduct of metabolic reactions (Metabolic water)
• Fluid is lost through the kidneys as urine, the intestines as feces, the skin by sweat and diffusion, and the lungs through expired air

Regulation of Intake and Output

• The body uses mechanisms that adjust fluid intake as well as urine output
• When total body water declines, physical changes stimulate the thirst center in the hypothalamus, salivation decreases causing dry mouth and thirst, and water consumption occurs
• To decrease urine output: physical changes stimulate the hypothalamus, ADH prompts the collecting ducts of the kidneys to reabsorb more water and produce less urine
• The rate of fluid loss slows until water is ingested

Disorders of Water Balance

• Three types of disorders: fluid deficiency, fluid excess, and fluid accumulation
• Fluid deficiency results when output exceeds intake
• Two types of fluid deficiency: volume depletion (blood loss or loss of fluids like diarrhea) and dehydration (losing more water than sodium)
• Dehydration can lead to hypovolemic shock
• Fluid excess occurs when fluid intake exceeds output.
• The kidneys usually compensate for excess fluid, but renal failure is one cause of fluid excess
• Fluid excess can manifest in: water intoxication (consuming excessive amounts of water or replacing water losses with only water) and fluid excess (ECF remains isotonic) - retention of both sodium and water
• Fluid accumulation leads to edema (fluid accumulates in interstitial spaces)
• Fluid can accumulate in organs and tissues of the body (commonly lungs and brain)

Electrolyte Balance

• Electrolytes are substances that dissociate into ions in water
• Electrolyte levels are measured in plasma
• Electrolyte Levels (plasma):

Chloride

• Most abundant extracellular anion

Phosphate

• Plasma level: 2.5 to 4.5 mg/dL

Sodium

• Main electrolyte in extracellular fluid, also the chief cation.
• Sodium levels are primarily regulated by aldosterone and ADH.
• Aldosterone adjusts sodium excretion
• ADH adjusts water excretion

Potassium

• Chief cation of intracellular fluid
• Works hand-in-hand with sodium
• Potassium imbalances are the most dangerous of any electrolyte imbalance, very high levels considered a medical emergency
• Imbalances can cause life-threatening cardiac arrhythmias

Calcium

• Functions: strengthening bones, muscle contraction, nerve transmission, blood clotting
• Plasma calcium levels are regulated by parathyroid hormone

Chloride and Phosphate

• Chloride is the most abundant extracellular anion.

Acid-Base Balance

• One of the most important factors influencing homeostasis is the body's balance between acids and bases.

Acids and Bases

• The pH of a solution is determined by its concentration of hydrogen (H+) ions.