Body Temperature Regulation & Metabolism

Body Temperature Regulation and Metabolism

• The lecture covers body temperature regulation and metabolic processes to maintain homeostasis and energy balance.

Body Temperature Regulation

• Mechanisms control and maintain core temperature via thermoregulatory centers in the brain, heat production, and heat loss.

Metabolic Processes

• Various metabolic pathways play a role in energy production through the use of carbohydrates, lipids, and proteins.

Homeostasis

• Maintaining a stable internal environment is important through the coordination of physiological systems and feedback mechanisms.

Factors Affecting Metabolism

• Age, gender, physical activity, and environmental conditions influence an individual's metabolic rate.

Normal Body Temperature

• Normal body temperature ranges from 96.5°F to 99.5°F, with an average of 98.6°F (36°C-38°C).
• Body temperature is usually lowest while sleeping.
• Infants and older adults may experience more temperature fluctuations.
• Infants have greater surface area, impacting temperature regulation.
• Thermoregulatory systems are less efficient in older adults.
• Exercise increases body temperature.
• Heat production and heat loss control body temperature.

Heat Transfer

• Heat transfer occurs through conduction, convection, and radiation.

Heat Production

• Cellular respiration is influenced by thyroxine (T3), epinephrine, normally active organs/skeletal muscles, food intake, and body temperature changes.

Role of Thyroxine and the Thyroid Gland

• Decreased thyroxine levels lead to decreased cellular energy production and a lower metabolic rate.
• The hypothalamus and pituitary gland influence thyroid gland stimulation.
• Increased thyroxine levels lead to increased cellular energy production and a higher metabolic rate.

Heat Loss

• Blood flow to the skin is critical in heat regulation.
• Vasoconstriction decreases heat loss.
• Vasodilation increases heat loss.
• Heat loss occurs through conduction, convection, and radiation.
• Sweating via eccrine glands results in fluid loss.
• Environmental temperature and humidity affect heat loss.
• Heat loss occurs through the warmth of the respiratory mucosa, which evaporates water during exhalation.
• Heat loss happens through the digestive and urinary tracts.

Heat Loss Pathways and Mechanisms

• Skin (major pathway): Heat is lost by:
  • Radiation & conduction to cooler air
  • Convection from air currents
  • Evaporation of sweat
• Respiratory tract (secondary pathway): Body heat evaporates water from the respiratory mucosa, exhaling water vapor.
• Urinary tract (minor pathway): Urine is at body temperature when eliminated.
• Digestive tract (minor pathway): Feces are at body temperature when eliminated.

Factors Affecting Heat Production

• Thyroxine & T3: Increase cell respiration and heat production in all tissues.
• Epinephrine & sympathetic stimulation: Increase cell respiration.
• Skeletal muscles: Require ATP and produce ~25% of total body heat at rest.
• Liver: Metabolically active, producing up to 20% of total body heat at rest.
• Food intake: Activates the GI tract, ATP, and heat production during peristalsis and synthesis of digestive enzymes.
• Higher body temperature: Increases metabolic rate, leading to increased heat production, which can be detrimental during high fevers.

Hypothalamus in Thermoregulation

• The hypothalamus functions as the body's thermostat by balancing heat production and heat loss.
• It receives information from skin peripherally and central receptors that detect blood flow temperature in the brain.

Mechanisms to Increase Heat Loss

• In a warm environment or during exercise, vasodilation in the dermis increases heat loss.
• If environmental temperature is close to or higher than body temperature:
  • Vasodilation becomes ineffective.
  • Evaporation of sweat is the primary mechanism for heat loss, but ineffective in high atmospheric humidity.
  • Evaporation doesn't readily occur.
• Reducing resting muscle tone induces heat production, causing sluggishness on hot, humid days.

Mechanisms to Conserve Heat

• In a cold environment:
  • Vasoconstriction in the dermis reduces heat loss.
  • Sweating decreases.
  • Increasing resting muscle tone produces more heat.
  • Shivering can increase heat production by up to 5x the normal amount.

Fever

• Fever is caused by pyrogens, substances that elevate body temperature.
• Pyrogens include endogenous pyrogens (made by the body), bacteria, foreign proteins, and chemicals released during inflammation.
• Pyrogens increase the thermostat setting, causing the body to match the new temperature.

Utility of Fever

• Low-grade fever increases white blood cells and metabolism to fight infection.
• High-grade fever can become stuck in positive feedback, damaging the hypothalamus' ability to regulate temperature and damage enzymes.

Metabolism

• Metabolism is life-sustaining chemical reactions in the body, focusing on breakdown, use, and elimination of food for energy.
• Anabolism involves the synthesis or formation of molecules and requires energy (ATP).
• Catabolism is the breakdown of molecules, releasing energy and often producing ATP.
• Anabolic and catabolic reactions are catalyzed by enzymes.

