Human Physiology Water Balance Quiz

Introduction to Human Physiology

• Physiology is the study of the biological functions of organs and their interrelationships.
• This study utilizes the many levels of organization of the human body.
• The internal environment remains remarkably constant despite changing external conditions.
• The interior of the body is separated from the external environment by a layer of epithelial tissue.
• The lumen of the respiratory, gastrointestinal, and urinary systems are part of the external environment.
• Internal environment is defined as the fluid surrounding cells (extracellular fluid).

Body Fluid

• A 70kg man has a total body water (TBW) of 42 liters
  • 28 liters of intracellular fluid (ICF)
  • 14 liters of extracellular fluid (ECF)
    • 3 liters are plasma
    • 11 liters are interstitial fluid (ISF)
• Intracellular fluid (ICF) comprises 2/3 of the body's water content.
  • ICF is approximately 40% of body weight in a person who is 60% water by weight.
  • Primarily a solution of potassium and organic anions, proteins, and other particles.
• Extracellular fluid (ECF) comprises 1/3 of the body's water content.
  • ECF is primarily a NaCl and NaHCO3 solution.
  • Subdivided into interstitial fluid (ISF) and plasma.
  • Interstitial fluid surrounds cells and does not circulate; approximately 3/4 of ECF.
  • Plasma circulates as the extracellular component of blood and is approximately 1/4 of ECF.
• Transcellular fluid are fluids outside of the normal compartments, including cerebrospinal fluid (CSF), digestive juices, mucus, etc.

Variation Due to Age

• Neonates contain more water than adults (75-80%).
• Extracellular fluid is proportionally larger at birth compared to adults.
• By 12 months of age, the proportion of body water has decreased to 60%, the adult value.
• Total body water decreases progressively with age, reaching only 50% of total body weight by the age of 60 in males, mainly due to an increase in adipose tissue.

Daily Intake and Output of Water

• Input:
  • Fluids ingested: 2100 ml/day
  • From metabolism: 200 ml/day
  • Total intake: 2300 ml/day
• Output:
  • Insensible-skin: 350 ml/day
  • Insensible-lungs: 350 ml/day
  • Sweat: 100 ml/day
  • Feces: 100 ml/day
  • Urine: 1400 ml/day
  • Total output: 2300 ml/day

Composition

• Electrolytes and anions (concentrations in mEq/L in parentheses):
  • Sodium (Plasma: 142, ECF: 153, ISF: 145, ICF: 10)
  • Potassium (Plasma: 4, ECF: 4.3, ISF: 4, ICF: 160)
  • Calcium (Plasma: 5, ECF: 5.4, ISF: 5, ICF: 2)
  • Magnesium (Plasma: 2, ECF: 2.2, ISF: 2, ICF: 26)

Inorganic Compounds

• Water is the most abundant and important inorganic compound in living materials, comprising 60-80% of most living cells.

Salts

• Salts are ionic compounds containing cations other than H⁺ and anions other than the hydroxyl ion (OH⁻).
• When dissolved in water, they dissociate into their component ions.
• Common body salts include NaCl, CaCO3, KCl, Ca3(PO4)2.
• Other important body ions include Fe, I, Mg, and Zn.

Acids

• Acids are substances that release hydrogen ions (H⁺) in detectable amounts.
• A hydrogen ion is a hydrogen nucleus and therefore, acids are also proton donors.
• Common body acids include HCl and H2CO3.
• Acids can be strong or weak depending on the amount of hydrogen ions liberated.

Bases

• Bases are substances which dissociate liberating hydroxyl ions (OH⁻).
• Bases remove H⁺ ions from solution and are often called proton acceptors.
• Common body bases include NaOH and ammonia (NH3).

pH

• The concentration of H⁺ in a solution is usually indicated in pH units on a pH scale that runs from 0 to 14.
  • pH = -log[H⁺]
  • [H⁺] = Molar H⁺ concentration
• Acidic solutions have a pH of less than 7.
• Basic solutions have a pH of greater than 7.
• A solution with a pH of 7 is neutral, for example, water.

Sources of H⁺ in the body

• Oxidation of carbon in organic foods produces CO2, which dissolves in fluids to form carbonic acid (H2CO3), releasing H+ and bicarbonate (HCO3-), about 300 Lit. CO2 & 13.000 mEq of H+ ion resulted per day in normal adults.
• Oxidation of sulfur and phosphorus in proteins and lipids leads to acidosis.
• Intake of acids (e.g. citric acid in lemons) or acidifying salts (e.g., NH₄Cl) also contributes.
• Anaerobic glycolysis in muscles during prolonged contractions or starvation releases lactic acid.
• Hydrolysis of high-energy phosphate bonds (e.g., ATP or CP) yields H⁺.

Body Defensive Mechanisms against pH Disturbances

• Body metabolism and muscle activity tend to add acids to body fluids.
• An alkaline reserve in the body system opposes this tendency.
• The maintenance of stable pH depends on: -[A] Chemical buffers (very rapid, in a fraction of a second). -[B] Respiratory regulation (1-12 minutes). -[C] Renal regulation (hours to days).

Homeostasis

• Homeostasis is the process of maintaining a stable internal environment, which is compatible for life.
• Most organ systems contribute to homeostasis.

Negative Feedback Control

• The primary mechanism for maintaining homeostasis is negative feedback control.
• External changes trigger changes in regulated variables within the internal environment.
• The reaction triggered by these changes opposes the change and returns the regulated variable back to its normal (set point).
  • Set point is the desired level of the regulated variable.
  • Sensors detect the level of the regulated variable and send information to the integrating center.
  • The integrating center compares the set point and the actual level.
  • If there is a difference (error signal), the integrating center sends instructions (output) to the effectors.
  • Effectors initiate corrective actions to return the regulated variable to the set point.

Thermoregulation

• Homeothermic animals regulate their body temperature within a narrow range.
• Poikilothermic animals do not regulate their body temperature.
  • Humans maintain a body temperature of 37°C (98.6°F).
  • Hypothermia is a decrease in body temperature.
  • Hyperthermia is an increase in body temperature; above 41°C is dangerous, and above 43°C is deadly.
• Components of the thermoregulatory system include thermoreceptors (sensors), hypothalamus (integrator), and sweat glands, blood vessels, and skeletal muscles (effectors).
• Heat transfer between the body and the external environment occurs via radiation, conduction, evaporation, and convection.
• The thermoneutral zone is the range of outside temperatures where body temperature is regulated only by blood flow to the skin.

Fever

• Fever accompanies infections.
• White blood cells secrete pyrogens.
• Body temperature set point increases.
• Fever enhances immune response.

Positive Feedback

• Positive feedback is a regulatory mechanism where the response to a stimulus causes the controlled variable to move farther from the set point.
• Usually does not maintain homeostasis; it's characterized by being short in duration and infrequent.
  • Example: blood clotting.