Dr. Ayisha Qureshi

Professor & Head of Department

Department of Physiology

INTRODUCTION TO PHYSIOLOGY &

HOMEOSTASIS

What is Physiology?

Physiology is the study of the normal functions of the living things:

their various systems & the different organs that make up the

systems!

WE SHIVER WHEN WE ARE COLD?

↓ Why

TO HELP WARM UP BECAUSE SHIVERING GENERATES HEAT

↓ How

INVOLUNTARY REPETITIVE CONTRACTION OF THE SKELETAL MUSCLES GENERATES HEAT

The Physiologists explain

WHY and HOW the processes occur.

Organization of the Human Body

CellsTissues

Organs

Organ

systems

Organisms

(Human body)

Body Systems (11 systems)

Body Systems (11 systems)

THE INTEGRATION B/W SYSTEMS OF

THE BODY.

Themes in Physiology:

Structure and function are related.

Living organisms need energy.

Information flow coordinates body functions.

Homeostasis maintains Internal stability.

INTERNAL ENVIRONMENT OF THE BODY

IF EACH CELL IS ABLE TO SURVIVE ON IT’S OWN, THEN WHY CAN’T THEY SURVIVE WHEN THE

BODY DIES?

IF EACH CELL IS ABLE TO SURVIVE ON IT’S OWN,

THEN WHY CAN’T THEY SURVIVE WHEN THE BODY

DIES?

The cells in a multicellular organism cannot live and function without contributions from the other body cells because the

vast majority of cells are not in direct contact with the external environment.

WHAT IS EXTRACELLULAR FLUID (ECF)?

WHAT IS INTRACELLULAR FLUID (ICF)?

WHAT IS PLASMA?

The fluid contained within

all body cells is called

intracellular fluid

(ICF).

The fluid outside the cells is

called extracellular fluid

(ECF).

THE ECF IS THE

INTERNAL

ENVIRONMENT OF

THE BODY.

ICF is inside the cells.

ECF is outside the cells.

As humans, the environment we live in is the external environment;

Our cells live in the body’s internal environment.

Unicellular organisms do NOT have an internal environment as they are cells placed directly in

their external environment. Their cell membranes are in direct contact with the outside.

Thus, all cells live in the Extracellular fluid (ECF) which is

essentially the Internal environment of the body, also called

the milieu interieur

(A term coined by the Physiologist Claude Bernard).

Maintenance of nearly constant conditions in the internal environment despite changes in the external environment is called Homeostasis.

It is a state of balance in the body.

Examples: Maintenance of body temperature & glucose levels in the body.

WHAT IS HOMEOSTASIS?

The word “Homeostasis” was coined by the American

Physiologist Walter Cannon in 1929.

He said that Homeostasis was a condition that:

“may vary, but which is relatively constant.”

Homeostasis means “keeping things the same” but “not keeping

everything the same”.

For Homeostasis, you need:

a VARIABLE which is a regulated condition of the ECF that is monitored & kept relatively constant

{e.g. Blood pressure}

a SENSOR that measures the value of the regulated variable

{e.g. Baroreceptors},

a SET POINT which is the normal range of the value for the regulated variable

{A Blood Pressure of 120/80 mmHg} ,

a CONTROL CENTER which interprets the error if there is a difference b/w the set point and the value of the variable

{CNS & Endocrine system}

and

an EFFECTOR that brings about the change to take the variable back to the Set Point

{cardiac muscle).

Regulated Variables in the Human Body:

Oxygen

Carbon dioxide

Na+ ion

K + ion

H + ion

Ca2 + ion

Glucose

Blood pressure

Blood volume

Blood Osmolarity

Core body temperature

Questions you should ask about any

homeostatically regulated system…

What and where is the sensor?

What and where is the control center?

What and where is the effector(s)? How do they alter their

activities so as to produce a response?

Does the response lead to a change in the regulated

variable/stimulus consistent with error signal reduction

(negative feedback)?

TYPES OF CONTROL SYSTEMS IN THE BODY

These are brought into action after a change has been detected.

1. NEGATIVE

FEEDBACK

2.

POSITIVE

FEEDBACK

3.

ADAPTIVE CONTROL

1. NEGATIVE FEEDBACK LOOP

If some variable becomes

excessive or deficient (moves

away from the set point), the

control system initiates a series

of changes that returns the

system to the set-point from

where it started.

Thus, the changes occur

opposite to the direction of the

stimulus.

Examples of Control Systems of the body aiding in

Homeostasis using Negative Feedback:

Body temperature Regulation (Failure of Homeostasis results in Heat Stroke or Hypothermia)

Blood Pressure Regulation (Failure of Homeostasis results in Hypertension or Hypotension)

Blood Glucose Regulation (Failure of Homeostasis results in Diabetes Mellitus)

Potassium ion concentration (Failure of Homeostasis results in Hyperkalemia or Hypokalemia)

Oxygen and Carbon Dioxide in the Blood (Failure of Homeostasis results in Hypoxia & Hypercapnia)

2. POSITIVE FEEDBACK LOOP If some variable becomes excessive or deficient, the control system causes more of the same initiating stimulus so that the condition worsens and the system moves further away from the homeostasis.

So, response occurs in the direction of the stimulus or change, thus, enhancing or exaggerating it.

E.g. person bleeds, heart weakens, can’t pump enough blood, heart weakens further as less blood reaches it.

HOWEVER, IT CAN BE USEFUL IN SOME INSTANCES.

Example of Positive Feedback Cycle

BLOOD CLOTTING

3. Adaptive Control

Adaptive control is delayed negative feedback. Also

contains feedforward mechanisms.

QUESTION 1

What are the 3 different types of control

systems working to bring about homeostasis in

the body.

Give one example of each.

1. Negative Feedback:

-Temperature Regulation

- Blood Pressure Regulation

2. Positive Feedback

- Childbirth

- Blood Clotting

3. Feed Forward

- Salivation on sight of food

- Release of Insulin from pancreas as soon as food is placed in the mouth.

QUESTION 2

Exposure to a hot environment causes the body to sweat. The hotter the environment, the greater the sweating.

Two students are arguing about it. Student A says that it is Positive Feedback while student B insists that it is Negative Feedback.

Who do u agree with and Why?

QUESTION 3

What factors are homeostatically maintained?

QUESTION 4

Compare Negative and Positive Feedback systems?