Physiology and Homeostasis

Introduction to Physiology and Homeostasis

• Physiology studies functional "nature" of organisms.
• Physiology comes from Greek words "physis" (nature) and "logos" (science).
• In East Asian countries using the Chinese alphabet, "physiology" consists of "logic, life, and science".
• The goal of physiology is to explain physical and chemical factors responsible for origin, development, and progression of life.
• Human physiology studies the characteristics and mechanisms of the human body.
• Subsets of physiology include human, plant, animal, cell, system, sports, environmental, and clinical.

The Cell

• The cell functions as the basic structural and functional unit of living things.
• Cells form tissues, tissues form organs, and organs form systems.
• Tissues are groups of cells performing specific functions.
• The human body is composed of cells and intercellular substances holding those cells together.
• There are approximately 100,000,000,000,000 cells
• "Cellula" means small chamber in Latin.
• Cell shapes vary: round, flat, filamentous, spindle, prismatic, and cubic, depending on tissue and organ structure.
• Cells perform a variety of tasks like fighting diseases, carrying oxygen, providing mobility and producing substances such as enzymes and hormones.
• Cells also store food and create new lives, ensuring human generation continuation.

Cell Composition

• Water is the basic fluid medium of the cell, in which many substances are dissolved.
• Electrolytes such as Sodium (Na⁺), Potassium (K ⁺), Magnesium (Mg ⁺²), Calcium (Ca ⁺²), Chloride (Cl¯), Phosphate (PO4¯³) and Sulphate (SO4¯²) are essential for chemical reactions.
• Proteins are the most abundant cell substances after water and can be structural (shaping the cell) or functional (ion channels, receptors, enzymes).
• Lipids do not dissolve in water, but in fat solvents, and they make up the cellular membrane.
  • Phospholipids make up the cell membrane.
  • Cholesterol also makes up the cell membrane.
  • Triglycerides make up 95% of fat cells and provide energy.
• Carbohydrates play a major role in cell nutrition instead of structural functions.
  • Glucose is dissolved in extracellular fluid and stored inside the cell as glycogen.
• Parts comprising the structure of the cell include the membrane itself, the cytoplasm and organelles, and the nucleus.

Cell Membrane Structure

• The membrane surrounding the cell is a semi-permeable structure made up of fats and proteins.
• The cell membrane is also called the plasma membrane.
• Cell membranes enable inner/outer cell communication for movement.
• Cell membranes are approximately 55% protein, 25% phospholipid, 13% cholesterol, 4% other lipids and 3% carbohydrates.
• The phospholipid molecule's head is polar/hydrophilic (partially soluble in water), while its tail is non-polar/hydrophobic (relatively insoluble).
• Large globular proteins reside between the membrane’s lipid double layer.
• Hydrophilic parts of lipid molecules face the aqueous environment outside/inside the cell, while hydrophobic parts face the water-poor interior.
• Lipid layer in the middle of the membrane is not permeable to water-soluble substances like glucose, urea, or ions but it is permeable to fat-soluble substances like oxygen, carbon dioxide or alcohol.
• Cholesterol molecules make up about 1/10 of the membrane and determine its water-soluble permeability and fluidity with its ratio to phospholipids determining membrane flexibility.

Cell Membrane Functions

• Arranges transitions, separates the inside and outside, drains secretion material out, transmits electrical impulses to neighboring cells, maintains physical shape, and carries external warnings to the intracellular space.
• Most membrane proteins are glycoproteins: proteins with carbohydrate chains attached.
• Integral proteins cross the membrane and peripheral proteins do not.
• Integral proteins create structural channels for water and water-soluble passage, and also act as carrier proteins specific to substances that can't pass through otherwise, such as glucose.
• Integral proteins also act as receptors for some hormones, as well as carriers, enzymes and antigens.
• Channel proteins extend across the membrane's width and form water-soluble channels, especially ones used by ions, and prioritize passage based on substance properties.
• Carrier proteins combine with transported substances and carry it to the membrane's opposite side and are integral proteins.
• Enzyme proteins catalyze reactions near the cell surface.
• Receptor proteins act as receptors for chemical messages coming to the membrane's surface, forming intracellular responses.
• Cell identification proteins provide identification information to distinguish certain proteins from normal, cancer, or invading organism proteins with the immune system.
• Membrane carbohydrates form glycoproteins and glycolipids.
• Carbohydrate molecule portions extend out of the cell, forming a covering called the glycocalyx.
• Glycocalyx are responsible for cell attachment and binding various hormones as a receptor.

Cell Skeleton

• The cytoskeleton found in all cells consists of fibrillar proteins organized into filaments or tubules.
• Functions include protecting general cell structure, changing cell shape, and ensuring movement.
• Components are Microtubules, intermediate filaments, and microfilaments.
• Tubules are tube-like structures that hold the cell in place.
• Filaments are like spider webs and related to cell shape.
• Centrioles and mitotic spindles during division are composed of microtubules.
• Intermediate filaments connect cell and nuclear membranes.

