Physiology Thermoregulation and Fluid Balance

Introduction to Human Physiology

• Physiology is the study of the biological functions of organs and their interconnectedness.
• To understand human physiology, the multilevel organization of the human body will be utilized.
• The internal environment of the body maintains remarkable consistency despite external environmental changes.

The Body's External Environment

• The interior of the body is separated from its surroundings by a layer of epithelial tissue.
• The lumens of the respiratory, gastrointestinal, and urinary systems are considered part of the external environment.

The Body's Internal Environment

• The internal environment is the fluid surrounding cells.
• In a 70 kg human, total body water amounts to 42 liters.
• Intracellular fluid (ICF) makes up 28 liters of this.
• Extracellular fluid (ECF) comprises 14 liters.
  • Plasma accounts for 3 liters of ECF.
  • Interstitial fluid (ISF) accounts for 11 liters of ECF.

Intracellular Fluid (ICF)

• ICF constitutes about 2/3 of body water.
• Approximately 40% of a person's weight is ICF.
• ICF is primarily composed of potassium ions, organic anions, and proteins.
• ICF composition is not uniform throughout, but a collection of fluids from different cells.

Extracellular Fluid (ECF)

• ECF comprises 1/3 of body water.
• ECF is approximately 20% of a person's weight.
• It primarily consists of sodium and bicarbonate ions.
• ECF is further divided into interstitial fluid and plasma.
• Interstitial fluid surrounds cells and makes up 3/4 of the ECF.
• Plasma is the extracellular component of blood, accounting for 1/4 of ECF.

Transcellular Fluid

• Transcellular fluid is situated outside the typical compartments.
• These fluids sum up to 1-2 liters, encompassing fluids like cerebrospinal fluid (CSF), digestive juices, and mucus.

Variation in Body Fluids due to Age

• Neonates possess a higher proportion of water (75-80%) compared to adults.
• Interstitial fluid is significantly greater in infants than in adults.
• By the age of one year, the proportion decreases to the adult value of 60%.
• Water content in the body decreases with increasing age.
• By age 60, total body water (TBW) comprises only 50% of total body weight in males, mostly due to increasing adipose tissue.

Daily Intake and Output of Water (ml/day)

• Daily water intake consists of ingested fluids (2100 ml) and metabolic water (200 ml).
• Total intake totals to 2300ml
• Daily water output includes insensible losses through skin and lungs (350ml each), sweat (100 ml), feces (100 ml), and urine (1400 ml).
• Total output totals to 2300ml

Body Fluid Composition (mEq/L)

• Data presented in a table shows the relative concentrations of electrolytes and anions in plasma, interstitial fluid, and intracellular fluid.

Inorganic Compounds

• Water is the most abundant and crucial inorganic compound in living materials.
• It represents 60-80% of most living cells.

Salts

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

Acids

• Acids are substances that release hydrogen ions (H+) in measurable amounts.
• Because a hydrogen ion is simply a hydrogen nucleus, acids can also be defined as proton donors.
• Common body acids include HCl and H2CO3.
• Acid strength is determined by the degree of H+liberation.

Bases

• Bases are substances that release hydroxyl ions (OH-) when dissolved in water or remove H+ ions from solution.
• They are sometimes referred to as proton acceptors.
• Common bases in the body include NaOH and ammonia (NH3).

pH

• pH scale measures the hydrogen ion (H+) concentration in a solution.
• The range of the pH scale is 0 to 14.
• A pH of 7 is neutral.
• pH below 7 is acidic, above 7 is alkaline.
• The pH formula is pH = -log[H+].

Sources of H+ in the body

• Oxidation of carbon in foods creates CO2, which dissolves into fluids, forming carbonic acid (H2CO3) and releasing H+ and bicarbonate (HCO3-).
• Oxidation of sulfur and phosphorus in proteins and lipids can lead to acidosis.
• Ingestion of acids like lemon juice or acidifying salts like NH4Cl.
• Anaerobic glycolysis in muscles during prolonged exertion can lead to lactic acid build-up.
• Hydrolysis of high-energy phosphate bonds in ATP or CP releases H+.

Body Defensive Mechanisms Against pH Disturbances

• Body metabolism and muscle activities produce acids, so the body has an alkaline reserve to counteract.
• Chemical buffering, respiratory regulation (1-12 minutes), and renal regulation (hours to days) are crucial for pH homeostasis.

Homeostasis

• Maintaining a stable internal environment optimal for life is called homeostasis.
• Most organ systems contribute to maintaining homeostasis.

Negative Feedback Control in Homeostasis

• Negative feedback is the main mechanism for achieving homeostasis.
• External changes trigger adjustments in the internal environment.
• Regulatory responses oppose the change and return the regulated variable to its normal set point.

Negative Feedback Mechanisms

• Set point is the desired level of a regulated variable.
• Sensors detect the regulated variable level and feed the information to an integrating center.
• The integrating center compares the input to the set point.
• The difference between the actual and set point values is the error signal.
• The integrating center controls effectors.
• Effectors make adjustments to return the variable toward the set point.

Negative Feedback Example (Blood Glucose)

• Insulin and glucagon regulate blood glucose levels by either storing glucose as glycogen or releasing stored glucose into the blood to maintain normal values.

Thermoregulation

• Homeothermic animals maintain a narrow range of body temperature, while poikilothermic animals do not regulate their body temperature.
• In humans, the normal body temperature is 37°C (98.6°F).
• Hypothermia is a decrease in body temperature, while hyperthermia is an elevation of body temperature above 41°C (dangerous), and above 43°C (deadly).
• Mechanisms of heat transfer include radiation, conduction, evaporation (sweating), and convection.
• Thermoregulatory components include thermoreceptors, hypothalamus, and effectors (sweat glands, blood vessels).
• The thermoneutral zone is the range of environmental temperatures where blood flow controls the body temperature.

Fever

• Fever is often associated with infections.
• White blood cells release substances (pyrogens) which increase the body's set point for temperature.
• Fever enhances immune responses.

Positive Feedback

• Positive feedback loops typically do not maintain homeostasis, but are characterized by short duration and infrequent use.
• An example is blood clotting, where the response to a stimulus (blood vessel damage) causes a variable to move further away from its normal level, such as a blood clot formation until the damage is sealed.
• Similar to blood clotting, there are other positive feedback loops (such as uterine contractions during childbirth) in the body .