Introduction to Homeostasis Notes

Learning Outcomes

• Describe human physiology
• Outline the structure and function of key systems
• Describe homeostasis
• Explain positive and negative feedback systems

Human Physiology

• Physiology - Study of life processes
  • Movement: Movement of substances, locomotion
  • Respiration: Gas exchange
  • Sensation: Responsiveness, ability to sense changes and react
  • Nutrition - Digestion: Break-down and delivery of nutrients
  • Nutrition - Metabolism: All chemical reactions within the body
    • Production of energy
    • Making body structures
  • Excretion: Elimination of waste from metabolic reactions
  • Reproduction: Production of future generation
  • Growth: Increasing of cell size or number
  • MRS. NERG (Mnemonic for the above)

Physiology: Structural Organisation

• Molecular Level: Atoms, Chemical level, Molecule (DNA)
• Cellular Level: Cellular level, Smooth muscle cell
• Tissue Level: Tissue level, Smooth muscle tissue, Epithelial tissue
• Organ Level: Serous membrane, Stomach
• System Level: Digestive system (Esophagus, Liver, Stomach, Pancreas, Gallbladder, Small intestine, Large intestine, Rectum)
• Organismal Level: Whole organism
• Physiology requires understanding of anatomy and explains it

Homeostasis

• Maintenance of a stable internal environment = a dynamic state of equilibrium
  • Homeo- = Greek ‘The same’
  • -stasis = Greek ‘Equilibrium’
  • It is NOT static, but attempts to achieve equilibrium
• Homeostasis must be maintained for normal body functioning and to sustain life
• Homeostatic imbalance – a disturbance in homeostasis resulting in disease (a.k.a. Pathophysiology)

Homeostasis of Organ Systems

• Nervous
• Blood
• Respiratory
• Reproductive
• Skeletal
• Endocrine
• Muscular
• Cardiovascular
• Digestive
• Urinary

Examples of Organ System Functions in Homeostasis

• Muscular System:
  • Allows locomotion, facial expression
  • Maintains posture – supports other organ systems
  • Thermoregulation - Produces heat
• Cardiovascular System:
  • Heart and vessels that transport blood (Control of blood pressure)
  • Blood – Haematology (Maintains nutrient balance + immunity)
• Respiratory System:
  • Gas homeostasis
  • Adds oxygen and removes carbon dioxide
• Digestive System:
  • Breaks down food + absorbs nutrients
  • Maintains blood nutrient homeostasis
  • Eliminates indigestible material (excretion)
• Nervous System:
  • Fast-acting control system
  • Responds to internal and external change
  • Activates muscles and glands
  • Controls homeostasis of multiple systems
• Endocrine System:
  • Slightly slower acting control system
  • Controls sleep and wakefulness
  • Controls homeostasis of multiple systems
• Urinary System:
  • Eliminates nitrogenous wastes
  • Maintains acid–base balance
  • Maintains osmolarity

Organ Systems are Connected

• Digestive system: Takes in nutrients, breaks them down, and eliminates unabsorbed matter (feces)
• Respiratory system: Takes in oxygen and eliminates carbon dioxide
• Cardiovascular system: Via the blood, distributes oxygen and nutrients to all body cells and delivers wastes and carbon dioxide to disposal organs
• Urinary system: Eliminates nitrogenous wastes and excess ions
• Integumentary system: Protects the body as a whole from the external environment
• Nutrients and wastes pass between blood and cells via the interstitial fluid

terminology

• Gastr- Stomach
• Cerebro- Brain
• Hepa- Liver
• Cardio- Heart
• Nephr- Kidney
• Pneumo- Lung
• Entero- Intestine
• Derm- // Cut- Skin

Homeostatic Control Mechanism

• Stimulus: Produces change in variable
• Receptor (sensor): Detects change
• Input: Information sent along afferent pathway to Control center
• Output: Information sent along efferent pathway to Effector
• Response of effector feeds back to influence magnitude of stimulus and returns variable to homeostasis

Example: Glucostatic Control

• Homeostasis: Normal blood glucose level (about 90 mg/100 ml)
• High blood glucose level detected by insulin-secreting cells of pancreas
• Insulin-secreting cells of pancreas stimulated to release insulin into the blood
• Most body cells take up more glucose
• Liver takes up glucose and stores it as glycogen
• Blood glucose level declines to a set point; stimulus for insulin release diminishes and body returns to homeostasis
• Declining blood glucose level, Low blood glucose level detected by glucagon-releasing cells of pancreas
• Glucagon-releasing cells of pancreas stimulated to release glucagon into the blood; target is the liver
• Liver breaks down glycogen stores and releases glucose to the blood
• Blood glucose level rises to set point; stimulus for glucagon release diminishes and body returns to homeostasis

