TOPIC 4: EXCRETION | BIOLOGY FORM 3

TOPIC 4: EXCRETION | BIOLOGY FORM 3

Excretion is the removal of metabolic waste products from the body of an organism.

Egestion (defecation)

Is the removal of undigested food from the alimentary canal through anus.

QUESTION: Why defecation is not excretion?

Answer: Because it involves the removal of undigested food materials from the body which are not metabolic wastes as excretion.

Secretion

Is the production and release of useful material in the body of an organism.

Example of useful material

— Enzymes

— Hormones

— Mucus

IMPORTANCE OF EXCRETION

1. It helps to remove waste product and toxic materials e.g. urea, carbon dioxide gas etc.

2. It eliminates the excess materials from our body, like, soluble vitamins, drugs.

3. Maintain the pH of body fluids by removing excess bile pigment through liver.

4. Maintains water balance in the body. Excess water is removed as sweat or urine.

5. Regulation of blood pressure by removing excess salt and water in the body.

6. It gives chance for absorption of other materials.

7. Regulates the salt content in the body.

EXCRETORY PRODUCTS

Are the waste products produced from metabolic activities of the body.

THE MAJOR EXCRETORY PRODUCTS

The major excretory products are;-

i. Carbon dioxide

ii. Excess water

iii. Nitrogenous compounds like ammonia, urea, uric acid, etc.

Other excretory products include:

— chemicals from medicines

— toxic substances

— Hormones

THE TABLE BELOW DESCRIBE VARIOUS EXCRETORY PRODUCTS

ELIMINATED BY ORGANISMS

Carbon dioxide

This is a by-product of respiration of both plants and animals.

It is excreted through stomata in plants

In man, carbon dioxide is eliminated from the body by lungs

Excess Water:

Excess water is lost from the surface of gaseous exchange in both plants and animals.

In mammals, water is also lost through sweat, water vapour or urine

Nitrogenous wastes

Are wastes formed from the breakdown of excess proteins and amino acids.

Amino acids cannot be stored in the body because their accumulation is toxic.

Deamination is the process whereby excess proteins and amino acid are broken down in the liver to form ammonia.

FORMATION OF AMMONIA AND UREA

i. Amino group is removed from amino acid to form ammonia

ii. Ammonia formed combine with carbondioxide from respiration to form urea(CO(NH₂)₂).

TYPES OF NITROGENOUS WASTE

The main nitrogenous wastes excreted by living things are:

i. Ammonia

ii. Urea

iii. Uric acid.

I. AMMONIA

This waste is the results of broken down proteins and amino acid in the liver.

Ammonia is high toxic and soluble in water

It requires a large amount of water to be eliminated.

Ammonia is excreted mostly by aquatic organisms e.g. fish, amphibians

NB: Organisms that excrete ammonia are called ammoniotelic

II. UREA

Isthe nitrogenous waste formed when ammonia combine with carbon dioxide in the liver.

Urea is less toxic and less soluble in water.

It does not require a large amount of water to be eliminated.

Urea is excreted by many aquatic and terrestrial animals. Eghuman being.

 III. URIC ACID

Is a major nitrogenous waste of terrestrial animals such as birds, reptiles, and insects.

Uric acid is much less toxic and insoluble in water.

It requires less amount of water to be eliminated

NB: Animals that excrete uric acids as their waste product are called uricotelic animals

EXCRETION IN HUMAN

In human, the removal of excretory products is done through excretory organs.

EXCRETORY ORGANS

Are special organs concerned with removal of excretory products.

The following are excretory organs in an animal’s body:

i. The kidney

ii. The skin

iii. The lungs

iv. The liver.

The table below shows excretory organs and its corresponding excretory products

THE ROLE OF THE SKIN, LIVER AND KIDNEYS IN EXCRETION

Functions of the skin in excretion

(i) It has sweat glands which excrete excess water, minerals salts, and traces of urea.

Functions of the liver in excretion

(i) Detoxification

(ii) Deamination

NB: Detoxification: is the process whereby toxic and harmful substances are made harmless by the liver cells.

Other functions of the liver

i. Production of bile

ii. Storage of vitamins. E.g. fat-soluble vitamins A,D, E and K

iii. Carbohydrate metabolism

iv. Production of heat

v. Hormone breakdown

vi. Breakdown of red blood cells

vii. Storage of blood

Function of the lungs in excretion

(i) It helps in the removal of carbon dioxide from the body through exhalation.

