Introduction to Physiology and Homeostasis

Introduction to Physiology and Homeostasis

• Physiology is the study of how living organisms function
• The human body is organized into multiple levels, starting from cells and ending with the whole organism
• In humans, there are four main types of tissues: epithelial, connective, muscle, and nervous
• Epithelial tissues cover the surface of the body and line internal organs and cavities
• Epithelial tissue functions include protection, secretion, absorption, and filtration
• The two main fluid compartments in the body are the intracellular and extracellular fluids
• Homeostasis is the maintenance of a stable internal environment within the body
• Feedback mechanisms are crucial for maintaining homeostasis, with negative feedback systems working to suppress changes and positive feedback systems amplifying changes
• Negative feedback is used for frequently adjusted conditions like body temperature, blood pressure, and glucose regulation
• Positive feedback is typically short-lived and does not require continuous adjustments, examples include blood clotting, urination, and labor
• Feed-forward control prepares the body for a change before it occurs, examples include shivering before entering cold water and salivation when smelling delicious food

Cell Structure and Membrane Transport

• The human body has around 50-100 trillion cells
• Cells differentiate to specialize in specific functions
• The cell membrane is a selectively permeable barrier that allows lipids to pass but not water-soluble substances
• The cell membrane is composed of a phospholipid bilayer with hydrophilic heads that face outwards and hydrophobic tails that face inwards
• Movement across the cell membrane can be passive or active
• Passive processes include simple diffusion (following a concentration gradient), facilitated diffusion (using a carrier protein), and osmosis (water movement)
• Active processes require energy and include primary active transport (direct ATP use) and secondary active transport (using the concentration gradient of another molecule)
• Osmosis is the movement of water across a semipermeable membrane from a region of high water concentration to a region of low water concentration
• Osmosis is important for cell volume and for fluid exchange at the tissue level
• The osmotic pressure inside human cells is equivalent to that of a 0.9% sodium chloride solution
• Hypotonic solutions have a lower osmotic pressure than cells, causing water to enter the cell
• Hypertonic solutions have a higher osmotic pressure than cells, causing water to leave the cell
• Isotonic solutions have the same osmotic pressure as the cells
• Active transport requires carrier proteins and moves substances against the concentration gradient
• Examples of primary active transport include the Na+/K+ pump and the active calcium transport in muscle cells
• Secondary active transport uses the energy stored in the concentration gradient of another molecule, like sodium, for transport
• Examples include the Na+/glucose co-transport system in the intestines

Membrane Potential

• The cell membrane is polarized with a negative charge on the inside compared to the outside
• This potential difference across the membrane is called the membrane potential
• The membrane potential is created by the distribution of ions across the membrane and the permeability of the membrane to those ions
• The Na+/K+ pump is crucial for maintaining the membrane potential by pumping sodium ions out of the cell and potassium ions into the cell
• The membrane potential is essential for nerve impulse transmission, muscle contraction, and other physiological processes

Body Fluids

• Extracellular fluid (ECF) makes up 1/3 of total body water and surrounds cells.
• ECF is comprised of plasma and interstitial fluid.
• Intracellular fluid (ICF) makes up 2/3 of total body water and is found within cells.
• ICF has higher concentration of potassium ions (K+) and lower concentration of sodium ions (Na+) compared to ECF.

Balancing Internal and External Environment

• Cells exchange nutrients and waste with their surroundings.
• ICF is conditioned by ISF, which is conditioned by plasma, which is conditioned by the organ systems it passes through.
• The external environment is made up of the organ function & the external world surrounding the body.

Homeostasis

• Homeostasis is about maintaining a stable internal environment, primarily focused on ECF.
• Processes involved in upholding homeostasis:
  • Regulating body temperature
  • Maintaining blood pressure
  • Regulating blood glucose levels
  • Balancing electrolytes

Levels of Organization

• Cell: The basic unit of life
• Tissue: A group of similar cells performing a common function
  • Examples: epithelial, connective, muscle, nervous
• Organ: Composed of two or more tissue types working together for a specific function
  • Examples: stomach, heart, kidney
• Body System: A collection of organs with a common function
  • Examples: digestive system, skeletal system
• Organism: A complete living individual

Body Fluids: Water

• Total body water in an adult is 60% of body weight.
• Water content decreases with age; newborns have about 80% body water.
• Females have a lower body water content due to higher body fat.
• Water is essential because:
  • High specific heat
  • Capillary action
  • Excellent solvent
  • Redox reaction to kill bugs

Basic Cell Functions

• Obtaining nutrients and oxygen from the environment
• Performing chemical reactions for energy production
• Eliminating waste products like carbon dioxide
• Synthesizing necessary cellular components such as protein
• Controlling movement of materials in and out of cells
• Sensing and responding to environmental changes
• Reproduction (Except for nerve and muscle cells)

Specialized Cell Functions

• Different cell types have specific roles.
  • Example: white blood cells fight infections while red blood cells carry oxygen.

Cell Membrane

• Thin and mechanically weak structure surrounding the cell.
• Important for:
  • Maintaining cell integrity
  • Controlling substance movement in and out of the cell (selective permeability)
  • Regulating cell-cell interactions
  • Serving as an interface between cytoplasm and external environment

Cell Membrane is Important

• It creates a barrier between ICF and ECF.
• It maintains concentration differences between ICF and ECF:
  • High potassium (K+) concentration inside the cell
  • High sodium (Na+) concentration outside the cell
  • Higher chloride (Cl-) concentration outside the cell
  • Higher calcium (Ca++) concentration outside the cell
  • Higher protein concentration inside the cell
  • Outside is slightly alkaline (pH=7.4), inside is neutral (pH=7)

Composition of the Cell Membrane

• 55% Protein

• 25% Phospholipids

• 13% Cholesterol

• 4% Other lipids

• 3% Carbohydrates

• Lipids play a critical role in cell membrane structure, facilitating the passage of lipid-soluble molecules.

Cell Membrane: Phospholipids

• Phospholipids are amphipathic (have both hydrophilic and hydrophobic parts).
• Phospholipid bilayer:
  • Hydrophilic heads face outwards
  • Hydrophobic tails face inwards
  • This structure allows for selective permeability, letting lipid-soluble substances pass through.

Movements Into and Out of the Cell

• Passive Processes:
  • Movement down concentration gradient
  • Does not require energy (ATP)
  • Types:
    • Simple diffusion
    • Facilitated diffusion
    • Osmosis (water movement)
• Active Processes:
  • Movement against concentration gradient
  • Requires energy
  • Types:
    • Primary active transport
    • Secondary active transport

1. Simple Diffusion

• Movement of substances directly through the cell membrane (intermolecular spaces).

• Permeable to:

  • Non-polar molecules (oxygen)
  • Lipid-soluble molecules (steroids)
  • Small polar molecules (carbon dioxide)
  • Water (small size, lack of charge)

• The rate of diffusion is determined by lipid solubility.

• Impermeable to:

  • Large polar molecules (glucose)
  • Charged inorganic ions (sodium)

1. Simple Diffusion: Summary

• Simple diffusion occurs when there's a concentration difference across the membrane, and the membrane is permeable to the substance (energy independent).

2. Facilitated Diffusion

• Movement of lipid-insoluble or water-soluble substances across the membrane down their concentration gradient.
• Requires specialized membrane proteins (facilitators) to aid in transport.
• Substances involved: potassium, sodium, calcium, glucose, amino acids, urea.