A&P CH 25

Explain the 1 concept of 4 balance with respect to fluids and electrolytes and acids and bases.

• Fluid balance 

  • Study of maintaining appropriate volume and concentration of body’s intracellular and extracellular fluids; largely study of water balance 

  • Imbalances that body is unable to compensate for due to inadequate or excessive amounts of water in body that can have serious homeostatic consequences

• Functions of water in body include: 

  • Polar solvent; allows water to transport and deliver large number of solutes throughout body

  • Distributes body heat

  • Cushions organs and tissues

  • Lubricates organs and tissues as they move

Define the terms body fluid, electrolyte, acid, base, pH scale, and buffer.

• Body fluids—all of body’s water-based liquids

  Blood plasma, Interstitial fluid, Cytosol, Cerebrospinal fluid, Lymph, Exocrine secretions, Other specialized fluids

• Electrolytes—substances that dissociate into ions, or charged particles, when placed in water; particles conduct electricity in solution of water; in large quantities throughout body 

• Acid—chemical that dissociates in water to release hydrogen ion (H⁺)

• Base (alkali)—chemical that accepts hydrogen ion in solution; generally results in salt and molecule of water

• pH scale—used to measure hydrogen ion concentration of solutions

  • An increase in hydrogen ion concentration results in solution with lower pH; solution with lower hydrogen ion concentration has higher pH

• Blood pH is maintained by respiratory system, urinary system, and two types of buffer systems:

  • Chemical buffer systems—chemical systems that buffer body fluids

  • Physiological buffer systems rely on functions of organ systems to buffer fluids 

Describe the fluid compartments, and explain how each contributes to the total body water.

• Total body water—reference volume that uses “standard man” of 70 kg (154 lb), where amount of water in body is about 60% of total body weight or 42 kg (92.5 lb)

• One kilogram of water is equal to one liter; equates to 42 liters (11 gal) or total body water of a 70 kg man

• Actually, total body water varies between individuals based on gender, body mass, age, and amount of adipose tissue present 

Body fluids are found within two compartments:

• Intracellular compartment—composed of trillions of cells and their cytosol (intracellular fluid (ICF)); accounts for about 60% of body fluids or 26 liters or Cytosol

• Extracellular compartment—filled with extracellular fluid (ECF); composed of variety of body fluids

  • Blood plasma—about 8% (3 liters) of total body water

  • Interstitial fluid—about 32% (13 liters) of total body water 

Compare and contrast the relative concentrations of major electrolytes in intracellular and extracellular fluids.

• Solute composition of plasma and interstitial fluid is similar; main difference in composition is in protein content; plasma has much higher protein content than interstitial fluid (with virtually no proteins)

• However, solute composition of ECF and cytosol varies starkly 

  • Concentrations of sodium, chloride, calcium, and bicarbonate ions are much higher in ECF than in cytosol

  • Proteins and potassium, magnesium, sulfate, and monohydrogen phosphate (HPO₄²⁻) ions have much higher concentration in cytosol than in ECF

Explain how osmotic pressure is generated, and compare and contrast the roles that hydrostatic and osmotic pressures play in the movement of water between fluid compartments.

• Direction water that moves between compartments is influenced by two gradients:

  • Hydrostatic pressure gradient—force that fluid exerts on cells; tends to push water away from area of higher hydrostatic pressure to one with lower hydrostatic pressure

    • Hydrostatic pressure of plasma is higher than that of interstitial fluid; normally water is forced out of capillary

    • When hydrostatic pressure of plasma increases due to higher water volume, more water is pushed out of capillary into interstitial fluid

    • Increases hydrostatic pressure of interstitial fluid above that of cytosol, creating gradient that drives water into cell

    • Generally, gradient is known to reverse—water moving from interstitial fluid into plasma—only in extreme cases, such as during severe blood loss (hypovolemic shock)

  • Osmotic pressure gradient—force of solutes in solutions

    • Tends to pull water toward solution with higher osmotic pressure by osmosis

    • Solution’s osmotic pressure is determined by its osmolarity; number of solute particles present in solution 

    • Osmotic pressure of ECF and cytosol is normally equal (isotonic), resulting in no net movement of water between compartments 

    • When solute concentration of ECF is altered, its osmotic pressure changes, and so its ability to cause water movement by osmosis also changes (next slide)

Describe the routes of water gain in and loss from the body.

