Positive and Negative Feedback Mechanisms

By Patrick Lee,2014-11-25 19:08
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Positive and Negative Feedback Mechanisms

    Positive and Negative Feedback Mechanisms.

    Learning Objectives

    By the end of the tutorial , the student should be able to :

     Appreciate the different levels of organization in the human body.

     The different body systems of a human body.

     Appreciate extracellular fluid and the internal environment

     Define homeostasis and its example

     Understand the origin and removal mechanisms of the ECF

     The pivotal role of two body systems in the maintenance of homeostasis

     Homeostasis control systems

     Feedback loops types with examples

     Example when positive feedback loop causes death

    Levels of Organization





     System Level

     Organismic Level

    Cells as the living units of the body Cells as the living units of the body

    Body Systems

     Basic living unit of the body is the cell

     Each organ is an aggregate of many different cells held together by intercellular

    supporting structures

     Red blood cells, numbering 25 trillion in each human being, transport oxygen from

    the lungs to the tissues

     The entire body contains about 100 trillion cells.

    Body Systems

     Groups of organs that perform related functions and interact to accomplish a common activity essential to survival of the whole body

Do not act in isolation from one another

Human body has 11 systems

    Body Systems

    Body Systems

Extracellular Fluid—The ―Internal Environment‖

About 60 per cent of the adult human body is fluid, mainly a water solution of ions and

    other substances

Although most of this fluid is inside the cells and is called intracellular fluid

About one third is in the spaces outside the cells and is called extracellular fluid

    The Extracellular Fluid

     Extracellular fluid is in constant motion throughout the body

     It is transported rapidly in the circulating blood and then mixed between the blood

    and the tissue fluids by diffusion through the capillary walls

     In the extracellular fluid are the ions and nutrients needed by the cells to maintain

    cell life.

    Differences between Extracellular and Intracellular


     The extracellular fluid contains large amounts of sodium, chloride, and bicarbonate ions

    plus nutrients for the cells, such as oxygen, glucose, fatty acids, and amino acids

     It also contains carbon dioxide that is being transported from the cells to the lungs to be

    excreted, plus other cellular waste products that are being transported to the kidneys for


     The intracellular fluid differs significantly from the extracellular fluid; specifically, it

    contains large amounts of potassium, magnesium, and phosphate ions instead of the

    sodium and chloride ions found in the extracellular fluid.

     Special mechanisms for transporting ions through the cell membranes maintain the ion

    concentration differences between the extracellular and intracellular fluids.


Defined as maintenance of a relatively stable internal maintenance of a relatively stable internal

    environment environment

     Does not mean that composition, temperature, and other characteristics are

    absolutely unchanging

Homeostasis is essential for survival and function of all cells

     Each cell contributes to maintenance of a relatively stable internal environment


     Body cells are in contained in watery internal environment through which life-sustaining exchanges are made

     Extracellular fluid (ECF) - Fluid environment in which the cells live (fluid outside the cells)

     Two components:


     Interstitial fluid

Intracellular fluid (ICF) - Fluid contained within all body cells


    Balancing the Internal and External Environment Balancing the Internal and External Environment Cells, the fundamental units of life, exchange nutrients and Cells, the fundamental units of life, exchange nutrients and

    wastes with their surroundings: wastes with their surroundings:

     The intracellular fluid is conditioned by The intracellular fluid is conditioned by

     the interstitial fluid, which is conditioned by the interstitial fluid, which is conditioned by

     the plasma, which is conditioned by the plasma, which is conditioned by

     the organ systems it passes through. the organ systems it passes through.

    ICF ISF plasma organs


    environment internal environment

    Extracellular Fluid Transport and Extracellular Fluid Transport and

    Mixing SystemThe Blood Mixing SystemThe Blood

    Circulatory System Circulatory System

     Extracellular fluid is transported through all parts of the body in two stages

The first stage is movement of blood through the body in the blood vessels

The second stage is movement of fluid between the blood capillaries and the intercellular

    spaces between the tissue cells

    Origin of nutrients in the Extracellular Fluid Origin of nutrients in the Extracellular Fluid Respiratory System ( O2 & CO2)

     Gastrointestinal Tract (Carbohydrates, FA, AA)

     Liver and other organs that perform primary metabolic functions

     Musculoskeletal system (Movement , Protection from adverse events)

    Removal of Metabolic End Products

     Removal of CO2 by the lungs

     Kidney perform their functions in which most of the substances that are not needed by the

    body, esp the metabolic end products such as urea, are reabsorbed poorly and pass through

    the renal tubules into the urine

    Regulation of Body Systems

     Nervous system

     Controls and coordinates bodily activities that require rapid responses

     Detects and initiates reactions to changes in external environment

Endocrine system

     Secreting glands of endocrine regulate activities that require duration rather than


     Controls concentration of nutrients and, by adjusting kidney function, controls

    internal environment’s volume and electrolyte composition


    Factors homeostatically regulated include

     Concentration of nutrient molecules

     Concentration of water, salt, and other electrolytes Concentration of waste products

     Concentration of O = 100mmHg and CO= 40 mmHg 2 2

     pH = 7.35

     Blood volume 4-6 L and pressure 120/80 o Temperature = 37 C

    Control of Homeostasis

     Homeostasis is continually being disrupted by

     External stimuli

     heat, cold, lack of oxygen, pathogens, toxins

     Internal stimuli

     Body temperature

     Blood pressure

     Concentration of water, glucose, salts, oxygen, etc.

     Physical and psychological distresses

Disruptions can be mild to severe

If homeostasis is not maintained, death may result

    Control of Homeostasis

    Homeostatic Control Systems

     In order to maintain homeostasis, control system must be able to

     Detect deviations from normal in the internal environment that need to

    be held within narrow limits

     Integrate this information with other relevant information

     Make appropriate adjustments in order to restore factor to its desired


    Homeostatic Control Systems

     Control systems are grouped into two classes

     Intrinsic controls

     Local controls that are inherent in an organ

     Extrinsic controls

     Regulatory mechanisms initiated outside an organ

     Accomplished by nervous and endocrine systems

    Homeostatic Control Systems Feedforward - term used for responses made in anticipation of a Feedforward


Feedback - refers to responses made after change has been Feedback


     Types of feedback systems



    Examples of Control Mechanisms Examples of Control Mechanisms

     Regulation of Oxygen and Carbon Dioxide Concentrations in the Extracellular Fluid

     Regulation of Arterial Blood Pressure

    Feedback Loops: Types

Negative feedback loop Negative feedback loop

     original stimulus reversed

     most feedback systems in the body are negative

     used for conditions that need frequent adjustment

Positive feedback loop Positive feedback loop

     original stimulus intensified

     seen during normal childbirth

    Negative Feedback Loop

     Negative feed back loop consists of:

     Receptor - structures that monitor a controlled condition and detect changes

Control center - determines next action


     receives directions from the control center

     produces a response that restores the controlled condition

    Negative Feedback Loop

    Negative Feed Back Loop

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