Introduction to Human Physiology and Homeostasis

Introduction to Human Physiology and Homeostasis

• Human physiology is the study of how the body functions and survives in constantly changing environments

Levels of Structural Complexity

• Chemical Level: Involves atoms, ions, and molecules essential for life.
• Cellular Level: Cells are the fundamental structural and functional units of the body, with various specialized types (e.g., muscle, nerve).
• Tissue Level: Tissues are groups of similar cells performing a specific function, including epithelial, muscle, nervous, and connective tissues.
• Organ Level: Organs consist of two or more tissues working together for a specific function (e.g., heart, liver, stomach).
• System Level: Systems are groups of organs with a common function. The human body has 11 major systems: digestive, nervous, endocrine, circulatory, respiratory, urinary, reproductive, muscular, lymphatic, skeletal, and integumentary (skin and glands).

Body Composition

• The average young adult male has 60% water, 18% protein, 7% minerals, and 15% fat.

Cells

• The basic living unit of the body is the cell.
• Organs are aggregates of various cells held together by supporting structures.
• Each cell type is specialized for specific functions.
• Cells share basic characteristics.

Body Fluids (Internal Environment)

• Intracellular fluid (ICF) is the fluid within cells.
• Ribosomes, responsible for protein synthesis, are found free in the cytosol or attached to the rough endoplasmic reticulum (ER).

Basic Structure of the Cell Membrane

• Lipid Bilayer: A thin, double-layered film of lipids forming a continuous barrier over the cell surface.
  • Contains large globular proteins embedded within the lipid layer.
  • Phospholipids have a hydrophilic (water-soluble) head and a hydrophobic (fat-soluble) tail.
• Membrane Carbohydrates: (Glycoproteins and Glycolipids) Small amounts are found only on the outer surface.
• Membrane Proteins: Attached to or inserted within the lipid bilayer.
  • Peripheral Proteins: Loosely attached to other membrane proteins or the lipid bilayer.
  • Integral Proteins: Tightly bound within the phospholipid bilayer.
    • Transmembrane Proteins: Integral proteins spanning the entire membrane.
• Some membranes have microvilli, cilia, or flagella.

Membrane Proteins Functions

• Hormone Receptors: Bind to hormones to initiate cellular responses.
• Pumps: Transport substances across the membrane against concentration gradients.
• Ion Channels: Form water-filled pathways allowing selective movement of ions across the membrane.
• Adhesion Molecules: Hold cells together in tissues.
• Cell Recognition Antigens: Identify "self" cells, preventing the immune system from attacking them.

General Functions of the Cell Membrane

• Physical Isolation: Separates the cell’s internal environment from the external environment.
• Structural Support: Maintains cell shape and holds the cytoplasm.
• Transport: Controls the passage of substances between the cell and its surroundings.
• Recognition: Enables the cell to respond to changes in its environment (e.g., the immune response).

Membrane Transport

• Membrane Permeability: Refers to the membrane’s ability to allow substances to pass through—permeable, impermeable, or selectively permeable.
• Factors influencing permeability:
  • Solubility of particles in lipids: Highly lipid-soluble particles can pass through the membrane easily.
  • Size of particles: Smaller particles can pass through.

Terms to Know

• Concentration gradient: Difference in concentration of a substance between two areas.
• Solution: A homogeneous mixture of two or more substances.
• Solvent: The substance dissolving the solute.
• Solute: The substance being dissolved by the solvent.

Two Types of Membrane Transport

• Passive Transport: Does not require energy input, relying on the energy of molecular movement.
  • Includes: Diffusion, facilitated diffusion, and osmosis.
• Active Transport: Requires energy (ATP) to move substances across the membrane.
  • Divided into primary and secondary active transport.

Passive Transport (Diffusion)

• Simple Diffusion: Movement of molecules or ions through membrane openings or intermolecular spaces without carrier protein involvement.
  • Pathways:
    • Through the lipid bilayer for lipid-soluble substances.
    • Through water-filled channels within transport proteins.
• Facilitated Diffusion: Requires interaction with a carrier protein. The carrier protein binds to molecules and shuttles them through the membrane.
• Factors Affecting Diffusion Rate:
  • Surface area of membrane
  • Concentration gradient magnitude
  • Molecule size and weight
  • Temperature
  • Cell membrane thickness
  • Lipid solubility of substance

Carrier-Mediated Transport (Co-Transport or Assisted Membrane Transport)

• Large, poorly lipid-soluble molecules require carrier proteins for transport across the membrane.
• Carrier proteins bind to specific substances and change shape for transport.
• Properties of carrier-mediated transport:
  • Specificity: Each carrier binds to a specific substance.
  • Competition: Different substances competing for the same carrier.
  • Saturation: Transport rate reaches a maximum when all carriers are occupied.

Properties of Facilitated Diffusion

• Cannot cause net transport from a low to a high concentration.
• Does not require ATP energy.
• Transport rate saturates when all carriers are being used.
• Highly specific, enabling cell selection of substances.
• Sensitive to inhibitors that react with carrier protein side chains.

Passive Transport (Diffusion) cont. - Osmosis

• Osmosis: Diffusion of water across a selectively permeable membrane down its concentration gradient.
• Cells often contain aquaporins, channels facilitating water passage.
• Water moves to areas of higher solute concentration (dilution).
• Osmolarity: Total concentration of solutes in a solution measured in osmoles/liter (osmotic pressure).
  • Normal osmolarity of body fluids is 300 mOsm/L.

Osmotic Effects on Cell Volume

• Isotonic: External solution balances the osmotic pressure of the cytoplasm.
• Hypotonic: External solution is more dilute than the cytoplasm, causing cell swelling.
• Hypertonic: External solution is more concentrated than the cytoplasm, causing cell shrinkage.

Active Transport

• Movement of molecules or ions against a concentration gradient, requiring energy input.
• Divided into two types based on the energy source:
  • Primary Active Transport: Energy derived directly from ATP hydrolysis.
  • Secondary Active Transport: Energy derived from stored ionic concentration differences created by primary active transport.

Properties of Active Transport

• Can transport substances from a low to a high concentration ("uphill" transport).
• Requires ATP energy.
• Examples: Na+/K+-ATPase (sodium pump), Ca2+-ATPases (calcium pump), H+/K+-ATPases, H+-ATPases, multidrug resistance (MDR) transporters.
• Transport rate saturates when all carriers are being used.
• Highly specific.
• Sensitive to inhibitors that react with carrier protein side chains.

Sodium-Potassium Pump (Example of Primary Active Transport)

• Most crucial active transport mechanism in cells.
• Maintains concentration gradients of Na+ and K+ across the cell membrane.
• Pumps 3 sodium ions out of the cell and 2 potassium ions into the cell.
• Results in a high Na+ concentration outside the cell and a high K+ concentration inside the cell.
• Represents a significant energy expenditure for cells, accounting for about 24% of cellular energy.

Explore the fascinating world of human physiology and how our bodies maintain homeostasis amidst changing environments. This quiz covers the levels of structural complexity, including chemical, cellular, tissue, organ, and system levels, as well as body composition. Perfect for students looking to delve deeper into the functions and organization of the human body.