Chapter 1 Lecture 3

Physiological Variation and Homeostatic Balance

Physiological Variation

• Physiology varies based on several factors:
  • Gender
  • Age
  • Weight
  • Diet
  • Physical activity
  • Environment
• Anatomical variation can also contribute to physiological differences.
• Understanding physiological variation is important for healthcare decisions, including medication dosage and treatment plans.

Reference Values for Physiological Parameters

• Typical physiological parameters, such as blood pressure, are based on values from healthy young adults.
• Reference man:
  • 22 years old
  • 154 lbs
  • Ambient temp 20°C
  • Light physical activity
  • Intake = 2,800 calories/day
• Reference woman:
  • 22 years old
  • 128 lbs
  • Ambient temp 20°C
  • Light physical activity
  • Intake = 2,000 calories/day

Homeostasis

• Coined by Walter B. Cannon in 1932.
• Refers to the body's maintenance of relatively constant internal states.
• The environment around body cells remains constant within a certain range, despite external variations.
• The body detects changes and activates mechanisms to oppose them, thus stabilizing conditions.
• Example: Body temperature remains fairly constant (36-37°C; 97-99°F), even with changing environmental conditions.

Dynamic Equilibrium

• The body adjusts variables from a normal "set point" within an acceptable range.
• This range fluctuates slightly, leading to a state of dynamic equilibrium.
• Examples:
  • Temperature regulation
  • Regulation of blood carbon dioxide level
  • Regulation of blood glucose level
  • Regulation of blood pressure

Homeostatic Control Mechanisms

• What is the appropriate response of the body when it moves away from its normal range?
  • Activation of mechanisms to reverse the change.
  • OR
  • Activation of mechanisms to enhance the change.

Basic Components of Homeostatic Control Mechanisms

1. Sensor mechanism (receptor): Sends afferent signal.
2. Integrating, or control, center: Analyzes signal and sends efferent signal.
3. Effector mechanism: Directly influences controlled physiological variable.

Feedback Control Mechanisms

• Homeostasis is maintained or restored via self-regulation through feedback control mechanisms.
• Information flows from a sensor to the integrator to effect a response.
• Loss of these control mechanisms causes illness or death (e.g., heat stroke).

Negative Feedback Mechanisms

• Inhibitory: the body senses a change and activates mechanisms to reverse it
• Stabilize physiological variables
• Produce an action opposite to the change that activated the system
• Responsible for maintaining homeostasis
• More common than positive feedback control systems

Temperature Regulation Example

• Temperature increase generates heat, which is then detected by sensors.
• A correction signal is sent to effectors to decrease temperature.
• Muscles shivering is an effector that increases temperature.
• Temperature receptors are the sensors.
• The hypothalamus acts as the integrator.
  • The set point is 37°C, while the actual value may fluctuate (e.g., 36°C).

Visual Representation

• Diagram of sensors, integrators, and effectors connected via nerve fibers and electrical wires.

Human Thermoregulation

• The brain (hypothalamus) senses changes in blood temperature.
• If overheating, vessels dilate in the skin and sweating begins.
• If too cold, vasoconstriction in the skin and shivering begins.
• The set point can change under special circumstances, e.g., bacterial infection.

Human Blood Pressure Control

• Stretch receptors in blood vessels detect a rise in blood pressure.
• The cardiac center in the brainstem sends out nerve signals via the vagus nerve to slow heart activity.
• Heart rate slows, and blood pressure falls.

Postural Control of Blood Pressure

• When a person rises from bed, blood drains from the upper body, creating a homeostatic imbalance.
• Baroreceptors above the heart respond to the drop in blood pressure.
• Baroreceptors send signals to the cardiac center of the brainstem.
• The cardiac center accelerates heartbeat.
• Blood pressure rises to normal; homeostasis is restored.

In-class Activity: Pharmaceutical Interventions for Blood Pressure

• When the body is unable to carry out homeostatic functions effectively, pharmaceutical interventions may be needed.
• Examples of blood pressure medications:
  • Lisinopril
  • Lisinopril hydrochlorithiazide
  • Amlodipine
  • Norvasc
  • Prazosin
  • Carvedilol
  • Losartan
  • Benicar HCT
  • Lasix
  • Toprol XL
  • Doxazosin
• Research how assigned medications return blood pressure to "normal."

Beta Blockers

• Largest class of blood pressure medication.
• Function by inhibiting the activity of adrenaline (epinephrine).
• Epinephrine speeds up the heart rate and increases blood pressure (fight or flight response).
• Released from the adrenal glands, epinephrine acts on cardiac muscle to promote greater force of contraction by binding to beta receptors.
• Beta blockers bind to epinephrine receptors (beta receptors), preventing epinephrine from stimulating the cardiac muscle.

Hypertension

• Why do people develop hypertension despite homeostatic regulation mechanisms?
  • Primary (essential) hypertension:
    • Develops gradually over time with no definitive cause.
  • Secondary hypertension:
    • Caused by an underlying condition, with a sudden onset and acute presentation.
    • Underlying causes:
      • Adrenal gland tumors
      • Congenital blood vessel defects
      • Hormonal imbalance caused by medications (e.g., birth control pills, over-the-counter pain relievers, some prescription drugs, and narcotics).

Positive Feedback Loops

• Self-amplifying change.
• Reinforces the initial change.
• Normal way of producing rapid changes in the body.
• Examples:
  • Childbirth
  • Blood clotting
  • Protein digestion
  • Generation of nerve signals

Positive Feedback Mechanisms

• Stimulatory
• Amplify or reinforce the change that is occurring
• Tend to produce destabilizing effects and disrupt homeostasis
• Bring specific body functions to swift (rapid) completion

Childbirth: Positive Feedback Example

• Contractions of the uterine wall force the baby's head or body into the cervix, increasing stretching of the cervix.
• Stretch-sensitive nerve cells in the cervix send nerve impulses to the brain.
• The brain interprets input and releases oxytocin.
• Oxytocin causes muscles in the uterine wall to contract more forcefully.
• Increased stretching of the cervix causes the release of more oxytocin, resulting in more stretching of the cervix.
• The cycle is interrupted by the birth of the baby, which decreases stretching of the cervix.

Negative Effects of Positive Feedback

• In the case of homeostatic imbalance leading to life-threatening fever (heat stroke):
  • Body temperature > 104°F
  • Increases metabolic rate
  • Body produces heat even faster
  • Cycle continues to reinforce itself
  • Becomes fatal at 113°F

Assignment

• Provide a detailed account of what occurs in the body during normal thermoregulation (at the molecular, cellular, organ-system levels) and what happens when someone is suffering from heat stroke. Include details such as why there is no sweat even though the body is overheating.
• Log on to Canvas and explain (250 words)

Homeostatic Imbalances

• Disorder: Abnormality of structure or function
• Disease: Illness with set of signs and symptoms
• May be local or systemic
• Symptoms: Subjective (headache, nausea, anxiety)
• Signs: Objective
  • Anatomical (swelling, rash)
  • Physiological (fever, paralysis, high blood pressure)

Cycle of Life Considerations

• The structure and function of the body undergo changes over the early years (developmental processes) and late years (aging processes).
• Infancy and old age are periods of time when the body functions least well.
• Young adulthood is the period of greatest homeostatic efficiency.
• In latter years, atrophy—the term used to describe the wasting effects of advancing age—ensues.

Summary

• Physiological processes are controlled at many levels, from the chemical to organismal levels
• Homeostasis is maintained via negative feedback mechanisms
• Loss of homeostasis leads to illness; studied in pathophysiology