Chapter 24

• Fluid Compartments: Major Components of the Body

  • 65% Intracellular Fluid (ICF) -> Cytoplasm
  • 35% Extracellular Fluid (ECF)
• Fluid Compartments: Extracellular Fluid (ECF)

  • 35%
  • 25% tissue (interstitial) fluid
  • 8% blood plasma and lymph
  • 2% transcellular fluid
• Transcellular Fluid:

  • Cerebrospinal, synovial, and serous fluids
  • Humors of the eye
  • Fluids of the digestive, urinary, and reproductive tracts.
• Fluid Compartments:

  Fluid is continually exchanged between compartments

  • water moves by -> osmosis
• Fluid Compartment:

  If imbalance occurs, osmosis restores balance quickly, so the ICF and ECF osmolarity are -> equal

  • If osmolarity of the tissue fluid (ECF) rises, water moves out of the cell (hypertonic).
  • If it falls, water moves into cells (hypotonic)
• Osmosis from one fluid compartment to another is determined by the concentration of solutes in each compartment.
• The most abundant solute particles is?

  Electrolytes
• Electrolytes:

  • Sodium (Na+) salts in ECF
  • Potassium (K+) salts in ICF
• Electrolytes play a major role in controlling body water distribution and total water content.
• Water Gain & Loss:

  When daily gains and losses are equal (about 2500 mL/day)?


  Fluid Balance
• Water Gains & Loss:

  Gains come from 2 places:

  • Preformed Water
  • Metabolic Water
• Water Gains & Loss:

  Preformed Water:

  • 2300 mL/day
  • Ingested Food (700 mL/day)
  • Drinks (1600 mL/day)
• Water Gains & Loss

  Metabolic Water:

  • 200 mL/day
• By-product of aerobic metabolism and dehydration is called?

  Metabolic Water
• Sensible water loss is observable.
• Water Gains & Loss:

  Sensible Water Loss:

  1500 mL/day is in the urine

  200 mL/day is in feces

  100 mL/day is sweat
• Water Gains & Loss:

  Insensible water loss is unnoticed:

  • 300 mL/day in expired (exhaled) breath
  • Cutaneous Transpiration is 400 ml/day
  • Water diffuses through epidermis and evaporates
  • does not come from sweat gland
  • Loss varies greatly with environment and activity
• Output that is unavoidable is called?

  Obligatory Water Loss
• Obligatory Water Loss:

  • Expired air, cutaneous transpiration, sweat, fecal moisture, and minimum urine output (400 mL/day).
• Thirst mainly controls and influences fluid intake.
• Dehydration:

  • Reduces blood volume and blood pressure
  • Increases blood osmolarity
• In hypothalamus, respond to Angiotensin II produced when blood pressure drops and respond to rise in osmolarity of ECF is called?

  Osmoreceptors in Hypothalamus
• Osmoreceptors communicate with other hypothalamic neurons and with the cerebral cortex.
• Hypothalamus produces the Antidiuretic Hormone (ADH), which promotes water conservation (reabsorbed back into the blood from tubules).
• Cerebral cortex makes us feel thirsty

  • Intense sense of thirst with 2% to 3% increase in plasma osmolarity or 10% to 15% blood loss
• Salivation is inhibited with thirst, which causes dry mouth.

  • Sympathetic signals from thirst center to salivary glands
  • Water is ingested so rehydration occurs.
• Regulation of Fluid Output:

  Only way to control water output significantly is through changes in urine volume.

  • Kidneys cannot replace water or electrolytes
  • Can only slow rate of water and electrolytes loss until water and electrolytes can be ingested.
• Control of water output: Changes in urine volume linked to adjustments in Na+ reabsorption.

  • when Na+ is reabsorbed or excreted, water -> follows
• water output is slowed by the action of ADH.

  • ADH secretion is triggered by hypothalamic osmoreceptors in response to dehydration.
• Regulation of Fluid Output:

  Aquaporins are synthesized in the DCT and Collecting Duct in response to ADH.
• Regulation of Fluid Output:

  Collecting Duct channels allow water to diffuse back into the renal medulla.

  • Na+ is still excreted, so urine's osmolarity increases.
• Physiological Functions of Electrolytes:

  • Chemically reactive and participate in -> metabolism
  • Determines electrical potential across cell membranes
  • Strongly effects osmolarity of body fluids
  • Affects body's water content and -> distribution
• Sodium:

  • Functions: One of the principle ions responsible for resting membrane potential
  • Inflow of sodium through membranes gates is an essential event in depolarization that is the basis of nerve and muscle function
  • Principle cation in -> ECF
  • Most significant solute in determining total body water and distribution of water among fluid -> compartments.
• Potassium: K+

  • Functions: Produces the resting membrane potentials and action potentials of nerve and muscle cells
  • Greatest abundant cation of -> ICF
  • Greatest determinant of intercellular osmolarity and cell volume.
• Chloride:

  • Functions: Most abundent in -> ECF
  • Major contributions to EFC -> osmolarity
  • Required for the formation of stomach acid (HCl)
  • Chloride shift occurs during CO2 loading and unloading in RBCs
  • Major role in regulating body -> pH
• Calcium:

  • strengthens the -> skeleton
  • Activates exocytosis of neurotransmitters and other cellular secretions
  • Essential factor in blood -> clotting
  • Cells maintain very low intracellular Ca2+ levels.
• Magnesium: Mg2+

  • a little over half of Mg2+ in bone, the rest is in -> ICF
  • most intracellular Mg2+ is in a complex with -> ATP
  • Mg2+ serves as a cofactor for enzymes, transports, and nucleic acids.
• Phosphates: Pi

  • Functions: Concentrated in ICF due to hydrolysis of ATP and other phosphate compounds.
• Phosphate Components of:

  • Nucleic acids, phospholipids, ATP, GTP, cAMP, and creatine phosphate
  • Activates many metabolic pathways by phosphorylating enzymes and substrates like glucose.
  • Buffers that help stabilize the pH of body fluids.
• Acid-Base Balance: pH is one of the most important aspects of homeostasis.