FORM TWO BIOLOGY NOTES.
TOPIC: EXCRETION AND HOMEOSTASIS
Objective:
At the end of this topic, the learner should be able to;
- i) Distinguish between excretion and homeostasis
- ii) Explain various methods of excretion in plants
iii)Describe excretory products in plants and their uses.
- iv) Understand different excretory organs in mammals, and how they work, that is
The skin
The Kidney
The liver
The lungs
v)Describe mechanisms of homeostasis; negative and positive feedback.
v)Describe the skin, liver, and kidney roles in homeostasis
- vi) Describe disorders of various excretory organs.
Introduction
Excretion and homeostasis are characteristics of living things as it was introduced early in form
one. Excretion is a very essential characteristic of living things because it helps organisms to get
rid of metabolic waste products. These two words are defined as follows;
Excretion-Is the process by which living organisms remove their metabolic waste products.
Homeostasis-Is the maintenance of the internal environment of the body of the organism at a
constant state for the normal functioning of the cell.
EXCRETION IN PLANTS
Plant do not have complex excretory organs because;
i)They can recycle their waste products, for example, oxygen which is a waste product during
photosynthesis is used for respiration.
ii)They deposit their waste products in their various parts like the leaves and fruits which drop off
or are utilized by man for food
iii)They do not move about hence have less toxic substances.
Method of Excretion in plants
- i) Recycling; plants re-use some of their waste products in other processes. For instance, oxygen
produced as a waste product is used for respiration during the day.
- ii) Exudation; some plants release fluids at the back of their stems, iii)Transpiration; plants remove excess salt and water through transpiration
- iv) Leaf fall, fruits fall, thereby releasing waste products deposited in them.
v)Diffusion of gases from the leaves to the atmosphere through the stomata.
EXCRETION IN ANIMALS
a)Excretion in unicellular organisms
Unicellular organisms like amoeba use simple diffusion to remove their waste products because of
their small size.
b)Excretion in multicellular organisms
Multicellular organisms possess complex excretory organs due to increased complexity in these
organisms. We will describe the excretory organs in mammals like a man and their functions.
Excretory organ. Excretory product(s)
i)Skin Excess salt, excess.water
Urea
ii)Kidney. Excess water, excess salt
Urea
iii)Liver. Urea
iv)Lung. Carbon(iv) Oxide, water
In this topic, we will describe the skin, liver, and kidney structure and functions. The lung is
described under the gaseous exchange topic.
KIDNEY
Kidney is an excretory organ that filters blood in the body. Man has two kidneys; left kidney and
the right kidney.
Expand your knowledge
“One can survive with one kidney.”
Anatomy of the Kidney
The kidney is bean-shaped.
It has three regions;
i)Cortex
ii)Pelvis
iii) Medulla
-The basic functional unit of the kidney is the Kidney nephron.
Structure of the kidney nephron
Kidney receives blood through the renal artery.
-Renal vein takes away the blood from the kidney.
How kidney nephron works
Kidney nephron receive blood from the afferent arteriole.
The blood entering the kidney is composed of molecules of both large and small molecular sizes.
Small molecular sized(liquid part) molecules of blood
-Dissolve mineral salts
-amino acids
-Glucose
-Ion like sodium, chlorine, potassium
-Water.
Large molecular sized molecules in blood; -Blood plasma
-Blood proteins.
Why do we say kidney filters blood?
Where is the sieve, the residue and the filtrate?
As described above, the kidney separates the liquid and the large molecular sized molecules of
blood.
As the blood enters the kidney through the afferent, it is composed of both small and large
molecular sized molecules. Blood and its components are filtered at the glomerulus with network
capillaries. At this point, the liquid part of blood is forced out of the blood into the Bowman’s
capsule. This process is termed as ultrafiltration.
The fluid part of blood that is forced out into the Bowman’s capsule is called glomerulus filtrate.
