ANIMALS AND PLANTS EXCRETION SYSTEM

 

Excretion is the removal of waste substances from the body, which are produced by the cells activity (metabolism) and substances of which the body has an excess. Waste substances are usually harmful to the living body. So the body separate, detoxicate, and remove them. 
Excretion is the removal of unwanted substances from the body as fast as they exceed a certain concentration.

Where unwanted substances come from;

There are many chemical reactions in the organism. All these reactions give rise to the end products, some of which are poisonous or could affect the normal chemical reactions in the body if they were allowed to accumulate.

· From the food we eat; indigestible substances are removed from the body through defecation (faecal matter, egestion).

· Digested substances which is in excess and can not be stored in the body are removed from the body together with useless and poisonous by products of metabolism by the process called excretion..

· All these reactions are useful to the body;

-eg –some produce energy (respiration).

--Some build new body tissues.

--Some produce useful chemicals, ie. enzymes and hormones.

· But yet, some of these reactions give rise to by products which are useless or even harmful to the body. These are called excretory products and are removed from the body by excretory organs, ie. kidney, lungs, skin, and liver.

Excretory Products.

Respiration – produces energy which is useful, but it also produces carbon dioxide and water.

Carbon dioxide is useless in the body but it can also be harmful if allowed to accumulate in large quantities, ie. forming carbonic acid which can damage body cells.

-So carbon dioxide is an excretory product and is removed by the lungs.

Water; normally it is useful to the body but it can be harmful if allowed to accumulate to the point that it dilutes the blood and tissue fluid. So it has to be removed as an excretory product.

Metabolism; some of the by-products of metabolism though useless when they are produced, they are not excreted because they can be turned into useful materials. Example lactic acid formed in the muscles of vertebrates during vigorous exercises.

Lactic acid is useless until converted back into glucose where it is used in respiration or stored as glycogen.

 The most poisonous by product of metabolism is ammonia. It is formed during breakdown of excess amino acid in the liver. Ammonia is very soluble and kills the cells if its concentration rises above 1 part in 25,000. Liver converts ammonia into urea (relatively harmless) which is released into the blood. Kidney extracts it from the blood and excretes as a part of urine.

Other examples are hormones, cholesterol (fats), bilirubin.

   TERMS,

Defecation; is the removal from the body of indigestible substances, ie. passage of faecal matter through anus. Faecal matter is not true excretory product because it is not produced by metabolism.

   Secretion; is the production by metabolism of useful substances like enzymes and hormones.

Excretory Organs.

1.      Kidneys: They remove nitrogenous (urea and ammonia) compounds from the blood              and eliminate excess water and salt.

2.      Liver: excretes bile pigments derived from   the decomposition of haemoglobin and urea.

3.      Lungs: Excrete carbon dioxide and water vapour.

4.      Skin: for water, mineral salts and trace of urea excretion.

Importance of excretion;

To provide suitable working environment for the cells.

   Kidney; The Urinary System:

 The human kidney is a bean shaped organ. A man has two kidneys. From each kidney comes a thin tube called ureter which carries the urine to the bladder. The ureter is about 25 cm long. The bladder is the muscular bag which stores urine for sometime, before it is passed out through urethra (by urination). The bladder can retain about 0.25 litre of urine. The bladder end of the urethra is normally held close by the means of a ring muscle called a sphincter.

   In babies sphincter muscle is controlled by a reflex action triggered off by nerve endings in the stretched walls of the bladder. After about two years or less the muscle can be controlled voluntarily.

   The blood circulation in the kidney is made possible by through renal artery and renal vein. Each kidney is about 150g, it is enclosed in a thin fibrous coat.

Internal Structure Of A Kidney.

There is a dark coloured outer zone, cortex; and pale coloured inner zone medulla which consists of cone shaped areas ie pyramids. Urine drains from the pyramids. Ureter leaves the kidney in the space called pelvis and into this project cones (pyramids) of kidney tissues. The renal artery brings oxygenated blood to the kidney and the renal vein takes deoxygenated blood away.

    The renal artery divides up into a great many arterioles and capillaries, each arteriole leads to glomerulus which is almost entirely surrounded by cup shaped Bowmans’s capsule. Glomeruli were discovered by Marcello Malpighi. Each kidney tubule emerges from the Bowmans’s Capsule and after a complicated series of coils and loops, it joins the wider collecting duct which collect urine from the kidney tubule (Nephron).

                           
 Functions Of The Kidney.

Excretion: of waste products (removal of nitrogenous waste products) like ammonia and urea formed in the liver.

