Homeostasis - Human Anatomy and Physiology B. Pharma 1st Semester

Homeostasis

Objectives

At the end of this lecture, students will be able to –

• Explain Homeostasis

• Describe the importance of homeostasis

• Explain role of different organ systems in maintaining body’s environment

• List out components of homeostasis system

• Explain mechanism involved in the internal environment maintenance

• Describe feedback loop

Content

• Homeostasis

• Different organ and organ system involved in the maintenance of body’s internal environment

• Functioning of various organ system in homeostasis

• Components involved in homeostasis

• Mechanism involved in the Homeostasis

• Positive feedback mechanism

• Negative feedback mechanism

LIFE’S BALANCING ACT

• If you are too hot or too cold your body temeruture is out of balance.

• When we are too cold we shiver, it warms us up

• When we are too hot we sweat, it cools us down.

Homeostasis

• Greek words; Homeo = ‘unchanging’ and Stasis = ‘standing’

• In its simplest form it means ‘staying the same’

• It is an organism’s internal environment which ‘stays the same’

• So, a better definition of ‘homeostasis’ is the maintenance of the internal environment within narrow limits

• In practice, it comes down to providing a stable environment for the cells of the body

• The living processes in the cells depend on the activity of enzymes.

• These enzymes work best in specific conditions such as those of temperature and pH.

• Any change in these conditions affects the function of the enzymes and may lead to the death of the cells or, ultimately, the whole organism

• Internal conditions are not absolutely constant, but allowed to vary within very narrow limits.

Ex: Human body temperature varies between 36.1 – 37.8 o C.

      The average temperature is usually expressed as 36.8o C

• There is no specific organ which controls homeostasis except, possibly, the brain

• The skin, kidneys, liver, endocrine system, nervous system and sensory system all play a part in maintaining the internal environment within narrow limits

ALL ORGANS AND ORGAN SYSTEMS OF THE BODY HELP IN MAINTENANCE OF HOMEOSTASIS

• Cardiovascular system.

• Respiratory system.

• Nervous system.

• Endocrine system.

• Gastrointestinal system.

• Excretory system.

• Skeletal system.

• Integumentary system.

• Reproductive system.

CARDIOVASCULAR SYSTEM

• Transports oxygen, carbon dioxide, nutrients and hormones to and from the body cells.

• Helps regulate pH and temperature.

• Provides protection against diseases.

• To carry out its living processes, every cell in the body needs a supply of oxygen and food. Waste products such as carbon dioxide have to be removed before they reach harmful levels and damage the cell

• These conditions are maintained by tissue fluid bathing the cells. Tissue fluid is derived from the blood

• The tissue fluid contains oxygen and food which the cells can absorb, and also accepts the carbon dioxide and other waste products produced by the cells

• The blood system maintains the composition of the tissue fluid cells

• One important method of achieving homeostasis is negative feedback

• It applies to many systems in the body. Temperature regulation is one example

• If the blood temperature rises, a ‘heat gain’ centre in the brain sends nerve impulses to the skin, which causes vasodilation and sweating, which cool the blood

• If the blood temperature falls, it stimulates a ‘heat loss’ centre in the brain which sends impulse to the skin causing vasoconstriction and the cessation of sweating. These changes reduce heat loss from the skin

• The warmer or cooler blood provides negative feedback to the thermoregulatory centre in the brain

RESPIRATORY SYSTEM

• Exchange the gases between atmospheric air and blood.

• Help adjust the pH of the body fluids.

NERVOUS SYSTEM

• Generates nerve impulses (Action Potential) that provide communication and regulation of most body tissues.

ENDOCRINE SYSTEM

• Regulates the activity and growth of target cells in the body.

• Regulate metabolism

• The glucose concentration in the blood is controlled by the pancreas

• The pancreas contains cells which produce digestive enzymes but it also contains cells (alpha and beta cells) which produce the hormones insulin and glucagon

• If the glucose concentration rises, the beta cells release insulin

• If the glucose concentration falls, the alpha cells release glucagon

• Insulin stimulates the liver and muscles to remove excess glucose from the blood and store it as glycogen

• Glucagon stimulates the liver to convert its stored glycogen into glucose which is then released into the bloodstream

• In this way, the concentration of glucose in the blood is kept within narrow limits

GASTROINTESTINAL SYSTEM

• Breaks down food into absorbable form.

