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HomHomeostasiseostasis

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HomeostasisHomeostasis• Maintenance of Relatively Constant Chemical/Physical

Conditions of the internal environment. of the internal environment.

SStable =/= rigidity, can vary within narrow limittable =/= rigidity, can vary within narrow limit (normal physiological range)(normal physiological range)

The golden goal of every organ : to maintain homeostaThe golden goal of every organ : to maintain homeostasissis (concept of REGULATION) (concept of REGULATION)

HoHomeostasismeostasis

Claude Bernard (mid 1800s)◦pancreas, liver◦brain, smooth muscle.◦internal environment.

The father of modern PhysiolThe father of modern Physiologyogy

◦The internal environment reThe internal environment remains relatively constant thomains relatively constant though there are changes in the ugh there are changes in the external environmentexternal environment

HomeostasisHomeostasis

Walter Cannon◦Coined -homeostasis◦sympathetic nervous

system Bodily Changes in Hunger,

Fear, and Rage

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In fasting blood In fasting blood

Arterial pHArterial pH 7.35-7.457.35-7.45

BicarbonateBicarbonate 24-28 mEq/L24-28 mEq/L

OO22 content content17.2-22.0 ml/100 ml17.2-22.0 ml/100 ml

Total lipidTotal lipid 400-800 mg/100 ml400-800 mg/100 ml

GlucoseGlucose 75-110 mg/100 ml75-110 mg/100 ml

Normal Normal Physiological rangesPhysiological ranges

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Homeostasis & Controls

•Successful compensation

•Homeostasis reestablished

•Failure to compensate

•Pathophysiology

•Illness

•Death

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Regulation of the Body Functions

Regulation- the ability of an organism to maintain a stable internal conditions in a constantly changing environment

-Three types:

1. Chemical (hormonal) Regulation- a regulatory process performed by hormone or active chemical substance in blood or tissue.

-It response slowly, acts extensively and lasts for a long time.

2. Nervous Regulation- a process in which body functions are controlled by nerve system

- Pathway: nerve reflex

- Types: unconditioned reflex and conditioned reflex

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- Example: baroreceptor reflex of arterial blood pressure

- Characteristics: response fast; acts exactly or locally, last for a short time

3. Autoregulation – a tissue or an organ can directly respond to environmental changes that are independent of nervous and hormonal control

In the human body these three regulations are coordinated and acts as one system, “feedback control system”.

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Feedback Control

The word “feedback” means a process in which a part of output (feedback signal) from controlled organ returns to affect or modify the action of the control system.

Feedback control mechanism consists of two forms: negative and positive feedback control.

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The feedback signals from controlled The feedback signals from controlled system produces effect opposite to the system produces effect opposite to the action of the control system.action of the control system.

The opposite effect is mainly The opposite effect is mainly “inhibitory action”.“inhibitory action”.

Negative feedbackNegative feedback

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Control SystemsControl Systems(Negative feedback in cardiovascular and (Negative feedback in cardiovascular and

endocrine systems)endocrine systems)

Reference Signal Comparator Controller Effectors Regulated

Variable

Sensor

FeedbackSignal

Error Signal

Hyperthermia Heat receptors in the skin

Hypothalamus

StressSensors Control Center

Increased activity of

sweat glands

Increased blood flow to the skin

Effectors

Perspiration evaporates

cooling the skin

Effect

Stress is reduced shutting down mechanism

Homeostatic Regulation of Body Temperature through Negative Feedback

Homeostasis Using a Neural Pathway

Control center

Many homeostatic mechanisms use a nerve pathway in which to produce their effects. These pathways involve an afferent path which brings sensory messages into the brain and an efferent path which carries outgoing nerve messages to effectors.

Hyperglycemia Pancreas-beta cells

Sensor and Control center

Insulin is releasedinto blood

Liver and Muscle cells take up glucose from

the blood

Effectors

Blood glucose is reduced

Stress is reduced shutting down

mechanism

Stress

Homeostatic Regulation of Blood Sugar through Negative Feedback

Negative Feedback Via a Hormonal PathwayRegulation of Blood Sugar

Hormones play an important role in many homeostatic pathways. Hormones are produced by endocrine glands. They enter the blood after being produced and travel throughout the body. However, hormones have their effect on specific target tissues.

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Positive feedback Positive feedbackThe feedback signal or output from the The feedback signal or output from the controlled system increases the action of controlled system increases the action of the control systemthe control system

Examples: Blood clotting, Micturition, Examples: Blood clotting, Micturition, defecation, Nadefecation, Na++ inflow in genesis of nerve inflow in genesis of nerve signals, Contraction of the uterus during signals, Contraction of the uterus during childbirth (parturition).childbirth (parturition).

Positive Feedback Mechanisms

Homeostatic systems utilizing positive feedback exhibit two primary characteristics:

1. Time limitation – Processes in the body that must be completed within a constrained time frame are usually modified by positive feedback.

2. Intensification of stress – During a positive feedback process, the initial imbalance or stress is intensified rather than reduced as it is in negative feedback.

Typical Positive Feedback Process

Stress Sensor Control Center

EffectorIntensifies

Homeostatic Regulation of Child Birth through Positive Feedback

Pressure of Fetus on the Uterine Wall

Nerve endings in the uterine wall carry afferent messages

to the Hypothalamus

Production and Release of Oxytocin into the

BloodIncreasing strength of uterine contractions

Intensifies

The birth of the child will bring this process to a close. Other examples of positive feedback regulation occur during milk letdown and blood clotting.

Positive feedback “mini-loops” are built into pathway to speed up production of chemicals needed to form the clot. Entire sequence of clotting is a negative feedback pathway:

Feedback in Coagulation

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Importance:

1) Enhance the action of original stimulus or amplify or reinforce change, promote an activity to finish

2) It is known as a vicious circle because it can lead to instability or even death

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Feed-forward control

Concept: a direct effect of stimulus on the control system before the action of feedback signal occurs.

Here the direct effect of the stimulus is termed disturb signal or interfere signal.

Example: Shivering before diving into the cold water

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Significance of Feedback-forward :

adaptive feedback control.

makes the human body to foresee and adapt the environment promptly and exactly

(prepare the body for the change).