The large molecular sized molecules remains in blood and goes back to circulation through the
efferent.
The process of ultrafiltration is achieved due to the following factors
i)The entering efferent is wider than the leaving efferent which build up pressure within the
network of capillaries at the glomerulus
It is important to note that, the glomerulus filtrate is composed of both nutrients (amino acids,
mineral salts, ions, glucose, some amount of water) and waste products (excess salt, excess water,
urea).
The body is in need of these nutrient and the waste products need to be excreted. Therefore, at
various parts of the nephron, different nutrients are reabsorbed and the waste products left to flow
until the collecting tubule where they form urine.
N/B. What kidney does is reabsorption and NOT absorption. Remember the absorption took place
at the ileum before these nutrients reaches the kidney.
The summary of parts of nephron and what they reabsorb.
i)proximal convoluted tubule
reabsorbs,amino acids, glucose some amount of water and vitamins
- ii) Loop of Henle
Reabsorb ions mainly sodium ions by the help of aldosterone hormone secreted by the adrenal
glands of the kidney.
iii)Distal convoluted tubule
Reabsorb water back to the body
It is regulated by hormone called antiduratic hormone secreted by the pituitary glands.
After reabsorption of water at the distal convoluted tubule, the remaining waste products(excess
salt, excess water and urea) collects at the collecting tubule moves to the pelvis and then drained
into the urinary bladder through the ureter and later released outside the body through the urethra.
Reabsorption at the proximal convoluted tubule is similar to absorption at the ileum. Therefore
proximal convoluted tubule and ileum have many common adaptations.
Adaptation of proximal convoluted tubule
i)Numerous villi and micro-villi to increase the surface area for reabsorption
ii)Numerous mitochondria to provide energy for active transport
iii)It is long to give more time for efficient reabsorption.
iv)It is highly coiled to reduce speed of flow of glomerulus filtrate for efficient reabsorption.
THE SKIN
It is the largest organ in the body.
General functions of the skin
i)Synthesis of Vitamin D
ii)Prevent dehydration by preventing loss of water.
ii)thermoregulation through vasodilation, vasoconstriction, sweat which loses with latent heat of
vaporization and the hair on the skin
iii)Perceive changes in the environment through nerve endings
iv)Protect the inner delicate tissues and organs of the body from physical damage
v)Prevent of entry of disease causing organisms and ultra violet rays.
Skin Anatomy (Structure of the Skin)/ Adaptation of the skin to its functions.
DIAGRAM
Skin is made up of two major layers;
i)Epidermis layer-outer layer
ii)Dermis layer-inner layer.
N/B- below the dermis is a layer of fats called hypodermis. Its considered to be another layer of
thits not a
Expand your knowledge
Do you know what the meaning of the pre-fixes below is?
Epi-means above
Hypo-means below.
Therefore epidermis means above the dermis and hypodermis means below the dermis.
Epidermis layer
Is the outer most layer of the skin
It is made up of three sub-layers
i)Horny layer/Cornified layer
Is a layer of dead cells.
The cells of this layer are made up of keratin.
This layer protects the inner tissues of the skin.
ii)Granular layer.
This layer is made up of actively dividing cells that gives rise to new cornified layer.
iii)Malphighian layer
Contain pigment called melanin which;-prevents the entry of ultraviolet rays and gives the skin its
colour.
Dermis layer
Is composed of various components as described below;
i)Nerve endings which perceive changes within the environment
ii)sweat glands for thermoregulation; when it is hot, excess salt and excess water are accumulated
in these glands to form sweat. Sweat moves onto the skin surface through the sweat pore the sweat
then evaporates with the latent heat of vaporization as it absorbs the body temperature.
iii)Blood vessels which;
-supply nutrients to the tissues of the skin
-Remove wastes of metabolism from the skin
-Supply oxygen to the body tissues.
iv)Hair and erector pilli muscle; originates from the dermis and extends to the outer part of the
skin. Hair is used for thermoregulation; when it is cold, erector pilli muscles contract, making the
hair to stand straight/erect, the hair traps air which acts as an insulator hence, reduced heat loss.