• Factors that influence water loss—majority of water lost daily is through urine via kidneys

  • Obligatory water loss—normally about 500 ml of urine produced daily irrespective of fluid intake; required to prevent toxic buildup of molecules and electrolyte imbalances 

  • Sensible water loss—usually about 100 ml; noticeable (sensed) amount of water lost in feces daily

  • Insensible water loss—usually 600 ml from skin in form of sweat and evaporation and 300 ml lost in expired humidified air; unnoticed (not sensed) amount of daily water loss

  • Most people lose about 2.5 liter of water daily although this fluctuates with water intake, physical activity, and food intake, among other possible factors 

• Factors that influence water gain

  • Body gains about 2.5 liters of water daily from three main sources:

    • Cumulative water formed by all catabolic reactions in body, called metabolic water (water of oxidation); equals about 250 ml daily

    • Water ingested from foods adds about 750 ml daily 

Water intake from ingested liquids; driven by thirst mechanism

Describe the mechanisms that regulate water intake and output, and explain how dehydration and overhydration develop.

• ADH (antidiuretic hormone)—plays the most important role in balancing water intake with water loss (fluid balance)

  • Produced in hypothalamus and released from posterior pituitary; acts primarily on kidney cells of distal tubule and collecting ducts

  • Triggers insertion of water channels (aquaporins) in plasma membranes of these kidney cells, allowing for water reabsorption from kidneys back into ECF by osmosis 

  • Increased ADH leads to more water reabsorption that increases ECF volume and decreases urine volume

  • Decreased ADH leads to more water elimination via urine and decrease in ECF volume

• Angiotensin-II, aldosterone, atrial natriuretic peptide (ANP) are remaining hormones involved in fluid balance

• Dehydration—decreased volume and increased concentration of ECF

  • Common causes include: profuse sweating, diarrhea and/or vomiting, some endocrine conditions, and diuretic overuse

  • Water loss decreases plasma volume and increases solute concentration; increases osmotic pressure

As ECF osmolarity increases, water moves out of cells by osmosis and cells crenate, leading to both water and electrolyte imbalances; can be fatal

• Overhydration (hypotonic hydration)—when ECF volume increases; decreases its osmotic pressure; water enters cells by osmosis, causing them to swell

  • Usually prevented by decrease in ADH production but can be compromised if renal function is impaired, ADH secretion is abnormal, or an extreme amount of water is consumed in brief time period (water toxicity)

  • Electrolyte imbalances, especially decrease in sodium ion (hyponatremia), result from diluted ECF

Cerebral edema and hyponatremia cause severe disturbances in homeostasis, particularly of CNS; causes mental status changes, seizures, coma, and death if untreated

Explain the factors that determine the pH of blood, and describe how it is maintained within its normal range.

• pH scale—used to measure hydrogen ion concentration of solutions

  • An increase in hydrogen ion concentration results in solution with lower pH; solution with lower hydrogen ion concentration has higher pH

    • Solutions with pH less than 7 are acidic 

    • Solutions with pH greater than 7 are basic

Solutions with pH of 7 are neutral

• Normal H⁺ level in body fluids equals pH range of about 7.35–7.45; must maintain slightly alkaline range to prevent cellular damage

• Blood pH is maintained by respiratory system, urinary system, and two types of buffer systems:

  • Chemical buffer systems—chemical systems that buffer body fluids

Physiological buffer systems rely on functions of organ systems to buffer fluids

Describe the buffer systems that help to keep the pH of the body’s fluids stable.

• Chemical buffer systems: rapid; first line of defense;  function to resist large swings in pH

• Physiological buffer mechanisms: slower; second line of defense; the most important buffer system in blood; consists of carbonic acid and its weak conjugate base, bicarbonate:

• Brain stem Respiratory Centers: act within 1–3 min; lungs directly control CO₂ levels in blood; also control amounts of both carbonic acid and hydrogen ions

• Renal Mechanisms: most potent, but require hours to days to effect pH changes; kidneys work with lungs in two ways: kidneys excrete fixed acids that lungs cannot, and they contribute to acid-base balance by controlling bicarbonate ion concentrations in blood

  • Urinary Mechanisms

    • 1. Secrete Hydrogen Ions

    • 2. Reabsorb/Produce Bicarbonate Ions

    • 3. Secrete Bicarbonate Ions   

Describe the relationship of PCO2and bicarbonate ions to blood pH.

An increase in PCO2 or a decrease in bicarbonate ions will lower pH (making the blood more acidic), while a decrease in PCO2 or an increase in bicarbonate ions will raise pH (making the blood more alkaline

Describe the role of the respiratory system in regulating blood pH, and predict how hypo- and hyperventilation will affect blood pH.

