Introduction to Human Physiology

Physiology Defined

• Physiology involves the Greek words "physis," meaning nature, and "logos," meaning science.
• Physiology refers to how living organisms, organs, tissues, and cells function normally.
• The study of the function of the "nature" of organisms.
• Biology branch deals with the processes, activities, and phenomena of living things.
• Focuses on elucidating the physical and chemical mechanisms responsible for the origin, development, and progression of life.
• Human physiology explains the specific attributes and mechanisms that enable the human body to function as a living entity.

Types of Physiology

• Human, plant, and animal physiology are distinct fields
• Cell and System physiology focus on specific levels of organization.
• Sports and environmental physiology study adaptations to specific conditions.
• Clinical physiology focuses on disease-related functional changes.

Cell Composition and Function

• Cells are the fundamental units of life
• 200 different kinds of cells exist, each differing in structure and function
• The human body comprises around 100 trillion cells
• "Cellula" means small chamber in Latin
• Cell shapes vary based on their role in tissues and organs: round, flat, filamentous, spindle, prismatic, cubic shapes exist.
• Cells perform various functions like fighting diseases, transporting oxygen, enabling mobility, producing biochemicals (enzymes, hormones), storing food, and creating new life.

Cellular Components

• Water forms the basic fluid medium of the cell, dissolving many substances.
• 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 processes
• Proteins, the most abundant substance after water, include structural proteins for cell shape and functional proteins such as ion channels, receptors, and enzymes
• Lipids, insoluble in water but soluble in fat solvents, are key components of cell membranes
• Lipids include Phospholipids and Cholesterol.
• Triglycerides, which make up 95% of fat cells, store energy
• Carbohydrates mainly play a role in cell nutrition rather than structure
• Glucose is found in extracellular fluid, while glycogen is stored inside cells.
• The parts that form the physical structure of the cell include: cell membrane, cytoplasm and organelles, and the nucleus.

Cell Membrane Attributes

• The cell membrane consist of lipids and proteins, and is a semi-permeable barrier
• The cell membrane is also known as the plasma membrane.
• The cell membrane facilitates communication and movement
• The cell membrane consist of 55% protein, 25% phospholipids, 13% cholesterol, 4% other lipids, and 3% carbohydrates
• Globular proteins reside within the lipid bilayer
• Phospholipids have partially soluble (polar, hydrophilic) head groups and insoluble (non-polar, hydrophobic) tail groups
• The phosphate end of phospholipid components is hydrophilic and the fatty acid component is hydrophobic
• Lipid molecule hydrophilic ends face outwards into the aqueous region, and hydrophobic ends face inwards away from water
• The lipid layer allows fat-soluble molecules like oxygen, carbon dioxide and alcohol to pass.
• Cholesterol, comprising about 10% of the membrane, regulates permeability and fluidity

Cell Membrane Functions

• Facilitates selective transport
• Separates the cell's internal and external environments
• Transports secretions
• Transmits electrical signals to adjacent cells
• Maintains cell shape
• Responds to external signals

Cell Membrane Protein Functions

• Proteins reside within the lipid bilayer
• Glycoproteins make up most membrane proteins, existing as transmembrane or surface-attached proteins
• Integral proteins create structural passageways allowing water and water-soluble substances through, while selectively binding carrier proteins transport non-permeable substances like glucose
• Some serve as hormone receptors, carriers, enzymes, or antigens
• Proteins function as channel, carrier, or enzyme proteins.
• Membrane carbohydrates combine with proteins and lipids to form glycoproteins and glycolipids.
• Carbohydrate extensions form the glycocalyx protective coating
• The glycocalyx facilitates cell adhesion and receptor binding for hormones.

Cytoskeleton

• Composed of fibrillar proteins organized as filaments and tubules
• Supports cell structure, modifies cell shape, and facilitates movement
• Intermediate filaments, microtubules, and microfilaments compromise the cytoskeleton -Tubules support the cell, while filaments support cell shape
• Microtubules form centrioles and mitotic spindles during cell division
• Intermediate filaments connect the nuclear membrane to the cell membrane.

Body Fluids Distribution

• Total body water in adults is approximately 40-42 liters, or 57-60% of body weight
• Body fluid percentage varies on age, gender, and fat composition.

