CONCEPT OF HOMEOSTASIS

• Homeo → same
• Stasis → standing
• The word homeostasis was introduced by Harvard Professor Walter B. Cannon in 1929.
• Importance of internal environment was notified by French Physiologist Claude Bernard in the 19th century.
• Claude Bernard found that multicellular organisms, including man, live in a perfectly organized and controlled internal environment.
• He called this internal environment “Milieu interieur”.

INTERNAL ENVIRONMENT

• Internal environment of the body → extracellular fluid (ECF) in which cells live.
• ECF is the fluid outside the cell and constantly moves throughout the body.
• ECF includes:
• Blood → circulates in vascular system.
• Interstitial fluid → present between the cells.
• ECF contains:
• Nutrients
• Ions
• All other substances necessary for survival of the cells.

Maintenance of Internal Environment

• Normal healthy living of large organisms, including human beings, depends upon constant maintenance of internal environment within the physiological limits.
• If internal environment deviates beyond the set limits:
  Body suffers from malfunction or dysfunction.
• Ultimate goal for an organism → normal healthy living.
• This is achieved by maintaining internal environment within set limits.

ROLE OF BODY SYSTEMS IN HOMEOSTASIS

Many systems of the body are involved in the homeostatic mechanism of each function.

1. DIGESTIVE SYSTEM

• Nutrients are essential for:
• Various activities of the cell
• Growth of tissues.
• Nutritive substances such as:
  • Amino acids
  • Carbohydrates
  • Fatty acids
  form the source of energy required for various activities of the cells.
• Adequate amount of nutrients must be:
• Digested
• Absorbed into blood
• Supplied to cells.
• Digestive system is responsible for:
• Digestion of nutritive substances.
• Absorption of nutritive substances into blood.
• Blood then transports and supplies nutritive substances to cells of the body.

2. RESPIRATORY SYSTEM

• Respiratory system is responsible for exchange of gases:
  • Oxygen
  • Carbon dioxide
• It supplies adequate amount of oxygen to cells for metabolism of nutrients.
• Simultaneously, carbon dioxide is:
  • Removed from cells.
  • Expelled out of the body via lungs.
• Respiratory system is also responsible for maintenance of:
  • Water balance
  • Acid balance
  • Temperature of the body.

3. EXCRETORY SYSTEM

• Kidneys and other excretory organs are involved in excretion of waste products.
• Kidneys are also responsible for maintenance of:
• Water balance
• Electrolyte balance
• Acid-base balance
• Osmolality of body fluids.

4. SKIN

• Skin covers and protects all organs of the body from:
  • Mechanical blow
  • Bacteria
  • Toxic substances
  • Ultraviolet rays.
• By secreting sweat, skin plays an important role in regulation of:
  • Body temperature
  • Water balance.
• Skin also excretes small quantities of waste materials such as:
  • Urea
  • Salts
  • Fatty substances.
• Skin also forms the largest sense organ.
• It has many receptors which convey many sensations to brain via afferent nerves.

5. BLOOD AND CIRCULATORY SYSTEM

• Circulatory system includes:
  • Heart
  • Blood vessels.
• Heart is responsible for flow of blood in blood vessels throughout the body.
• Blood transports:
• Nutritive substances
• Oxygen
• Other important substances such as hormones
• These are transported to all cells of the body.
• Simultaneously, blood removes from cells:
• Carbon dioxide
• Metabolic waste products
• Other waste products which are to be excreted.
• White blood cells are responsible for:
• Protection of body from invading organisms.
• Development of immunity against pathogenic agents.

6. LYMPHATIC SYSTEM

• Lymphatic system includes a closed system of:
  • Lymphatic vessels
  • Lymph nodes.
• Lymph flows through these vessels and nodes.
• Major function of lymph → return proteins from tissue space into blood.
• It is also responsible for redistribution of fluid in the body.
• Lymph nodes:
• Form defence barriers of the body by destroying bacteria and other toxic agents.
• Filter lymph.
• Retain proteins and lipids which are essential for the body.

7. ENDOCRINE SYSTEM

• Many hormones are essential for:
  • Metabolism of nutrients and other substances necessary for cells.
  • Growth and functions of various tissues of the body.
• Endocrine glands are responsible for:
• Synthesis of hormones.
• Release of hormones in appropriate quantities.
• Hormones then act on body cells appropriately.

