Untitled Quiz

Homeostasis

• Homeostasis is a state of maintaining a similar condition, or Internal Stability, to cope with external variability in the environment.
• The body monitors its internal state and takes action to correct disruptions that threaten normal function.
• Failure to maintain homeostasis of critical variables disrupts normal function and may result in disease or a pathological condition.

Intended Learning Outcomes

• Define homeostasis and explain its importance in health and disease.
• Describe the main features of control systems in the body.
• Define negative and positive feedbacks and their role in homeostasis.
• Describe examples of biological rhythm.

Variables under Homeostatic Control

• Environmental factors that affect cells (osmolarity, temperature, pH)
• Materials for cell needs (nutrients, water, sodium, calcium, other inorganic ions, oxygen)
• Internal secretions (hormones and other chemicals that communicate between cells, having general and continuous effects).

Diseases and Homeostasis

• Diseases arise either from internal failure of physiological processes or from external sources.
• When homeostasis is disturbed, the body attempts to compensate.
• Compensation can fail, leading to illness or disease, or succeed, maintaining wellness.

Body's Internal Environment

• Extracellular fluid (ECF) is the watery internal environment surrounding cells, acting as a transition between the organism's external environment and the intracellular fluid (ICF).
• The ECF acts as a buffer zone between cells and the outside world.
• Physiological processes keep the composition of the ECF relatively stable.

Fluid Content of the Body

• Total body water (TBW): 60% of body weight in a 70 kg man is approximately 42 liters.
• Intracellular fluid (ICF): About 28 liters.
• Extracellular fluid (ECF): Approximately 14 liters.
  • Intravascular fluid (plasma): ~3 liters.
  • Extravascular fluid (~11 liters)
    • Interstitial fluid (~10.5 liters).
    • Transcellular fluid (~0.5 liters).

Differences Between Extracellular and Intracellular Fluids

• Body compartments are in a dynamic steady state, not equilibrium.
• Ion concentrations are different in the ECF and ICF.
• Special mechanisms transport ions through cell membranes to maintain ion concentration differences between the ECF and ICF.
• Major cations in the ECF include Na+, Ca2+, and Mg2+. Major anions include Cl− and HCO3−.
• Major cations in the ICF include K+, Mg2+, and some Na+. Major anions include PO43−, organic anions and proteins.

The Extracellular Fluid

• Contains large amounts of sodium, chloride, bicarbonate ions and nutrients for the cells (e.g. oxygen, glucose, fatty acids, and amino acids).
• Contains carbon dioxide that is transported from cells to the lungs for excretion.
• Contains cellular waste products transported to the kidneys for excretion.

The Intracellular Fluid

• Contains large amounts of potassium, magnesium, and phosphate ions.

Normal Ranges and Physical Characteristics of Extracellular Fluid Constituents

• A set of values of different factors in the ECF are presented.

Homeostatic Control Systems

• Functionally interconnected network of body systems.
• Maintain given factors in the internal environment.
• Example: temperature, fluid, nutrient levels, chemical composition.
• Achieve steady state from cellular to system levels.
• Detect deviations from normal.
• Integrate information with other relevant information.
• Make appropriate adjustments to return factors to 'normal' or desired steady state levels.

Homeostatic Components

• Input signal (sensed by receptor).
• Controller/integrating center (integrates information; initiates response).
• Output signal (creates response by effector).

Components of Homeostatic Control Mechanisms

• Sensor/receptor: Detects changes in internal or external environment (e.g., chemoreceptors, thermoreceptors, baroreceptors).
• Integrating center/control center: Receives information from sensors, initiates response.
• Effector: Any organ or tissue that brings about the response.

Local Control

• Relatively isolated change occurs in a tissue.
• A nearby cell or group of cells senses the change, usually by releasing a chemical.
• The response is restricted to the region where the change took place.
  • Example: Dilation of regional blood vessels in response to tissue hypoxia.

Reflex Control

• Widespread or systemic changes require more complex control systems.
• Uses nervous and/or endocrine systems.
• Output signals may be chemical, electrical, or both.
  • Example: Low blood pressure sensed by receptors, signals to the heart, kidneys and blood vessels to increase cardiac output and blood volume (increasing BP).

Negative Feedback

• A initiating stimulus elicits a response that results in an opposite effect, bringing the system back to normal functioning.
• Example: blood pressure regulation, metabolism, body temperature.

Negative Feedback Mechanisms

• Prevent over-activity of hormone systems.
• Controlled variable is often the degree of activity in target tissues..
• Appropriate level of activity in target tissues triggers feedback signals to endocrine glands, slowing further hormone secretion.
  • Example: control of thyroid hormone secretion.

Positive Feedback

• Enhances or accelerates output created by an activated stimulus.
• Leads to instability in most cases, but has some exceptions.
• Example: blood clotting, childbirth.

Positive Feedback example in Blood Clotting

• When a blood vessel ruptures, multiple clotting factors are activated.
• These factors activate other factors in the adjacent blood, causing more clotting.
• This continues until the rupture is plugged.

Positive Feedback in Childbirth

• Cervical stretching during childbirth triggers oxytocin release.
• Oxytocin stimulates uterine contractions, further stretching the cervix, initiating more contractions.
• This continues until delivery.

Positive Feedback (Vicious Circle)

• Does not lead to stability, but to instability and can cause death under some circumstances.
  • Example: excessive bleeding (Shock) - Reduced blood pressure, reduced cardiac perfusion, decreased cardiac contractility and output, leading to more decrease in BP causing a drop in heart function, followed by death.

Biological Rhythms

• Rather than a fixed steady value, set points can vary over time, producing biological rhythms.
• Example: menstrual cycle, body temperature variations during the cycle, fluctuations in cortisol levels during the day.

Disruptions of Homeostasis

• The body maintains a range for each factor to prevent illness, disease, and death, with the following categories:
  • Ideal value: Where body functions are at their most efficient.
  • Optimal range: Where body functions are efficient.
  • Range of tolerance: Where body functions can still function, but not at optimal.
  • Minimum or maximum set point: The maximum or minimum set point where moving past may cause illness and/or death.