Osmoregulation: To regulate the composition of blood, ie. controlling the rate of osmosis by regulating amount of water and amount of salt in the blood.

           
 How kidney fulfil their work. 
The smallest function unit of the kidneys is the kidney tubules or nephrons. All nephrons of one kidney have the total length of 30 KMs. This immerse surface is needed to filter the blood well. This is done in two processes: filtration and reabsorption.

                  
 Filtration:

Blood from the renal artery enters the glomerulus through narrow capillaries. The smaller molecules of the blood are forced under pressure out of the capillaries into the Bowmans’s capsule. Capillary membrane and capsule’s membrane act as a filter. Thus only plasma and tiny molecules like glucose, minerals, vitamins, amino acids, waste products and water  pass into the capsule and are now called the glomerulus filtrate. Glomerulus and Bowmans’s capsule are called together as the malphigian Body.

                
 Reabsorption.

Nearly 100 litres of glomerulus filtrate are filtered a day in all nephrons (kidney tubules) but only 1.5 litres finally are excreted as urine. Therefore the rest of 98% of glomerulus filtrate is reabsorbed. The useful components re-enter the blood by diffusion, osmosis and active transport in different parts of the nephrone.

   
 Reabsorption occurs as glomerulus filtrate passes out of the Bowmans’s capsule and down the kidney tubules towards the collecting ducts. All along this route, cells in the kidney tubule’s walls extract useful substances from glomerulus filtrate so that by the time it reaches the collecting ducts a liquid called urine has been formed. Urine contains urea and other waste excretory substances.

The Liver.

The liver is a large, reddish-brown in colour owing to the large number of blood vessels it contains. It lies just below the diaphragm. It has two lobes, and gall bladder is placed between the lobes. The liver is the largest gland in our body. It is an enormous chemical factory which makes and release into the body many useful substances. As chemical factory, it needs a large supply of raw materials. The liver obtain its supplies from blood in Hepatic Portal vein, which brings all the digested food absorbed by the intestine.

   
 The liver regulates the amount of food which reaches the blood and tissue fluid. It does this by absorbing and storing food which it receives, and then releasing it into the circulatory system at a rate which depends upon the body’s current needs.

Structure of the Liver.

-Has many capillaries which gives it reddish-brown colour.

-Has two lobes, and gall bladder is between the two lobes.

-Is made up of small groups of cells called lobules. Each lobule is composed of great number of liver cells, between which are small blood vessel and very small bile ducts.

-The blood vessels: Hepatic artery- brings oxygenated blood.

                                 : Hepatic portal vein- brings blood rich in digested food from the

                                   limentary canal for control.

                                 : Hepatic vein. Carries away the deoxygenated blood.

Functions of the Liver.

1. Regulation of blood sugar: Excess glucose is converted into glycogen and fat under the influence of insulin. In case of decrease of blood glucose; the stored glycogen and fat are converted into glucose ready for respiration.

2. Deamination; Protein is digested into amino acids. Deamination is the removal of amino group from an amino acid, ie. changing Amino group (NH₂) which would form poisonous Amonia (NH₃) into less poisonous urea to be excreted through the kidney.

3. Detoxification; Is the process whereby harmful (poisonous) compounds (Drugs)  are converted to less  toxic compounds in the liver, which are then excreted through urine.

4. Manufacture of plasma protein- plasma proteins like prothrombin and fibrinogen (for blood clotting) are manufactured in the liver from the synthesise of amino acids.

5. Storage of minerals and vitamins such as A, B, D,E,K,B12 for red cells in the bone marrow.

6. Storage of blood; it is highly vascularized, so able to hold large volume of blood.

7. Formation of Erythrocytes in the liver of the foetus. The liver also breaks down the old erythrocytes.

8. Regulation of body heat.

9. Storage of iron: iron from the haemoglobin which comes  from the decomposition (breaking) of red blood cells is stored in the liver.

10. Formation of bile; when red blood cells break down, the green and yellow pigments are formed. These pigments are removed from the blood by the liver and excreted in the bile. Bile is stored in the gall bladder and is used for emulsifying fats.

     Diseases of the liver

The Skin.

 The skin is the largest organ of the body. Is continuous layer of tissue over the surface of the body.

Skin structure.

Skin consists of two main layers-outer-skin called epidermis.

                                                    -Inner skin called dermis.

