WEEK 4 Dr.Zain, Homeostasis 2014_.ppt

Homeostasis“Regulation of Body Functions”

Objectives:• To define homeostasis.• To know the different regulatory systems. • To understand the principle feedback

mechanisms.• To understand the importance of the

different feedback mechanisms involved in regulation of the body functions

2

Homeostasis• The underlying principle of physiology is homeostasis

• Homeo : same stasis : standing still• Homeostasis is the ability to maintain a relatively

stable internal environment in an ever-changing outside world

• The internal environment of the body is in a dynamic state of equilibrium (Dynamic Constancy)

• Chemical, thermal, and neural factors interact to maintain homeostasis

• Loss of homeostasis results in disease or death

Homeostasis

The relatively stable internal environment needed to

I. Maintain life.

II. Provides an optimal internal environment for cell function

Prof.M.Barbary 4

Importance of Homeostasis• Metabolic reactions are controlled by

enzymes• Enzymes work best in a narrow range of

temperature & pH only• Important to keep internal environment

as steady as possible• Maintain body systems within ranges

necessary to maintain the homeostatic state.

• Alterations in the homeostatic state or levels indicate the presence of disease or dysfunction.

5

1-6

Homeostatically Regulated Variables:-as

• Body Temperature• Blood Composition (ions, sugars, proteins)

• Concentrations of O2 and CO2 in the blood

• Acid-Base balance (pH)• Blood osmolarity• Blood pressure, cardiac output, Heart rate• Respiratory rate and depth• Secretions of endocrine glands

1-7

Homeostatic Control Mechanisms

E.g. Increase in blood sugar levels or body temperature

• The three interdependent components of control mechanisms are:

• Receptor – monitors the environments and responds to changes (stimuli)

• Control center – determines the set point at which the variable is maintained

• Effector – structures that provide the means to respond to the stimulus and restore the variables to the optimal physiological range.

Stimulus:Produceschangein variable

1

2

3

Changedetectedby receptor

Input:Informationsent alongafferentpathway to

5 Response ofeffector feedsback to influencemagnitude of stimulus andreturnsvariable tohomeostasis

Variable (in homeostasis)

Imbalance

Imbalance

Receptor (sensor)

Controlcenter 4 Output:

Information sentalong efferentpathway to

Effector

Homeostatic Control Mechanisms

• Values of variables fluctuate around the set point to establish a normal range of values.

• Set point :The ideal normal value of a variable.

• What is the set point for body temperature? For blood sugar levels? For blood pH? For blood osmolarity?

Change in setpointThe set point for some variables may

be ‘reset’ -that is, physiologically raised or lowered (e.G., Bp)

Changes in set point developed by– Acclimatization :  occurs by slowly

altering that 'set point‘ to cope with long-term changes to its environment. Eg (raised Bp)

– Biorhythms: circadian rhythm, sleep-awake, menopause cycle

Control System Action

Prof.M.Barbary 11

REGULATION FACTOR

R = CHANGE WITH REGULATION

CHANGE WITHOUT REGULATION

GAIN

G = CORRECTION APPLIED

RESIDUAL CHANGE ErrorCorrectionGain

Increase the Gain ----- more effective control system

Prof.M.Barbary 12

VARIABLE SET POINT DISTURBANCE CHANGEWITHOUT

REGULATION

CHANGE WITH

REGULATION

BP 120 mmHg BLOOD LOSS (-60) 60 mmHg (-20)100 mmHg

BODY TEMP

37OC COLD EXPOSURE (-17) 20OC (-0.5) 36.5OC

•FIND OUT THE REGULATION FACTOR, AND GAIN FOR BOTH CONTROL SYSTEM

•WHICH SYSTEM IS MORE EFFICIENT?

Prof.M.Barbary 13

CORRECTION BY THE

CONTROL SYSTEM

REGULATION FACTOR

GAIN

40 mmHg -20/-60=1/3 40/20=2

16.5 OC -0.5/-17=1/34 16.5/0.5=33

Prof.M.Barbary 14

Negative feedback mechanismThe control center activates the effector system

generating a response which cancels the effect of the stimulus (change) and reestablishes the homeostatic condition.

Gravitational effects in cardiovascular system

Immediately on standing, the arterial pressure in the head and upper part of the body tends to fall, caused by pooling of blood in leg veins reducing venous return and marked reduction of this pressure could cause loss of consciousnessHomeostatic mechanism on controlling this

effect Stimulation of baroreceptors stimulate vasomotor

center to elicit an immediate reflex resulting in: 1. Heart Rate increase2. Venoconstriction3. Arteriolar constriction4. Increase level of renin & aldosteroneThis minimizes the decrease in pressure in the head

and upper body.

2. Positive Feedback

• In contrast to negative feedback, positive feedback is relatively uncommon in the human body.

• The control center activates effectors which generate a response which increases the stimulus further reinforcing a the initial change ( vicious cycle)

• Positive feedback does not lead to stability

POSITIVE FEEDBACK MECHANISM

UNUSUAL P

HENOMEN

A

Prof.M.Barbary 20

Examples of positive feedback

• The upstroke of the action potential in nerve and muscle.

• The action of the hormone oxytoxin on the uterus during birth.

• Surges of hormones can occure with of positive feedback.

• Lactation • Blood clotting

EXAMPLE OF POSITIVE FEEDBACK

22

Pituitary glandPituitary gland

Osmoregulation (Water & Mineral salts)Osmoregulation (Water & Mineral salts)

Regulate water potential in Tissue Fluid,Organ involved: KIDNEYS

Osmoregulation (Water & Mineral salts)Osmoregulation (Water & Mineral salts)

• After having a very salty meal

produce concentrated urine to remove

excess salts in solution form

extra water is needed to be excreted

along with the excess salts

sensation of thirst (drink more water to

compensate for the water loss)24

Maintenance of Body TemperatureMaintenance of Body Temperature

Control Body Temperature in hot and Cold Control Body Temperature in hot and Cold ConditionsConditions

Regulation of Blood Glucose Level

Controlled by negative feedback mechanism

Controlled by insulin secreted from the islets of langerhans in pancreas

Diabetes - malfunction of pancreas (does not secrete enough insulin)

Feedback LoopA rise in blood glucose causes release of insulin from beta cells the pancreas, promoting glucose uptake in cells and storage as glycogen in the liver .

A fall in blood glucose stimulates alpha cells in the pancreas to secrete glucagon, which causes the liver to break down glycogen and release glucose .

Summary• Homeostasis• Homeostatic control system-Gain• Negative feedback-examples• Positive feedback-examples• Body temperature regulation• Blood pressure regulation• Body fluid regulation• Blood glucose regulation