URINARY SYSTEM AND ITS ADAPTIVE FEATURES

THE URINARY SYSTEM

Is a system concerned with production, storage and removal of urine.

FUNCTIONS OF THE URINARY SYSTEM

i. Excretion of nitrogenous metabolic waste products such as ammonia and urea.

ii. Regulates the concentration of salts in the body fluids.

iii. Maintains balance of water in the blood.

iv. Plays a role in controlling blood composition, blood pressure and volume of plasma.

Components of the urinary system

i. Kidneys

ii. Ureters

iii. Blood vessels

iv. Urinary bladder

v. Urethra

vi. Sphincter muscles

DIAGRAM OF HUMAN URINARY SYSTEM

URETERS

Is the pair of ducts or tubes that carries urine from the kidneys to the bladder.

Ureters wall consist of smooth muscles which contract to force urine downward, away from the kidneys.

URINARY BLADDER

Is the muscular sac that stores urine temporary before excreted out of the body.

SPHINCTER MUSCLES

Are the circular muscles that help to keep urine from leaking by closing tightly around the opening of the bladder

URETHRA

Is a tube which carries urine from the bladder to outside the body.

In female, urethra carry urine outside the body

In maleurethra carry urine and sperm outside the body.

KIDNEYS

Are the bean-shaped organs which help the body to eliminate urea in form of urine.

The kidneys are the main organs of excretion.

Each kidney is enclosed in a thin, fibrous covering called the capsule.

LAYERS OF THE INTERNAL STRUCTURE OF THE KIDNEYS

The kidney has three distinct regions, namely;- (i) The cortex

(ii) The medulla

(iii)The pelvis

DIAGRAM OF THE INTERNAL STRUCTURE OF THE KIDNEY

THE CORTEX

Is the outermost layer of the kidney.

It contains billions of glomeruli where blood is filtered.

THE MEDULLA

Is the middle layer of the kidney, normally red in color

It consists of billions of loops of Henle where the amount of salt and water are controlled.

The surface of medulla is folded to form projections called pyramids.

THE PELVIS

Is the space inside the kidney which collects the urine and leads it to the ureter.

FUNCTIONS OF THE KIDNEY IN EXCRETION

i. They help to remove excess water and dissolved urea in form of urine from the blood.

ii. They maintain appropriate water-salt balance in the blood.

iii. They are important in regulation of blood pressure.

They filter blood to remove wastes and reabsorb useful substances such as water and salts.

Absorbing minerals.

RENAL ARTERY

Is the blood vessel that carries blood from aorta to the kidneys.

Blood carried toward the kidneys contains more urea than that blood carried away from the kidneys

RENAL VEIN

Is the blood vessel that carries blood away from the kidney where waste products have been removed

Some animals do not have a well developed kidney, they have structures called nephridia.

Nephridia have the same role as the nephron in the kidneys.

Example of animals that use nephridia as their excretory organ

— Earthworm

NEPHRON

is the structural and functional unit of the kidney.

It act as filters and remove the waste products from blood and forms urine.

Each kidney possesses a large number of nephrons approximately one million.

DIAGRAM OF NEPHRON

PARTS OF NEPHRON

Each nephron is divided into two portions. (a) Malpighian body or organ (b) Renal tubules.

(a) MALPIGHIAN ORGAN

Is the anterior rounded part of the nephron.

Malpighian organ is formed of two parts, namely:

i. Glomerulus

ii. Bowman’s capsule

NB: Glomerulus and Bowman’s capsule together are called Malpighian body or Malpighian Organ.

(i) GLOMERULUS

Is a fine interconnected network of blood capillaries enclosed by Bowman’s capsule. Functions/Roles of glomerulus

It receives and filters blood from afferent arteriole.

AFFERENT ARTERIOLE

Is the branch of renal artery which carries blood to the glomerulus

Function of afferent arteriole

Carries blood to the glomerulus

EFFERENT ARTERIOLE

Is the branch of renal artery which carries blood away from the glomerulus to different part of the body.

Function of efferent arteriole

Carries blood with large particles away from the glomerulus to different part of the body.

(ii) BOWMAN’S CAPSULE

Is a round cup-shaped structure that encloses the glomerulus.

Function/role of Bowman’s capsule

It serves as a filter to remove organic wastes, excess inorganic salts, and water.