A.     Epidermis.

1. The cornified layer: the horny layer; Is the outmost layer which consists of dead cells that form a tough outer coat. The cells are continually worn away and replaced from beneath. It may become very thick on the palms of the hand (when used for heavy works) and soles of feet. It contains keratin. The layer offers resistance from damage, bacteria, and from drying up.

2. The granular layer: contains living cells; towards the outside it gives way gradually to the cornified layer.

3. The malphigian layer. Contains living cells that divide actively to give rise to the upper layers of the skin. They contain pigments called melanin which gives colour to the skin and act as a screen against ultraviolet radiations/rays that can cause cancer.

B.     Dermis: 
The dermis is a thicker layer of connective tissue with many elastic fibres in it. It contains sweat glands, receptor cells, blood vessels, lymphatic vessels, adipose tissue, hair follicles and sebaceous glands.

1. Blood capillaries: supply skin with the necessary food and oxygen and remove its excretory products. Sweat glands and hair follicles have net work of capillaries.

-capillaries beneath the epidermis play an important role in the temperature control.

2. Sweat glands: they are in deeper layers of the inner skin. Sweat gland is a coiled tube of one cell thick. It opens at the surface of the skin through sweat–pore. Sweat gland is surrounded by network of blood capillaries from which it extracts water containing waste substances. It absorbs excess water, mineral salts, traces of urea and lactic acid from blood vessels. It functions when the body temperature rises above the normal by between 0.2C to 0.5C.

3. Receptor cells: found within the dermis are able to detect different stimuli such as cold, heat, pressure, touch and pain.

4. Hair follicles: originate from the epidermis. Their bases are surrounded  by malphigian  and glanular layers. These are deep pit like where hair lies. Hair is made up of dead cells containing keratin. Hair root is embedded in the follicle.

  -The erector hair (pili) muscle is attached to the hair follicle and to the epidermis on the other end. The hair raise and lowers as muscle contracts and relaxes respectively.

  -The granular and malpighian  cells multiply and build up a hair inside the follicle. The constant adding of the new cells to the base of the hair causes it to grow.

5. Sebaceous gland: it produces oily sebum which keeps the epidermis supple (not crack), makes hair water proof, reduces tendency of skin becoming too dry after evaporation.

-Also has antiseptic substances which protects the skin against micro organisms.

6. Sub-cutaneous fat; is the layer beneath the dermis containing fat cells, ie. adipose tissue where fat is stored. The fat insulate the body against heat loss. In the presence of sunlight the layer also manufactures vitamin D.

Functions Of The Skin.

1. Against desiccation- prevents loss of water from the body by evaporation.

2. Protects body against entry of micro organism and damage due to friction.

3. Excretion of salts, excess water, traces of urea, lactic acid and carbon dioxide.

4. Regulation of body temperature.

5.      Prevention against  ultra violet rays penetrating the body through melanin pigment.

6.      Reception of stimulus, ie. cold, heat, pain, touch and pressure.

7.      Synthesis of vitamin D.

8.      Stores fats in the adipose tissue.

HOMEOSTASIS-REGULATION

In co-ordination we see how organisms react to internal & external stimuli in order to find optimal living conditions. Regulation deals with the control of the internal environment in organisms.

Homeostasis: is the ability of organisms to maintain a stable internal environment.  This enables organs (cells) to function efficiently, hence the body functions normally.

It makes animals more independent of external environment (being stable in changing environment.)

The organs with homeostatic functions are liver, skin, lungs & kidney.

The ability to control internal environment is best developed in most mammals (humans) and birds. The organs with homeostatic functions in these animals keep the following features of blood & tissue fluid at a fairly constant level i.e. temperature, dissolved substances like CO2, O2, food (glucose concentration), urea, various poisonous substances and osmotic pressure, PH & water content.

Homeostasis includes excretion: that is the removal of waste products from the body. Waste products are substances arising from metabolic processes which are not needed or which are harmful when they get accumulated in the organism.

Examples of waste products are excess of water, CO2 & nitrogenous compounds (ammonium, urea, and uric acid)

Why regulate internal environment?

Internal environment needs to be regulated so that the body can function normally e.g.

*        Terrestrial organisms dry out by loosing water to the air.

*        Fresh water organisms, too much water enters the body by osmosis.

*        Seawater organisms loose a lot of water from their bodies by exosmosis.

-Despite all these problems water content of an organism must be regulated for the body to function normally.

Þ    Size of organism: small animals have large surface area compared to volumes than large animals. They loose more heat per unit volume.

However all (large  and small) have a mechanism of regulating body temperature in order to keep their bodies alive.