• Absorbs various nutrients.

• Eliminates waste from the body.

EXCRETORY SYSTEM

• Helps eliminate the waste products from the body.

• Maintains the blood pH, volume, pressure, osmolarity, electrolyte composition etc.

• Produces hormones.

• In the cells, the chemical breakdown of proteins produces the nitrogenous compound, urea. If this were allowed to accumulate in the cells it would damage or kill them.

• The concentration of the blood and tissue fluids tends to vary. If water is lost through evaporation and sweating, the fluids become more concentrated. The intake of water dilutes the fluids

• It is important that the concentration of blood and tissue fluid is kept within narrow limits if the cells are to function properly

• These homeostatic functions are carried out by the kidneys

Microscopic slice of kidney tissue:

Blood pressure forces tissue fluid out of a clump of narrow capillaries

The fluid contains urea, salts, glucose and other soluble substances

The fluid trickles down this tubule and the useful substances such as glucose are reabsorbed into the blood

In this tubule more or less water is reabsorbed to maintain the blood concentration

Excess water, some salts and urea collect here before passing to the bladder as urine

SKELETAL SYSTEM

• Bones provide support, protection, the production of blood cells.

• Muscles produce body movements and produce heat to maintain the body temperature.

INTEGUMENTARY SYSTEM

• Contributes to homeostasis by protecting the body and helping regulate the body temperature

• It also allows you to sense pleasurable, painful and other stimuli in your external environment.

Section through skin

The sweat gland extracts sweat from the blood and passes it up the duct to the skin surface where it evaporates

Vasodilation

If the body temperature rises, the blood vessels in the skin dilate (become wider) and allow more blood to flow near the surface. The heat loss from the blood through the skin helps cool the circulating blood much heat lost

Vasoconstriction

If the body temperature falls. The blood vessels in the skin constrict. Less warm blood flows near the surface so less heat is lost

HOW HOMEOSTATIC CONTROL MECHANISMS WORK?

• Homeostatic control mechanisms work through ‘Feedback Mechanisms’.

• Status of a body condition is continually monitored, evaluated, changed, re-monitored & reevaluated.

FEEDBACK MECHANISM

• A feedback mechanism is a cycle in which the output of a system “feeds back” to either modify or reinforce the action taken by the system.

• A feedback mechanism may operate at:

– Tissue level

– Organ level

– Organ system level

– Body level, integrating with other organ systems.

• Feedback mechanism can be:

– Negative feedback (more common)

– Positive feedback

A FEEDBACK SYSTEM CONSISTS OF THREE COMPONENTS

1. SENSOR (RECEPTOR): detects specific changes (stimuli) in the environment.

2. INTEGRATOR: act to direct impulses to the place where a response can be made.

3. EFFECTOR: performs the appropriate response.

A FEEDBACK LOOP

NEGETIVE FEEDBACK

• Mechanisms that maintain the factor at some mean value.

• Reverse a change

• Restore abnormal values to normal

 

EXAMPLE: NEGATIVE FEEDBACK

BLOOD PRESSURE REGULATION

POSITIVE FEEDBACK

• Strengthens or reinforces a change.

• Makes abnormal values more abnormal.

• Produces ‘Vicious Cycle’.

• But in body a mild degree of positive feedback can be overcome by the negative feedback control mechanisms of the body, and the vicious cycle fails to develop.

POSITIVE FEEDBACK LOOP

EXAMPLE: POSITIVE FEEDBACK

MEMBRANE DEPOLARISATION

POSITIVE FEEDBACKS IN BODY

• Action potential

• Clotting of blood

• Parturition

• Release of calcium from SR

• Sexual arousal

• LH surge

Summary

• Homeostasis = Maintaining body’s internal environment within in a narrow limit.

• Homeostasis is very essential for normal functioning of body

• Various organs and organ systems play specific roles to achieve homeostasis

• Homeostasis is involved in continuous monitoring of body’s internal environment with respect to altering external environment

• Components of homeostasis are ESI (Effector, Sensor, and Integrator)

• Internal environment is maintained by –positive and negative feedback mechanisms

• Most of the homeostasis mechanisms involve negative feedback