When it is hot, erector pilli muscles relax, hair lies flat on the skin, less air is trapped, reducing
insulation effect hence reduced heat loss.
v)Sebaceous gland;secrets sebum which;
-Contain antiseptic substances which kills the disease causing microorganisms
-Soften the skin and makes it waterproof
- vi) Adipose tissue; is a layer of fats. Also called hypodermis; the layer of fats acts as an insulator
hence reduces hear loss when it is cold.
LIVER
Liver is the second largest organ in the body. It is a very important organ that controls many body
metabolic activities.
Blood vessel that brings blood to the lover from the heart is hepatic artery
Blood vessel that takes blood away from the kidney is the hepatic vein
Blood vessel that brings blood and absorbed nutrients from the small intestines is the hepatic
portal vein.
Major functions of the liver.
i)Detoxification; the removal of toxic substances which may enter the body through drugs and
other chemically
ii)Deamination; the breakdown of amino acids which involves the removal of amine group from
amino acids, combine it with carbon (iv) Oxide to form urea.
NH3 + CO2 -> HCOH + H2O
This process helps the body to get rid of highly toxic ammonia by converting it into less toxic
form urea which is later excreted by the kidneys.
iii) Regulation of blood sugar level.
Liver controls blood sugar level through the effect of two hormones; insulin and glucagon.
When blood sugar level goes down, glucagon us secreted to the liver, and liver intern breaks down
glycogen to glucose hence increasing blood sugar level to normal.
When blood sugar level rises, insulin is secreted which makes liver to increase intake of glucose
from the blood and convert the excess glucose to glycogen hence reducing the level of glucose in
the body.
iv)Elimination of the old red blood cells; Liver breaks down haemoglobin pigment in old red
blood cells to heme and globin. Iron is recycled for the production if new red blood cells.
vi)thermoregulation; when it is cold, liver increases the rate of metabolic activities to generate
heat while when it is hot, liver reduces the rate of heat generating metabolic activities.
Other roles of the liver include
-Elimination of excess cholesterol in the body
-Production of red blood cells in foetus when the bone-marrow is not yet developed.
-Storage of vitamins like vitamin K.
HOMEOSTASIS
Is the maintenance of the internal environment of the body at a constant state.
Excretion helps the body to achieve the homeostasis.
Organism is made up of cells which work at regulated conditions of their environment. However,
the cells can not control their external environment.
Internal environment of an organism is made up of factors like temperature, PH, water, dissolved
mineral salts and ions which need to be maintained a constant state.
Therefore, we will discuss the following under homeostasis;
i)Thermoregulation- regulation of body temperature
ii)Osmoregulation-Salt and water balance
iii)Blood sugar balance. -regulation of blood glucose level
iv)Ionic balance-regulation of ions in the body like sodium.
Some excretory organs already discussed are also essential in hemeostasis, for example.
Organ. Homeostatic role
i)Skin. Thermoregulation
ii)Liver Blood sugar balance, thermoregulation
iii)Kidney. Osmoregulation and ionic balance.
Mechanism of Homeostasis
There are two mechanisms through which the body corrects elevated levels or lower levels of
internal body environment. These responses that correct the changes are called feedback
mechanisms. Feedback can be positive or negative.
In the first case,the body responds to the change in internal environment by reversing the change.
This is called negative feedback.However, in some rare cases the body responds to the change in
internal environment by elevating that change, that is, the change goes to the same direction in
which it was originally. This is called positive feedback.
Example of negative feedback.
i)Thermoregulation-When body temperature goes up, the body responds by reducing the increased
temperature back to normal. When temperature goes down, below the normal, the body responds
by increasing the body temperature.
ii)Blood sugar level;The body reducing the blood sugar level when it is high and increasing it
when it is low.
iii)Ion balance-the body increases the level of ions when its low in the body and reduces it when
it is in high levels
iii)Blood sugar level.