Cellular Respiration

• Complex series of reactions represented by formula: C6H12O6 + 6O2 → 6CO2 + 6H2O + Chemical Energy (in ATP).
• Glucose is broken down piece by piece to produce ATP.
• ATP is produced when glucose is broken down.
• Energy is released when phosphate bonds from ATP are broken down and convert to ADP.

Stages of Cellular Respiration

• Glycolysis occurs first.
• The Krebs Citric Acid Cycle occurs second.
• The Cytochrome (electron) transport system is the final stage.

Glycolysis

• A six-carbon glucose molecule is broken down into two 3-carbon pyruvate molecules.
• The net gain is 2 ATP molecules per glucose molecule (2 ATP are needed to start, and 4 ATP are yielded).
• Two pairs of H+ ions are removed by NAD (carrier molecule containing niacin).
• NAD is reduced to become 2NADH2 and H pairs are transported to the cytochrome transport system.
• Enzymes for glycolysis reactions are in the cytoplasm.
• PFK is a rate-limiting enzyme.
• Oxygen is not required.
• If oxygen is present, pyruvic acid continues to the Krebs cycle.
• If oxygen is not present, pyruvic acid converts to lactic acid, contributing to muscle fatigue.

Krebs Citric Acid Cycle

• Produces coenzymes for continued energy production, including ATP.
• Reactions occur in the mitochondria and require oxygen.
• 2 pyruvate are converted to 2 Acetyl Co-A, carbon dioxide (CO2) and NADH in the transition reaction.
• CO2 is released, and Acetyl CoA moves to the mitochondria; NADH carries high-energy electrons to the electron transport chain.
• Oxaloacetate (OAA) is needed to break down Acetyl CoA.
• Cycle starts with Acetyl Co-A combining with OAA (4 carbon molecule).
• Citric acid (6 carbons) is produced.
• Citric acid is broken down in a series of reactions where energy is captured in NADH, ATP, FADH2.
• CO₂ is released as waste product.
• The final step regenerates OAA.

Cytochrome (Electron) Transport System

• Cytochromes are proteins containing iron or copper, located on mitochondrial membranes.
• H pairs are extracted from glucose and brought to cytochromes by NAD & FAD.
• Each H atom splits into its proton (H+ ion) and electron.
• H electrons are passed from one cytochrome to next, and then to oxygen.
• H+ ions accumulate, creating a concentration gradient.
• Ions flow through ATP synthase to areas of a lesser concentration creating energy.
• Around 25 molecules of ATP are synthesized (value variations).
• Each O atom that gains 2 electrons reacts with 2 H+ ions.
• "Metabolic water" is formed this way.
• This process contributes to intracellular fluid and prevents acidosis.

Alternate Energy Sources

• Proteins:
  • Are made from amino acids.
  • Cannot be stored, but can be deaminated (NH2 removed) and converted to fit into the Krebs cycle.
  • Excess amino acids can be converted to glucose to supply the brain.
• Fats:
  • Are made of glycerol and fatty acids.
  • Can also be converted and enter the Krebs cycle.
• Amino acids and fatty acids can be converted by the liver to ketones.
• 2- or 4-carbon molecules include acetone and acetoacetic acid.
• Body cells can slowly use ketones in cell respiration.
• Ketosis might occur when fats or amino acids are used for primary energy.

Gluconeogenesis and Glycogenolysis

• Gluconeogenesis (reverse glycolysis): A metabolic process that creates glucose from non-carbohydrate sources (amino acids, lactate, and glycerol).
• The liver and kidneys break down food sources and store them in the liver.
• Low blood sugar levels causes a glucose release.
• Regulated by insulin, glucagon, and cortisol.
• Glycogenolysis is where the liver converts glycogen to glucose, stimulated by low blood sugar.

Summary of Cell Respiration

• Glycolysis (cytoplasm):
  • Molecules entering the process include glucose with ATP for energy of activation.
  • The result is 2 ATP (net), 2 NADH2 (to cytochrome transport system), and 2 pyruvic acid.
  • Niacin (part of NAD) is needed.
• Krebs citric acid cycle (mitochondria):
  • Molecules entering include pyruvic acid (from glucose or glycerol or excess amino acids) or Acetyl CoA (from fatty acids or excess amino acids).
  • The result is CO2 exhaled, ATP (2 per glucose), 3 NADH and 1 FADH2 (to cytochrome transport system), and a regenerated 4-carbon molecule for the next cycle.
  • Includes thiamine to remove CO2, niacin, riboflavin, and pantothenic acid.
• Cytochrome/electron transport system (mitochondria):
  • NADH and FADH2 from glycolysis or the Krebs cycle enter the process.
  • The result is 25 ATP and metabolic water.
• Iron & copper are needed.