RENAL TUBULE

Is the duct like tubule behind the Bowman’s capsule

The renal tubule is divided into four regions, namely;

i. Proximal convoluted tubules

ii. Loop of Henle

iii. Distal convoluted tubule

iv. Collecting duct

I. PROXIMAL CONVOLUTED TUBULE

Is highly coiled tubule that extends from Bowman’s capsule to the descending loop of Henle.

Function/Role of proximal convoluted tubule

It helps in reabsorption ofglucose, amino acid, potassium and calcium ions.

II. LOOP OF HENLE

Is the U-shaped part of the nephron.

Function/Role of loop of Henle

It helps to reabsorb water and salt (Nacl).

NB:

Animals with long loop of Henle store large amount of water in their body for long period of time.

It is one of the adaptive feature for animals living in desert area.

PARTS OF LOOP OF HENLE

Loop of Henle is divided into two parts namely:

(i) Descending loop of Henle

(ii) Ascending loop of Henle.

DESCENDING LOOP OF HENLE

This part is permeability to water.

Function

To reabsorb water

ASCENDING LOOP OF HENLE

This part is permeability to sodium ions.

Function

To reabsorb sodium ions

III. DISTAL CONVOLUTED TUBULE

Is more highly coiled tubule that extends from the loop of Henle to the collecting duct.

Function/Role of distal convoluted tubule

To reabsorb sodium ions and water

IV. COLLECTING DUCT

Is tube that directs urine into the renal pelvis of the kidney for drainage into the ureter.

ROLES OF THE NEPHRON IN EXCRETION

i. At the distal convoluted tubule and collecting duct water re- absorbed under the influence of ADH.

ii. The afferent arteriole entering the Bowman’s capsule is wider than the efferent arteriole leaving it. This creates high pressure at the glomerulus.

iii. Used in the process called ultrafiltration. Whereby liquid part of the blood with dissolved substances (urea, glucose, salts and amino acid) forced out into the cavity of Bowman’s capsule.

iv. Reabsorption of salts such as Na+ ions and water at the loop of Henle under the influence of the aldosterone hormone.

ADAPTATIONS OF THE URINARY SYSTEM TO ITS FUNCTIONS

1. The urinary system has a large afferent arteriole, and narrow efferent arteriole, which allows build up of pressure, thus facilitating ultra-filtration.

2. The glomerulus capillaries are highly coiled and semi permeable, causing a build up of pressure in the glomerulus hence ultra filtration.

3. The glomerular capillaries are semi permeable to allow selective movement of materials in and out of the nephron (selective reabsorption).

4. The tubules’ epithelium is thin (1 cell thick) to reduce diffusion distance for faster passage and hence reabsorption of materials; and they are and leakier than normal capillaries.

5. It is connected to a collecting duct, which channels the filtrate (urine) out of the nephron to the ureter to allow for continous functioning of the nephron.

6. The proximal convoluted tubule and the distal convoluted tubule are coiled so as to increase the nephrons’s length and hence more surface area for efficient reabsorption to take place.

7. The Bowman’s capsule is cup-shaped to provide maximum surface area for filtration.

8. The tubule is supplied with a network of blood capillaries for maximum reabsorption.

9. The nephrons are numerous in number for efficient excretion of waste products.

THE PROCESS OF URINE FORMATION

The process of urine formation in the body involves three steps, namely:

1. Ultrafiltration

2. Selective reabsorption

3. Removal of urine

1. ULTRAFILTRATION (GLOMERULAR FILTRATION)

This involves forcing out liquid part of the blood into the cavity of Bowman’s capsule.

The process occurs as follows

1. The blood from renal artery carrying the urea, plasma, proteins, mineral ions, blood cells, dissolved food substances, hormones and oxygen enters the nephron through the afferent vessel.

2. The afferent vessel entering the glomerulus is wider than the efferent vessel leaving the glomerulus.

3. The narrowness of efferent vessel produces a resistance to blood flow and hence creating a high pressure in the glomerulus.

4. Due to the high pressure in the glomerulus the liquid part of the blood and dissolved substances of small molecular sizes such as urea, glucose, amino acids, salts, uric acid, vitamins, and hormones are forced out of the glomerulusinto the Bowman’s capsule.

5. The large sized molecules such as proteins and blood cells are not filtered because the walls of the capillaries of the glomerulus and Bowman’s capsule have very small pores.

6. The filtrate formed during this process is called glomerular filtrate.

NB: The blood that remains rich in plasma proteins and blood cells has very little water.