Body Fluid Function

• Transports nutrients, enzymes, hormones, blood cells, and neurotransmitters
• Provides a medium for chemical reactions
• Regulates body temperature and aids digestion and excretion.
• Functions as a solvant

Body Fluids Compartments

• Intracellular fluid (ICF) accounts for 40% of total body water
• Extracellular fluid (ECF) makes up 20% of total body water, comprised of:
• Intravascular (plasma) fluid accounts for 5%
• Intercellular (interstitial) fluid accounts for 15%

Body Fluid Balance

• Sodium (Na⁺) determines the osmolarity of the extracellular fluid
• Fluid moves across compartments to maintain osmotic balance
• Isotonic solutions like 0.9% NaCl do not shift fluid between ICF and ECF upon intravenous injection
• Hypotonic solutions cause water to enter cells, while hypertonic solutions draw water out

Kidneys Role in Body Fluids

• The kidneys serve to regulate water balance
• Kidneys conserve water by producing concentrated urine during low water intake or water loss
• Kidneys eliminate excess water by producing urine during high water intake
• Fluid loss leads to dehydration, while high fluid intake can cause overhydration

Interstitial Fluid and Cellular Waste

• The capillary vasculature filters blood and creates interstitial fluid.
• Cells exchange nutrients and waste products with the interstitial fluid
• The body maintains about 15 liters of interstitial fluid
• Interstitial fluid must have sufficient nutrients and not accumulate waste
• Since substances are continually exchanged amongst liquid compartments, disruptions in one compartment can have an impact on others
• Compartments are regulated as to not have changes
• The body ensures cellular stability by dividing labor among organ systems that compliment and interact with each other.

Dehydration

• Dehydration is caused by losing too much fluid due to vomiting, diarrhea, excessive urination (as in uncontrolled diabetes or diuretic use), and excessive sweating
• It may also be related to not drinking enough fluids because of nausea, lack of appetite, and mouth sores

Dehydration Signs

• The signs of dehydration include a dry or sticky mouth
• Reduced urine production and dark urine
• Lack of tear production
• Sunken eyes
• Sunken fontanelles
• Tiredness
• Mild cases usually resolve with fluid intake, while severe cases require intravenous fluids

Overhydration

• Overhydration, or excessive fluid in the body, is generally averted if the pituitary gland, kidneys, liver, and heart function normally
• Low blood sodium can occur with overhydration
• The brain is most sensitive to hydration extremes
• The brain can adapt to gradual electrolyte changes and as such, few symptoms occur
• Fast electrolyte changes will result in confusion, confusion, and seizures and coma may develop in severe cases
• Fluid intake restriction or drug administration are the mainstays of treatment

Homeostasis Defined

• Homeostasis refers to how organs, systems, and cells operate in harmony, balance, and coordination
• Structures synchronize to produce internal balance
• Internal balance is sustained by keeping structures within designated ranges.

Extracellular Fluid Role

• Extracellular fluid is paramount for constant conditions
• ECF volume is finite
• Extracellular fluid volume is considerably smaller than the intracellular fluid
• Cells rapidly deplete oxygen and nutrient supply while also releasing waste
• Homeostasis is a “stable internal environment”

Walter Cannon and Homeostasis

• Walter Cannon refers to homeostasis as "Wisdom of the Body"
• Homeostasis is the ability to maintain stable internal conditions when there are continuous external changes
• It is achieved through self-regulating physiological mechanisms
• Physiological activity maintains steady states
• Homeostasis is a dynamic equilibrium rather than a static state
• Internal conditions shift within narrow parameters

Feedback Loops

• Feedback loops function to control variables
• The control center defines the set point
• Receptors monitor and send changes to the control center
• Effectors impact the stimulus based on control center output

Negative Feedback

• Negative feedback stabilizes conditions, most homeostasis regulation uses negative feedback
• Original stimulus is reduced or stopped.
• Thermostat is an example of negative feedback
• The body controls temperature through negative feedback, shivering warms the body in the cold and sweating cools it in heat
• The stability of an organism is maintained by homeostasis
• The organism dies of there is no homeostasis

Factors Controlled by Negative Feedback

• Blood glucose
• Blood pressure
• Heart rate
• Respiratory rate
• Oxygen level in the blood
• Mineral level in the blood

Positive Feedback

• Positive feedback intensifies the original stimulus instead of homeostasis
• This is positive feedback from an external force
• Causes explosive events like avalanches and cascades
• Promotes transient infrequent processes, for explosive events that don't require maintenance
• Positive feedback stabilizes blood clotting

Oxytocin

• Oxytocin is the pregnancy hormone to enhance labor contractions
• The pressure receptors in the cervix send electrical impulses, after the baby moves down the birth canal
• Oxytocin forces the fetus down the birth canal
• During the childbirth process, it ensures the uterus contracts
• The pressure during birth triggers the stimulus for oxytocin release and the cycle shuts down.

Compensation and Homeostasis

• Homeostasis can be successfully compensated, restabilizing the body.
• Homeostasis, when not successful, leads to pathophysiology, illness and death.