• Respiratory acidosis and compensation: defined as decrease in pH of body fluids due to excess CO₂; caused when ventilation decreases (hypoventilation)

  • Leads to shift that favors production of carbonic acid and lowering of blood pH

  • Three general causes of this imbalance—suppressed ventilation from brainstem dysfunction, blockage of air passages in lungs, and decreased gas exchange in alveoli 

  • Respiratory compensation begins within minutes of decreased pH, as excess H⁺ in ECF stimulates chemoreceptors in brain; trigger increase in rate and depth of ventilation to increase amount of CO₂ exhaled; indirectly reduces H⁺

Renal compensation is more effective than respiratory compensation, but takes from hours to several days to have effect; kidney tubule cells absorb all available HCO₃^– from filtrate, secrete H⁺, and synthesize new HCO₃^– from glutamine catabolism

• Metabolic acidosis and compensation ‘cont:

• Hyperventilation is the first compensatory mechanism as respiratory system attempts to eliminate CO₂ to lower H⁺ level in blood and increase pH level

• We have just established that respiratory rate increases in metabolic acidosis as part of respiratory compensation; how does changing level of carbon dioxide in blood help when problem is metabolic in nature? 

• Respiratory compensation cannot correct actual cause of metabolic imbalance; can correct resulting pH imbalance

Remember that pH of solution depends only on number of hydrogen ions present, not on source of hydrogen ions

Explain the mechanisms by which the kidneys secrete or retain hydrogen and bicarbonate ions, and describe how these processes affect blood pH.

• Urinary system effects on blood pH: kidneys work with lungs in two ways: kidneys excrete fixed acids that lungs cannot, and they contribute to acid-base balance by controlling bicarbonate ion concentrations in blood:

  • Can manufacture new bicarbonate ion when pH of blood becomes acidic (pH decreases) 

  • Can secrete and eliminate bicarbonate ions when blood becomes too alkaline (pH increases)    

• Urinary Mechanisms

  • 1. Secrete Hydrogen Ions

  • 2. Reabsorb/Produce Bicarbonate Ions

  • 3. Secrete Bicarbonate Ions  

• Hydrogen ion secretion and bicarbonate reabsorption in proximal tubule: 

  • Secreted H⁺ binds to HCO₃^– in filtrate; forms carbonic acid; carbonic anhydrase catalyzes breakdown of H₂CO₃ to CO₂ and H₂O

  • Enter kidney tubule cells; CO₂ and H₂O reforms H₂CO₃ catalyzed by carbonic anhydrase; dissociates into H⁺ and HCO₃^–

  • Bicarbonate ion is released into blood while hydrogen ion is recycled to bind to another filtrate bicarbonate ion

• Formation of new bicarbonate ions occurs in proximal tubule cells as they secrete H⁺

• Hydrogen ion secretion in distal tubule and collecting system:

  • Carbon dioxide diffuses from blood and interstitial fluid into distal tubule cells where it reacts with water to form carbonic acid

Carbonic acid dissociates into H⁺ and HCO₃^–; H⁺ is transported into filtrate for elimination while HCO₃^–is transported into interstitial fluid, then blood

Discuss the concept of compensation to correct respiratory and metabolic acidosis and alkalosis.

Respiratory acidosis and compensation: defined as decrease in pH of body fluids due to excess CO₂; caused when ventilation decreases (hypoventilation)

• Respiratory compensation begins within minutes of decreased pH, as excess H⁺ in ECF stimulates chemoreceptors in brain; trigger increase in rate and depth of ventilation to increase amount of CO₂ exhaled; indirectly reduces H⁺

Metabolic acidosis and compensation: defined as addition of H⁺ to ECF (from acids other than CO₂) or loss of HCO₃^–

• Hyperventilation is the first compensatory mechanism as respiratory system attempts to eliminate CO₂ to lower H⁺ level in blood and increase pH level; dependent on kidney health

Respiratory alkalosis and compensation is caused by loss of CO₂ through lungs due to hyperventilation usually as result of variety of psychological states that increase respiratory rate

• excretion of bicarbonate ions and retention of hydrogen ions from kidneys

Metabolic alkalosis and compensation is caused by loss of H⁺ or excess of HCO₃^–

• slowed respiratory rate, retention of H+ by the kidneys, excretion of HCO₃^– by the kidneys