A-Level Bio Chapter 14: Homeostasis

Introduction

  • To function efficiently, organisms have control systems in place to keep internal conditions near constant → Homeostasis

  • Physiological factors that are controlled by homeostasis in mammals:

    • Core body temperature

    • Metabolic wastes (CO2 and urea)

    • Blood pH

    • Blood glucose concentration

    • Water potential of the blood

    • The concentration in the blood of respiratory gases (oxygen and carbon dioxide)

Internal Environment

  • All conditions in the body dictate the conditions the cell must function within

  • Three features of tissue fluid that influences how cells functions:
    1. Temperatyre (low temperatures slow processes, but too high can denature proteins)
    2. Water potential

    • If water potential decreases, water may diffuse out of cells, stopping or slowing metabolic processes.

      • If water potential increases, water will diffuse into cells that could swell or burst.

  • 3. Concentration of Glucose

    • Lack of glucose can slow or stop respiration (no energy source)

    • Too much glucose could cause water to move out of the cell.

Homeostatic Control

  • Negative Feedback Loop - most control mechanisms in living organisms use this.
    Step 1: Receptors detect stimuli (internal or external)
        - Information travels to a central control (brain/spinal cord) via nervous system → “input”
        - Stimuli: Any change in a factor (like a temperature of water content of blood)
    Step 2: Effectors [whatever is receiving] (like muscles and glands) receive information
        - Central control instructs an effector to carry out a specific action → the “output”
    Step 3: Corrective actions → work to correct or reverse the change that was detected
        - Factors fluctuate around a particular “ideal” value or set point
        - Negative feedback keeps factors near the ideal by minimizing the difference between the actual value and the ideal

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  • Positive Feedback
        - Positive feedback increases the effect of the change (instead of trying to minimize it)
        - Example: Breathing air with a high CO2 concentration → More CO2 in the blood
            - Sensed by carbon dioxide receptors → Breathing rate increases
            - The person breaths faster, taking in even more CO2 → leads to more CO2 sensed by receptors → continues to increase breathing rate

photo 2

  • Mammalian coordination systems:
        - Nervous system [instant]: Uses electrical impulses transmitted along nerve cells (neurons)
        - Endocrine system [takes long]: Uses hormones that travel in the blood as a form of long-distance signaling

Control of Body Temperature: Thermoregulation

  • Involves both coordination systems

  • All mammals generate heat and have ways to retain it (“warm-blooded’)

    • Heat generated during cell respiration - liver cells especially

  • Hypothalamus: “the body’s thermostat”

    • Receives a constant input of sensory information about the temperature of the blood-thermoreceptor cells; monitors the core temperature of the body (37*C in humans)

  • Skin receptors play a big role - skin temperature is the first to change if there is a change in surroundings “early warning” system

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Ways to REDUCE heat loss [aka you’re cold]:

  • Vasoconstriction - muscles in the walls of. your arterioles that supply blood to capillaries near the skin’s surface contracts; lumens narrow and reduces supply of blood to capillaries → reduces heat lost

  • Shivering - involuntary contraction of skeletal muscles (generates heat)

  • Raising Body Hairs [aka goosebumps] - Muscles at the base of hairs in the skin contract to increase the depth of fur-trapping air closer to the skin

  • Decreasing the production of sweat - reduces loss of heat via evaporation from the skin surface

  • Increasing the secretion of adrenaline - Hormone from the adrenal glands increases the rate of heat production in the liver

  • Animals can also curl up to reduce the surface area exposed to the air

Ways to INCREASE heat loss [aka you’re warm]:

  • Vasodilation - Muscles in arterioles close to the skin relax, allowing more blood to flow through the capillaries to increase heat lost

  • Lowering Body Hairs - reduce the depth of fur and the layer of insulating air

  • Increasing sweat production - sweat evaporates on the surface of the skin (removing heat from the body)

  • Limbs spread out to increase the body surface area exposed to the air

Gradual Temperature Changes

  • When environmental temperature decreases slowly (seasons changing), the hypothalamus releases a hormone → activates the anterior pituitary gland to release