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HOMEOSTASIS
OBJECTIVES:
- define homeostasis as the maintenance of a constant internal environment
- locate and name the main organs which maintain a constant internal
environment, namely pancreas and liver (blood glucose), kidneys (water content)
and skin (temperature).
- identify and label on a diagram of the skin: hairs, sweat glands receptors and
blood vessels - describe how the named parts are responsible for maintenance
of a constant temperature in Man
- discuss the co-ordinating role of the brain (hypothalamus) in maintaining a
constant body temperature.
This refers to as the maintenance of constant internal environment.
In mammals a variety of organs play specific roles in homeostasis.
Organs Concerned Factors controlled
Kidneys Water and pH
(urea, salts)
Lungs Carbondioxide & Oxygen
Skin & Liver Temperature
Liver Blood sugar
The Skin
The skin has two layers;
1. Epidermis
2. Dermis
Epidermis
This is the outermost layer of the skin. It protects the body from pathogens by
forming a barrier on top of one’s body. It is mostly made of dead cells
Dermis
This tissue has several structures such as hair follicle, sebaceous gland, nerve
endings, adipose tissue, blood vessels etc.
Functions of the skin
a) Protection – the outermost layer of the dead cells of the epidermis helps to
reduce water loss & provides a barrier. The melanin pigment produced by the
mulphigian layer protects the skin from damage by ultra-violet light rays.
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b) Sensitivity – scattered throughout the skin are a large number of tiny receptors
which give rise to sensations of pain, pressure, heat & cold. These make us
aware of changes in our surroundings & enable us to avoid danger.
c) Temperature Regulation - Keeping the human body temperature constant is
an example of a homeostatic mechanism. The skin plays an important role in
maintaining a constant body temperature by adjusting blood flow near the skin
surface & by sweating.
THE SKIN STRUCTURE
LEAVE 10 LINES FOR DIAGRAM
Hair Follicle – This gives rise to hair found on the surface
Sebaceous gland – Secretes an oily substance called sebum. This goes
onto the skin surface to keep it soft & supple
Sweat glands – Absorb excess water and salts to make sweat
Blood vessels – These supply the skin with useful substances such as
glucose, amino acids. oxygen & take away waste such as carbondioxide
from the skin.
Nerve endings – These are receptors to touch, pressure, heat etc; they
pick up stimuli in the skin & also bring motor impulses to the skin.
Adipose tissue – muscles on the skin which contract and relaxes
vigorously when shivering
subcutaneous fats - it is food store and also insulates the internal organs
& prevents excessive heat loss.
Maintenance of a constant body temperature in humans;
Overcooling;
If the temperature of blood falls below normal, the Hypothalamus in the brain
detects this, motor impulses are sent to the skin for the blood’s temperature to be
increased to normal. The skin will respond by;
i. Vaso-Constriction – this is the reduction on diameter of the blood vessels
running near the skin surface. This reduces blood flow near the skin
surface and less heat is lost by conduction, convection & radiation
ii. Sweat production is reduced – less heat is lost when there is less
evaporation of sweat from the skin.
iii. Hair on the skin surface stands up. This helps to trap air that insulates the
body therefore less heat is lost from the body. The hair also acts as heat
insulator therefore both the hair and trapped air provide double heat
insulation.
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iv. Shivering – shivering is an involuntary vigorous muscular contraction and
relaxation in response to a drop in body temperature. During these
vigorous muscular contractions a lot of heat energy is released.
Overheating
If the blood’s temperature rises above normal, the thermoregulatory centre of the
hypothalamus detects this, motor impulses are sent to the skin to drop the
temperature of blood to normal. The skin responds by;
i. Vaso-dilation – blood vessels in the skin increase in diameter. As a result,
more blood flows near the skin surface. More heat is lost by conduction,
convection & radiation.
ii. Sweat production increases-sweat glands absorb more water, excess
salts & urea from the blood to make more sweat. The sweat is transported
to the skin surface through the sweat duct to the sweat pore. As more
sweat evaporates, it draws a lot of heat from the blood capillaries and one
feels cooler as the temperature falls to normal.
iii. Hair on the skin lies close to the skin surface – these makes the air
currents to easily flow close to the skin surface therefore easily
evaporating sweat & heat energy away from the body.
The role of the brain in temperature regulation
The hypothalamus located in the brain, detects the temperature of the blood. The
thermoregulatory centre (in the hypothalamus) receives sensory impulses from
the skin. If the blood is too hot, the hypothalamus sends motor impulse to skin.
The effect of this motor impulse will cause the skin to respond through the
activities that cool the blood as outlined above. This mechanism of coordination
is known as “Negative Feedback Mechanism”. This whereby the outgoing
impulse counteracts the effects of the received impulse. If the temperature of
blood drops below normal, the outgoing impulse will rise the temperature of blood
to normal..
Another example of negative feedback mechanism is regulation of blood sugar
level.
When the concentration of blood sugar increases above normal, insulin is
released by the islets of Langerhans cells in the pancreas into the blood stream.
Upon reaching the liver, insulin stimulates the liver cells to take up excess
glucose & convert it to glycogen for storage. The concentration of blood sugar
level will then drop to normal
When the glucose levels in the blood drop below normal another hormone called
glucagon is released by the pancreas to stimulate the liver cells to convert stored
glycogen back to glucose. Then the glucose level in the blood will rise to normal
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Another example of negative feedback mechanism is regulation of water level.
Osmo-regulation
Water balance in the body is achieved by regulating the amount of water in the
urine. This is made possible by the action of ADH – anti-diuretic hormone
produced in the pituitary gland.
Importance of water balance
Concentrated blood
Water is gained by the body from drinks & food and also produced within through
respiration.
It is lost through urine, sweat, faeces & as vapour in the exhaled air. It is
imperative that there is balance between water gained & lost. If this balance
breaks down then the body cells will either gain or loss water through osmosis
resulting in malfunctioning.
Generally kidneys regulate the amount of water present in the body by variation
of the urine produced as depicted in the flow diagram below.
If there is less amount of water If there is more amount of water
in the blood, the hypothalamus in the blood, the hypothalamus
cells will detect that cells will detect that
Dilute blood
Pituitary gland produces more Pituitary gland produce less
ADH ADH
Kidney tubule cells become Kidney tubule cells become less
more permeable to water – more permeable to water – less water
water is re-absorbed into the is re-absorbed into the blood
blood
Small amounts of concentrated Large amount of dilute urine is
urine produced produced
Normal amount of water in the
body
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