Introduction to Physiology and Cell Structure

What is Physiology?

• Describes the functions of living organisms and their components: organs, cells, and molecules.
• Focuses on how the human body functions, including organ systems, cells, and molecular interactions.
• Serves as a foundation for many biological sciences.

Cell Structure

• The basic structural and functional unit of all life.
• Consists of cytoplasm encased within a membrane.
• Contains numerous organelles, each with a specific function (nucleus, endoplasmic reticulum, Golgi apparatus, mitochondria, chloroplasts, peroxisomes, lysosomes).
• Cytosol is the gel-like fluid that fills the cell and surrounds the organelles.

Cell Membrane Structure

• All cells are enveloped by a plasma membrane.
• Semi-permeable, allowing some substances to pass while excluding others.
• Contains ion channels and transport proteins, regulating permeability.
• Maintains a unique composition for intracellular fluid, distinct from extracellular fluid.

Membrane Lipids

• Phospholipids are the predominant membrane lipids.
• Cholesterol molecules are present within the phospholipid bilayer.
• Functions:
  • Forms a barrier around the cell.
  • Acts as a selective barrier for substance passage between intracellular and extracellular fluid.
  • Fluidity due to the movement of phospholipids and cholesterol, enabling cell shape changes (e.g., muscle contraction).

Membrane Proteins

• Functions:
  • Ion channels: Allow passage of ions in and out of the cell (downhill transport).
  • Carriers: Facilitate downhill transport.
  • Pumps: Utilize energy (ATP) for uphill transport.
  • Receptors: Bind neurotransmitters or hormones, influencing cellular functions.
  • Enzymes: Catalyze specific chemical reactions.
  • Cell adhesion molecules (CAMs): Maintain cell cohesion (intercellular junctions).

Membrane Carbohydrates (Outer Membrane)

• Include glycoproteins and glycolipids.
• Functions:
  • Receptors for certain hormones.
  • Self-identity markers for cell recognition and interaction, crucial for immune responses.
  • Antigen-like, exemplified by blood group markers.

Homeostasis

• The maintenance of a relatively stable (dynamic steady state) internal environment (extracellular fluid).
• Essential for survival and proper function of all cells.
• Cells and tissues function optimally only when internal conditions are maintained.
• Not unlimited: the body can resist changes within a specific range and duration.
• Disease often arises from failed homeostasis.

Factors Requiring Homeostatic Maintenance

• pH: 7.4
• Blood volume: 5 L
• Blood pressure: 120/80 mmHg
• Temperature: 37°C
• Partial pressure of oxygen (O2): 100 mmHg and carbon dioxide (CO2): 40 mmHg in arterial blood
• Concentration of water, salt, and electrolytes
• Concentration of nutrients and waste products

Body Systems Involved in Homeostasis

• Circulatory system: Continuous blood movement, transporting nutrients, oxygen, carbon dioxide, waste, and electrolytes throughout the body.
• Respiratory system: Oxygen uptake and carbon dioxide release into the alveoli as blood passes through the lungs, maintaining oxygen and carbon dioxide levels in the blood.
• Digestive system: Digestion and absorption of nutrients.
• Nervous system: Rapid control and coordination of bodily activities, detecting and responding to external environmental changes.
• Endocrine system: Slow regulation of activities demanding duration rather than speed, achieved through hormone secretion. Controls nutrient concentration and adjusts kidney function to regulate internal environment volume and electrolyte composition.
• Musculoskeletal system: Movement coordination and body stabilization.
• Immune system: Defense against infection.
• Skin: Biological barrier preventing entry of foreign bodies or particles, also involved in vitamin D activation.
• Urinary system: Excretion of waste, excess water, electrolytes, and hydrogen.

Homeostatic Control Systems

• Feedback: Responses made after a change is detected, consisting of:
  • Receptor/sensor/detector: Monitors a controlled condition and detects changes (stimuli).
  • Afferent pathway: Neural or hormonal communication.
  • Control center/integrator: Compares the condition to a set point and determines the required effector response.
  • Efferent pathway: Neural or hormonal communication.
  • Effector: Receives orders from the control center and produces a response to restore the controlled condition (muscles or glands).

Types of Feedback Systems

• Negative feedback: Response opposes the initial change, promoting stability. Most of the body's homeostatic mechanisms operate via negative feedback.
  • Examples: Blood pressure changes, body temperature changes, blood glucose changes, blood gases changes.
• Positive feedback: Amplifies the initial change, driving it further away from the original set point, often temporary, with physiological importance.
  • Examples: Normal childbirth, blood clotting, stress.

Feedforward

• Responds in anticipation of a change in a regulated variable, less common.
  • Examples: Increased saliva production due to sight, smell, or thought of food, rise in respiration before exercise, shivering before entering cold water, increased insulin secretion in response to food in the digestive tract, preventing excessive blood glucose rise after meals.