Þ    Living cells can only work properly within narrow limit of temperature i.e. human’s 37ºC-enzymes & physiological processes work at their best. Below 30ºC or above 42ºC the body dies, i.e. internal body temperature not external.

Temperature control.

A constant warm internal temperature permits efficient functioning of the body.

Homoiothermic (endothermic): these are birds and mammals that maintain their body temperature within certain limit independent of the external temperature (formerly known as warm-blooded). They are able to counteract fluctuating environmental temperature. They generate much internal heat and have good insulation.

Poikilothermic (ectohermic): are animals, which are unable to control their temperature or maintain it within certain limits. They produce little internal heat & have little insulation. Their temperature fluctuates with the external temperature, e.g. fish, amphibians & reptiles. These were formerly known as cold-blooded animals.

The terms warm-blooded & cold-blooded are very general & misleading, therefore unsuitable for scientific purposes.

Why? Because on very hot day a cold-blooded might have a higher body temperature than a warm-blooded animal e.g. a lizard sitting on a rock in a hot sunny day might have a higher body temperature than a man sitting on a similar spot.

Aquatic poikilotherms: water temperature does not fluctuate rapidly and water has a high specific heat capacity. So aquatic poikilotherms have little problems compared to those on the land.

Terrestrial poikilotherms: on the land the air temperature can change rapidly over a wide range. So terrestrial poikilotherms achieve some measure of temperature control by means of their behaviour.

E.g.: Some borrow the land and live there.

Some expose themselves to the sun according to their needs (nocturnal)

These help them to avoid the extremeness of temperatures, which would interfere with their metabolism.

Temperature of poikilotherms depends on different factors: -

1.      Amount of heat produced in its body by metabolism.

2.      The heat, which it receives & looses by radiation from the sun and near objects i.e. rocks.

3.      The heat, which it receives & looses by conduction with object touching it.

4.      The heat, which it receives & looses to the air.

5.      The heat, which it receives & looses by evaporation of water from its skin.

These factors also affect Homoiothermic animals but these animals have some internal control over the effects which these factors have upon body temperature.

Adaptation to cold climate:

Mammals, which live in very cold environments, are insulated by a thick layer of sub-cutaneous fat. The body shapes & sizes increase the ability to conserve heat, i.e. smaller surface area to volume ratio e.g. rounded bodies of whale, seals & polar bears which consists of thick thermal shell enclosing a substantial core.

Other animals without such adaptation can’t survive the coldest season unless they migrate or hibernate.

Hibernation: Is a state of sleep and greatly reduced metabolic rate, (reduce energy needed) that enables certain mammals to survive prolonged periods of low temperature & food scarcity e.g. hedgehog, rodents & bats.

Causes of hibernation: changes in day length & climate trigger hormonal changes, which slow down metabolism & lead to hibernation.

-In some species hibernation is striggered by an environmental stimulus such as food shortage. In certain cases hibernation is usually proceeded by extra feeding & the lying down of fat stores in the body. Small animals are prone to hibernate as they have a high basal metabolic rate and few food stores.

Small animals, Small birds (humming birds) and smallest mammals (shrew) have particular problems in maintaining high body temperature due to their large surface area to volume ratio, since they can’t carry thick insulation.

-Addition metabolism is necessary to produce required heat.

Young mammals: include newly born baby, have regions of special adipose tissue known as brown fat. These fats involve the rapid production of heat by special metabolism of free fatty acid; these pads of brown fat are also found in some hibernating animals.

Adaptation in hot climate.

Very few organisms can tolerate wide range of temperature like in the desert (0ºC-midnight to 55ºC at noon). Most desert animals survive by restricting their activities when conditions get less extreme e.g. kangaroo rats spend most of their day in deep burrows where temperature stays approximately constant.

Large animals like camels have thick non-wettable fur that is pale coloured to reflect heat and allow water to evaporate from their skin surface so that cooling occurs.

Surface area: elephants have large ears to increase surface area for evaporation. Ears are important sites for heat exchange.

Therefore changes in body colour and body size with climate is one of the adaptations to hot climate.

Human beings adopt different climate with type of clothes and food.

Aestivation: is a period of inactivity in some animals during summer or hot dry season e.g. fish, amphibians & reptiles who inhabit fresh water e.g. lugfish burying themselves in mud bottom at dry season and reactive as water return.

Control of over-heating:

How can the body be over-heated: -

1.    When environment is hotter than the body temperature i.e. absorption of heat energy from the sun, hot objects or hot animals.