Terminology

• Gluco- or glyco- comes from the Greek word glykys meaning "sweet".
• Geno- comes from the Greek word genesis meaning "birth, origin, creation".
• Lipo- comes from the Greek word lipos meaning "fat".
• Lysis comes from the Greek word lyein meaning "to unfasten, loose, untie".
• Neo- comes from the Greek word neo meaning "new, recent".
  1. Gluconeogenesis
  2. Glycogenolysis
  3. Lipolysis
  4. Glycogenesis
  5. Lipogenesis

Systems Working in Harmony: The Stress Response

• Blood
• Epinephrine and norepinephrine
• Adrenergic receptor activation
• Liver
  • Glycogenolysis
  • Gluconeogenesis
• Skeletal muscle
  • Glycogenolysis
• Adipose
  • Lipolysis
• Pancreatic β-cells
  • Insulin synthesis and release

Homeostasis: Feedback Loops

• Negative Feedback Loops – most homeostatic processes
  • Example: Glucostatic control (insulin and glucagon)
• Positive Feedback Loops – special scenarios e.g. blood clotting

Example of Positive Feedback

• Auto-amplification of cytokines in immunity
  • Cytokines e.g. TNF
  • Chemokines e.g. CCL2
• Always have inbuilt or external negative feedback loops – there is no exception in human physiology
  • E.g. clotting – if you don’t stop it – the entire vasculature will clot
• Positive feedback loops can shift homeostatic set-point to a new level
• IMPORTANTLY - Homeostasis must be maintained even if this is at a different level

New Homeostatic Set-Points

• Homeostasis = Dynamic equilibrium (Not static)
• Multifactorial
• Circadian rhythms
• Hormone rhythms (e.g. menstrual cycle)
• Varying environment

Equilibriums Vary

• Steady state over a day
  • Blood glucose
  • Cortisol release
  • Blood pressure
  • Core body temperature

New Homeostatic Set-Points

• Homeostasis = Dynamic equilibrium (Not static)
• New set points occur in both Physiology and Pathophysiology
• Usually as a response to triggers by a ‘regulatory system’ or failure of one
• Examples – Type II diabetes and the immune response

Immune System Activity

• Immune response - Sepsis
  • Immune hyperactivity, damage from inflammation
  • Organ failure, Early death
  • Immune suppression, vulnerability to infection
• Innate immune dysregulation
  • Persistent inflammation
  • Chronic catabolism
  • Decreased cytokine production
  • Myeloid cell immaturity
  • Reduced phagocytosis
  • Contracted antigen presentation
• Adaptive immune suppression
  • T cell anergy
  • Lymphocyte apoptosis
  • Diminished T cell cytotoxicity
  • Reduced T cell proliferation
  • Increased Treg suppressor function
  • T cell Th1-Th2 polarization
  • Recurrent infections
  • Continued organ injury
  • Poor tissue regeneration
  • Long-term death
  • Opportunistic infections
  • Long-term death

Immune Response – Sepsis

• 'Normal'
• New homeostatic set-point (shorter term)
• New homeostatic set-point (long term)
• Regulatory system = Extremely complex and poorly understood (immune response itself and hormones e.g. cortisol)

Diabetes – Type 2

• 'Normal'
• (Western) Healthy
• Diabetic
• Dynamic equilibrium
• Regulatory system = Insulin production (dysfunctional) and glucose sensitivity (receptors – sensor failure)

Diabetes – Type 1

• Diabetic coma – dependent on sugar from diet
• Loss of regulation results in a new homeostatic set-point (dynamic)
• Not tolerated by body organs (e.g. Brain) - Pathophysiology
• Regulatory system = Insulin production (Almost completely gone at end-stage of disease)

Homeostasis and Pathophysiology

• In terms of homeostasis - pathophysiology is an imbalance in ‘normal’ homeostasis that is not tolerated
• This can be due to:
  • A new homeostatic set-point caused by a permanent or long lasting alteration in regulation (e.g. type 2 diabetes)
  • An acute imbalance that overloads the system – regulatory systems can't cope (e.g. glycotoxicity)

Principles of Physiology

• Homeostasis a main principle of physiology
  1. The functions of organ systems are coordinated with each other.
  2. Most physiological systems are controlled by multiple regulatory systems, often working in opposition.
  3. Homeostasis is essential for health and survival.
  4. Information flow between cells, tissues and organs is essential – integration of physiological processes.
  5. Controlled exchange of materials occurs between compartments and across cellular membranes.
  6. Physiological Processes dictated by laws of chemistry and physics.
  7. Physiological Processes require the transfer and balance of matter and energy.
  8. Structure is a determinant of – and has coevolved with – function.

Summary

• Physiology is study of processes operating within an organism.
• Organ systems are interconnected and are formulated to maintain life
• Homeostasis attempts to keep the system in balance
• Positive and negative feedback maintain homeostasis by increasing or decreasing signals

Recommended Reading

• Marieb, 10th or 11th Edition: ESSENTIALS of Human Anatomy & Physiology
• Vanders, 12th-15th Edition: VANDER'S Human Physiology