Energy Availability from Nutrient Types

• Potential energy from food is measured in calories, in which 1 calorie is the amount of energy required to raise 1 gram of water 1 degree C. A kilocalorie (kcal or C) is equal to 1000 calories.
• 1 gram of carbohydrate = 4 C.
• 1 gram of protein = 4 C.
• 1 gram of fat = 9 C.
• "Empty calories" are less useful and more susceptible to being stored as fat (glycemic index).

Nutrient Synthesis

• Glucose is the raw material for pentose sugars.
  • Deoxyribose is a 5-carbon sugar found in DNA.
  • Ribose is found in RNA and ATP.
  • Glucose is important for cell division, protein, and ATP synthesis.
• Excess glucose is converted to glycogen and stored in the liver and skeletal muscle.
• Continued excess is stored as fat in adipose tissue.
• Amino acids are used for primary and non-essential protein synthesis.
• Proteins (keratin, melanin, collagen, myosin, GH, hemoglobin, albumin, pepsin, amylase, enzymes) are used for energy once new proteins needs are met.
• Fatty acids and glycerol:
  • Used for synthesis of phospholipids and are essential for cell membranes.
  • Broken down via beta-oxidation, resulting in acetyl groups that can be used to synthesize cholesterol (primarily in liver).
  • Cells can also synthesize cholesterol for their own cell membranes.
• Liver uses cholesterol to synthesize bile salts.
• Steroid hormones (cortisol, aldosterone, estrogen, progesterone, and testosterone) are also synthesized from cholesterol.
• Excess triglycerides may be stored in adipose tissue.

Hormone Regulation of Metabolism

• Thyroxine (thyroid gland): Increases uses of glucose, fats, and amino acids for energy, as well as increasing protein synthesis.
• Growth hormone (anterior pituitary): Increases amino acid transport into cells and protein synthesis and increases fat use for energy.
• Insulin (pancreas): Increases glucose transport into cells and its use for energy, increases conversion of glycogenesis in liver and muscles, and increases transport of amino acids and fatty acids into cells for synthesis.
• Glucagon (pancreas): Increases glycogenolysis and the use of amino acids and fats for energy.
• Cortisol (adrenal cortex): increases conversion of glycogenesis in the liver and the use of amino acids and fats for energy, and decreases protein synthesis except in the liver and GI tract.
• Epinephrine (adrenal medulla): Increases glycogenolysis and use of fats for energy.

Micronutrients - Vitamins

• Organic molecules are needed for body function, and can be coenzymes and antioxidants.
• Coenzymes are necessary for the function of certain enzymes.
• Antioxidants prevent damage from free radicals by combining with them.
• Vitamins C, E, and beta-carotene (precursor for vitamin A) are antioxidants.
• Plant foods (including coffee) are good sources of antioxidants.
• Deficiencies of vitamins can cause disease and or altered body function.
• Free radicals are highly reactive molecules that damage DNA, cell membranes, and cell organelles.
• Free radicals form during some normal body reactions, while smoking and exposure to toxins can also lead to their formation.

Micronutrients - Minerals

• Simple inorganic chemical elements.
• They have a variety of functions in the body: structural support (calcium in bone matrix) and as part of a process (calcium in clotting, chlorine in osmosis).
• Involved in maintaining fluid electrolyte balance.

Metabolic Rate

• Is usually expressed as heat production.
• Includes body processes of muscle contraction, pumping of the heart, and breakdown of cellular components.
• Energy expenditure is often measured in kcal.
• BMR (Basal Metabolic Rate) is energy expenditure at rest.
• MET (metabolic equivalent) represents caloric consumption by breathing related to rest.
• Measured O2 consumption in a 70kg man at rest is approx 3.5 ml/kg/min.
• 1 MET = 3.5 ml/kg of body weight/min.

Factors Affecting Metabolic Rate

• Exercise: Skeletal muscle contraction increases energy expenditure, increasing the rate.
• Age: Metabolic rate is highest in young children and decreases with age. The rate decreases by 2% per decade but 5% in less active decades.
• Body configuration: Greater skeletal muscle mass increases the metabolic rate. Tall, thin people have higher rates because they have a larger surface area.
• Sex hormones: Testosterone increases metabolic activity and men have a higher metabolic rate.
• Sympathetic stimulation: Increases metabolic rate in stress as well as hormones epinephrine and norepinephrine.
• Decreased food intake: A prolonged decrease causes a decrease.
• Climate: People in cold climates have 10-20% higher metabolic rates.