8. SKELETAL MUSCLES

• Skeletal muscles are also involved in homeostasis.
• They help the organism to:
• Move around in search of food.
• Protect itself from adverse surroundings.
• Thus, they help prevent damage or destruction.

9. NERVOUS SYSTEM

• Central nervous system (CNS), including:
  • Brain
  • Spinal cord
  plays an important role in homeostasis.
• Sensory system detects the status of:
• Body
• Surroundings.
• Brain integrates and interprets the pros and cons of this information.
• Brain commands the body to act accordingly through the motor system.
• Thus, body can avoid damage.
• Autonomic nervous system regulates all vegetative functions essential for homeostasis.

COMPONENTS OF HOMEOSTATIC SYSTEM

Homeostatic system includes 3 components:

1. Sensors
2. Control center
3. Effectors

1. SENSORS

• Sensors or detectors or receptors:
  • Recognize deviation in any activity in internal environment.
  • Transmit the message to the control center.

2. CONTROL CENTER

• Control center or integrator center:
  • Receives the message from receptors.
  • Immediately sends commands to concerned effectors.

3. EFFECTORS

• Effectors receive commands from the control center.
• They either:
• Accelerate the activity, or
• Inhibit the activity.
• Thus, normalcy is restored.

MECHANISM OF ACTION OF HOMEOSTATIC SYSTEM

• Homeostatic mechanism in the body is responsible for maintaining normalcy of various body systems.
• Whenever there is any change in the behavioral pattern of any system:
• Effectors bring back normalcy either by:
  • Inhibiting and reversing the change, or
  • Supporting and accelerating the change,
  depending upon the requirement of the situation.
• This is achieved by means of feedback signals or control system.

Types of Feedback Control Systems

1. Negative feedback control system
2. Positive feedback control system
3. Feed-forward control system

1. NEGATIVE FEEDBACK CONTROL SYSTEM

• After receiving a message, effectors send inhibitory signals back to the system.
• The system then stabilizes its own function by:
  • Stopping activities, or
  • Reversing activities.

Example: Secretion of Thyroxine

Example: Maintenance of Water Balance

If water content decreases

If water content increases

2. POSITIVE FEEDBACK CONTROL SYSTEM

• Positive feedback mechanism → system reacts in such a way as to amplify (increase the intensity of) the change in the same direction.
• Positive feedback is less common than negative feedback.
• However, it has its own significance, particularly during emergency conditions.
• One such positive feedback mechanism occurs during blood clotting.
• Blood clotting is necessary to arrest bleeding during injury.

Blood Clotting

Blood clotting occurs in 3 stages:

1. Formation of prothrombin activator.
2. Conversion of prothrombin into thrombin.
3. Conversion of fibrinogen into fibrin by thrombin.

• Thrombin formed in the second stage:
  • Stimulates formation of more prothrombin activator.
  • Converts fibrinogen into fibrin.
• It causes formation of more and more prothrombin activator.

Other Processes Where Positive Feedback Occurs

• Milk ejection reflex
• Parturition
• Both processes involve oxytocin secretion.

Positive Feedback Mechanism: Parturition

3. FEED-FORWARD CONTROL SYSTEM

• In addition to positive and negative feedback, homeostasis mechanism also has feed-forward control system.
• Feed-forward control system → control system in homeostasis that:
  • Anticipates the change or deviation that may occur at a later stage.
  • Takes appropriate control action to avoid the disturbance.
• Feedback control systems detect the deviation only when it happens.

Examples of Feed-Forward Control System

1. Preadaptation before onset of exercise
   • Changes in cardiovascular system occur in order to prepare the body for exercise.
2. Secretion of gastric juice
   • Gastric juice, along with salivary secretion, occurs even before entrance of food in mouth.
3. Secretion of luteinizing hormone
   • Increase in secretion of luteinizing hormone occurs prior to ovulation.

HOMEOSTATIC IMBALANCE

• It is the starting point of disorders and diseases in the body.

Common Causes of Homeostatic Imbalance

1. Stress
2. Altered eating habits
3. Alcohol consumption
4. Sleeplessness
5. Over-exercise
6. Lack of water intake
7. Hypoxia
8. Long-term illness
9. Genetic mutation
10. Aging

Common Consequences of Homeostatic Imbalance

1. Acidosis
2. Diabetes mellitus
3. Hypoglycemia
4. Hyperglycemia
5. Dehydration
6. Hypertension
7. Heart failure
8. Gout
9. Infections
10. Disease caused by toxins.