2.    Vigorous exercises increase heat production due to metabolic reactions especially in the liver & muscles.

3.     When the body is fighting a disease:

-       The blood temperature is constantly monitored by the lower part of the brain called Hypothalamus.

-       The process where chemical reactions of metabolism produce heat especially in active organs like the liver is called Thermogenesis.

Over-heating is adjusted through: -

1.      Sweating: is the production of water fluid containing dissolved salt from sweat glands in the skin. Evaporation of sweat is an extremely efficient cooling mechanism.

-          Sweat evaporates & its effectiveness in cooling the body depend on:

a)      Humidity = amount of water vapour in the air.

b)      Air movement = wind and fans.

-          Sweat evaporates and cools the body very rapidly in hot, dry, & windy conditions.

        Temperature above 41ºC will lead to heat stroke i.e. collapse & death.

®    In hot climate vigorous exercise may cause loss of up to 3 litres of water & 30 grams of salt per day. So it causes blood to be thick & concentrated (not able to circulate properly). Loss of salt causes muscle pains (heat cramp)

 So in hot climate a person must drink a lot of water and increase amount of salt in his diet.

2.      Panting: Animals of the dog family have sweat glands only in the pads of their paws. Their main method of loosing heat is to pant rapidly with the tongue hanging outside. This causes evaporation from the lungs & mouth & cools the body.

3.      Vasodilatation: superficial capillaries below the skin become wide so that more blood flows to the skin surface. Heat is lost through the skin surface, that’s why the skin feels hot and looks red.

4.      Relaxation of hair erector muscles: hence hair lies more or less flat against the skin, so heat is easily lost by radiation and convection.

5.      Feeding and food storage: rate of metabolism is reduced and this reduces the appetite for food. (the metabolic activity of the liver produces energy to be stored as ATP and not as heat to be released)

6.      People wear thin and light coloured cloths; cold drinks; rest in shade; use fan or air-conditioners; reduce muscular activity and taking cold shower/swimming.

Control of over-cooling:

As people keep their houses warm in winter, the homoiothermic animals also keep their bodies warm in cold weather. More heat is produced inside the body while at the same time loss of heat is reduced to the minimum.

1.    Increase in heat production. Through metabolism in the liver and muscles i.e. respiration = breakdown of food to produce energy/heat.

-  Through vigorous exercise which also increase the rate of respiration.

In cold weather rate of metabolism increases and hence increase appetite for food; increase food output.

2.    Shivering: is heat production process in which skeletal muscles are stimulated into uncontrolled activity. These muscles use up the metabolites and produce heat.

3.    Contraction of hair erector muscles: hairs are raised to make a thick coat thus trapping more air as an insulating layer. The contracted hair muscles in naked skin appear as “goose pimples.”

  Also the upright hairs prevent the cold winds from reaching the skin.

  Birds achieve a similar effect by means of muscles, which makes their hair fluff out.

4.    Adipose tissue: is a thick layer of fat beneath the skin in cold climate animals i.e. polar bears. The fat act as an insulating layer to prevent heat loss.

5.    In cold weather sweat glands cease to operate, so reduces heat loss by evaporation.

6.    Vasoconstriction: capillaries below the skin become narrow so that less blood flows through the skin and less heat gets lost through radiation. The skin looks pale.

7.    In animals of dog family; jaws are kept closed, breathing is slow and through nostrils.

8.    Man wears more thick clothes to keep his body warm; take hot drinks (tea).

Advantages of homoiotherms:

-Allow animals to exploit wide range of geographical habitats.

-Independent of external temperature.

-Enzyme- controlled reaction proceeds efficiently at optimum body temperature.

-Have high metabolic rate.

-Are active through out.

-Can respond to faster and move faster.

-Have great survival value.

Disadvantages of homoiotherms:

-High food consumption for generation of internal temperature: - metabolically expensive.

-Animals use food of high calorific valopue such as fat to keep warm especially in the cold season.

-There is a danger of overheating in tropical and desert areas: hence need of efficient means of controlling heat loss/gain by the body.

Advantages of poikilotherms

-Organism  can regulate behavior pattern to regulate body temperature.

-Low food consumption as they do not generate heat internally for temperature control.

-Organism can go in hibernation when the need arises.

-Insect larva have diapause (dormancy period) to avoid hostile temperature conditions.

Disadvantages of poikilotherms

-Low metabolic rate, therefore sluggish when external temperature are low and can be preyed upon easily.

-Depend too much on external temperature to react to external stimuli.

-When temperature are too low they go to hibernation and possibly leading to death.