Example of positive feedback
i)Blood clotting process-more and more plateletes are secreted in a damaged blood vessel until
clotting is achieved.
- ii) Labour pain during birth due to increased secretion of oxytosin hormone which increases
contraction and relaxation of uterine walls (leading to more and more pain) untill the baby comes
out.
THERMOREGULATION
Is the regulation of body temperature.
It is achieved through negative feedback.
Change in body temperature is detected by the thermoreceptors in the hypothalamus
After detecting the change, the hypothalamus sends the information to respective organs like the
skin and the liver and these organs respond to correct the change in body temperature.
Role of homeostatic organs
a)Roles of the skin in thermoregulation
i)sweat glands for thermoregulation; when it is hot, excess salt and excess water are accumulated
in these glands to form sweat. Sweat moves onto the skin surface through the sweat pore the sweat
then evaporates with the latent heat of vaporization as it absorbs the body temperature.
ii)Blood vessels which;
-supply nutrients to the tissues of the skin
-Remove wastes of metabolism from the skin
-Supply oxygen to the body tissues.
iii)Hair and erector pilli muscle; originates from the dermis and extends to the outer part of the
skin. Hair is used for thermoregulation; when it is cold, erector pilli muscles contract, making the
hair to stand straight/erect, the hair traps air which acts as an insulator hence, reduced heat loss.
When it is hot, erector pilli muscles relax, hair lies flat on the skin, less air is trapped, reducing
insulation effect hence reduced heat loss.
- iv) Adipose tissue; is a layer of fats. Also called hypodermis; the layer of fats acts as an insulator
hence reduces hear loss when it is cold.
b)Roles of the liver in thermorgulation
vi)thermoregulation; when it is cold, liver increases the rate of metabolic activities to generate
heat while when it is hot, liver reduces the rate of heat generating metabolic activities.
OSMOREGULATION
Is the salt and water balance.
Is achieved by the role of the kidney.
Hormone responsible is antidurati hormone (ADH).
How kidney controls salt and water balance in the body.
When the body has less amount of water, the osmotic pressure is increased. This is detected by the
hypothalamus which triggers secretion of antiduretic hormone. Antiduretic hormone is trnsported
to the kidney tubules (distal convoluted tubule). ADH increases the permeability of kidney tubules
thereby allowing more water to be reabsorbed into the blood stream thereby reducing the osmotic
pressure of the blood. This leads to excretion of less and more concentrated urine.
When the body has excess water, Osmotic pressure is low. In this condition,less ADH is secreted
thereby reducing the permeability of kidney tubules hence less water is reabsorbed. This leads to
excretion of more and less concentrated.
Ionic balance.
Done by the Loop of Henle of the kidney
When sodium ions level increase in the body, adrenal gland secrets less or no aldosterone hormone
and therefore, Loop of Henle reabsorbs less or no sodium ions.
When the level goes up, more aldostetone ion are secreted to increase reabsorption of the ions in
the loop of Henle.
Blood sugar regulation
Done by the liver
When blood glucose goes up, insulin hormone is secreted in blood and transported to the liver.
Insulin makes liver to reduce blood sugar level by
-Conversion of excess glucose to glycogen
-converting excess glucose to fats
-Reducing the rate of respiration.
When blood sugar is low, glucagon hormone is secreted and trnsported to the liver, liver responds
by ;
-Increasing the breakdown of glycogen to glucose
-Increasing conversion of fats to glucose
-Increasing the rate of respiration.
HEAT LOSS AND HEAT GAIN
Animal whose body temperature remain constant despite the change in environmental temperature
are called homoiotherms. Example are birds and mammals
Animal whose body temperature fluctuate with that of external environment are called
poikilotherms. Example are fish, reptiles, amphibians etc.