Body Fluids

• The total amount of water in an adult body is about 40-42 liters, depending on age, gender, and fat content, and it accounts for 57-60% of body weight.
• Body fluids transport nutrients, enzymes, hormones, blood cells, neurotransmitters, etc.
• Body fluids act as a liquid medium for chemical reactions, maintain body temperature, and help digestion and excretion.
• Body fluids act as a solvent for electrolytes.
• The fluid present is located in one of three compartments: intracellular, intercellular (interstitial), and intravascular.
• Intracellular fluid accounts for 40% (ICF), extracellular fluid accounts for 20% (ECF), intravascular fluid, also known as plasma, accounts for 5%, and intercellular (interstitial) fluid accounts for 15%.
• Sodium (Na+) is the leading determinant of extracellular fluid (ECF) osmolarity.
• Water can move freely between fluid compartments.
• ICF and ECF compartments remain osmotically balanced.
• Isotonic solutions, like 0.9% NaCl, do not cause fluid shifts between ICF and ECF after being injected intravenously.
• Hypotonic solutions cause the water to be carried into cells.
• Hypertonic solutions cause the water to come out of the cells.
• Kidneys regulate water balance to protect water in the body by controlling concentration.
  • Low water intake elevates hyperosmotic urine concentration.
  • High water intake reduces urine solute concentration creating dilute hypo-osmotic urine
• A loss of body fluid causes dehydration, and excessive fluid intake causes overhydration.
• Interstitial fluid is formed when circulating blood is filtered through the capillary vessels' holes.
• Cells exchange substances with the intercellular fluid.
• Trillions of cells share about 15 liters of interstitial fluid.
• Interstitial fluid is susceptible to nutrient consumption and accumulating waste.
• Constant liquid compartment change causes them to remain fixed.
• Organ cells work together to ensure cellular environment stability.

Body Fluid Imbalance

• Vomiting, diarrhea, uncontrolled diabetes, diuretic use, excessive sweating and fever are possible causes of losing too much body fluid.
• Nausea, lack of appetite, and sore throat may prevent adequate fluid intake.
• Low/no urine output, concentrated dark yellow urine, no tears, sunken eyes or fontanelles, and tiredness are all signs of dehydration.
• For severe dehydration, intravenous rehydration may be required under hospital conditions, whereas mild dehydration often responds to diligent fluid intake.
• Overhydration occurs when there is excessive fluid in the body.
• Overhydration often causes low sodium levels in the blood.
• Brain cells are particularly sensitive to overhydration (and dehydration).
• Drinking large amounts of water usually does not cause overhydration if the pituitary gland, kidneys, liver, and heart function normally.
• If overhydration occurs quickly, this could cause confusion, seizures and a coma.
• Fluid restriction or drug applications are used in treatment.

Homeostasis

• The process of Homeostasis involves cooperation, balance, and harmony from all organs, systems, and cells in the human body
• Internal balance, or homeostasis, is kept within specific limits via organs/systems.
• Extracellular fluid (ECF) enables stable internal environment, depletes oxygen and nutrient stores and floods with waste products because the volume is no longer infinite and is even smaller than intercellular levels.
• Walter Cannon termed homeostasis as the "wisdom of the body" in the early 20th century.
• Homeostasis involves maintaining relatively stable internal conditions in the face of continuous external change.
• Self-regulatory physiological mechanisms maintain steady states through coordinated physiological activity.
• Homeostasis is not static, but an unchanging state, as it is a dynamic state of equilibrium or balance where internal conditions change and vary but always within narrow ranges.
• Homeostasis helps each cell function and allows the organism to remain in balance with its environment.
• Each cell must receive nutrients and have waste removed in a stable fluid environment.
• Specialized activities ensure maintenance of the internal environment for all cells.
• Organ systems contribute to maintenance of stable internal environment as well:
  • Circulatory system
  • Nervous system
  • Endocrine system
  • Respiratory system
  • Digestive system
  • Excretory (urinary) system
  • Musculoskeletal system
  • Immune system
  • Reproductive system
• Constant inner cellular communication is vital for homeostasis, mainly through the nervous and endocrine systems.
  • Electrical stimuli are communicated by nerves.
  • Hormones are carried by blood in the circulatory system.
• All homeostatic control mechanisms must have a minimum of 3 interconnected components: a control center, receiver (receptor), and effector.
  • The control center determines a set point to maintain a variable, analyzes input, and determines appropriate response.
  • The receiver responds to stimuli by sending "afferent path" information and monitoring the environment.
  • An effector creates a response (output) to a stimulus with the "efferent pathway".

Feedback

• Negative feedback reduces the initial warning, working to slow down or stop the activity.
• A home thermostat is an example of a negative feedback mechanism.
• A body thermostat, located in the hypothalamus, operates similarly to regulate core temperature.
• Shivering warms body's core during cold weather.
• Sweating cools body's core during heat.
• Negative feedback mechanisms include blood glucose level, blood pressure, heart rate, respiratory rate, oxygen and mineral levels in the blood.
• Positive feedback intensifies the initial stimulus to increase efficiency, moving in the same direction as deteriorating homeostasis.
• Positive feedback mechanisms often control infrequent/transient events and do not require constant regulation.
• Positive feedback causes a series of self-stimulating and explosive events (cascades, avalanches).
• Blood clotting is a good example of positive feedback: once initiated, it gradually grows to stop bleeding.
• Oxytocin is a hormone that enhances labor contractions during childbirth and promotes milk delivery by contraction from baby suckling.
  • The baby moves down, increasing pressure on the cervix.
  • Pressure receptors send nerve impulses to the brain.
  • Hypothalamus releases oxytocin from the posterior pituitary.
  • Oxytocin stimulates the muscles in the uterine wall to contract even more vigorously.
• As the cyclic events cause contractions to become powerful and frequent, eventually it shuts off when the baby is born.

Compensation

• As balance is maintained, successful compensation leads to homeostasis re-establishment
• Otherwise, failure to compensate results in pathophysiology, illness, or death.