2. SELECTIVE REABSORPTION

This involves turning back of useful substances into the blood capillaries.

— During this process as the glomerular filtrate is flowing along the renal tubule, most of the filtered substances which are useful in the body are selected and reabsorbed back into the blood.

Selective reabsorption occurs along the renal tubules as follows

i. The glomerular filtrate from the Bowman’s capsule enters the proximal convoluted tubule whereby all Glucose, amino acid, vitamins, hormones and 80% of water and salts are reabsorbed through the process of active transport.

ii. From the proximal convoluted tubule, the glomerular filtrate flows into the loop of Henle whereby 5% of water and most salts are reabsorbed through the process of osmosis and active transport.

iii. From the loop of Henle, the filtrate moves to distal convoluted tubule where most of salts and water are reabsorbed.

iv. The remaining filtrate is now called urine.

3. SECRETION

The process where substances move out of the blood into the renal tubules (nephron).

Secretion takes place at distal convoluted tubule where potassium and acid in the form of hydrogen ions, are removed from the blood by the distal convoluted tubule, and are then added to the urine.

REMOVAL OF URINE

From the distal convoluted tubule, the urine moves to the collecting tubules.

From the collecting tubules the urine flows down into the collecting duct where it joins urine from collecting tubules of other nephrons.

The urine then flows into the pelvis via the pyramids and is finally emptied into the urinary bladder via the ureter.

When the bladder is full, the sphincter muscles relaxes and urine is allowed to pass out of the body via urethra.

Diagram to illustrate the process of urine formation

NB:

The process by which urine is passed out from the body is known as urination.

Human produce approximately 1.5 litres of urine per daily but may vary depending on the amount of liquid taken.

COMPONENTS OF URINE

POSSIBLE EXAMINATION QUESTIONS

1. Explain why:

(i) Desert animals have a long loop of Henle and fewer glomeruli. (ii) Proximal convoluted tubule is coiled.

i. Carbon dioxide

ii. Oxygen

iii. Water

3. The following table shows the approximate concentration of a certain substance in blood plasma, glomerular filtrate and urine. Study the table below carefully and answer the questions that follow.

4. Briefly explain why the concentrationof protein and blood cells in glomerular filtrate and urine is zero?

i. Because protein and blood cells are not filtered due to their large molecule size

ii. Because the walls of the capillaries of the glomerulus and Bowman’s capsule have very small pores.

Briefly explain why there is no glucose in urine?

iii. Because in proximal convoluted tubule all glucose are reabsorbed into blood capillaries.

5. By how many times is urea ore concentrated inuring ethane glomerular filtrate?

6.Explain why there is great concentration of urea in urine than in glomerular filtrate

7. Study the diagram below and answer the questions that follow:

a. Name the part A to G

b. Name the process which takes places between C and D

c. Name three materials reabsorbed at E and two at G

d. What is the function of F during urine formation

8. Explain what happens to excess amino acids in the liver.

9. The diagram represents the nephron. Use it to answer the questions that follow;

i. Name the parts labelled A, B, C and D

ii. Name the fluids founds in C and D

iii. What eventually happens to the fluid in D?

iv. Explain how the urinary system is able to carry out its functions

10. The diagram below represents a mammalian kidney. Study it carefully and answer the questions that follow.

Before the drains into the ureter, it is collected at part…………….

Blood vessel R carries blood from……. to …………..

Blood vessels S is called…………………..

b. Secretion

c. Deamination

d. Nephron

COMPLICATIONS AND DISORDERS OF THE EXCRETORY SYSTEM

The following are the common complications and disorders of the excretory system

1. Gout

2. Kidney failure

3. Kidney stone

4. Urinary tract infections (UTIs)

5. Liver cirrhosis

6. Hepatitis

7. Bladder Cancer

1. KIDNEY (RENAL) FAILURE

Is the failure of the kidney to function adequately due to partial or entire destruction of nephrons

Causes of kidney failure

i. Damage to the kidney due to accident or complications during surgery

ii. Low blood volume due to excessive bleeding

iii. Poor intake of fluids

iv. Medication, for example, diuretics (“water pills”) may cause excessive water loss
v. Obstruction of renal artery, causing blocking of blood flow to the kidneys
Kidney stones

vi. Chronic diseases that gradually cause the kidneys to stop functioning

vii Dehydration from loss of body fluid (for example, vomiting, diarrhoea, sweating, fever)

viii. Prostate cancer may block the urethra and prevent the bladder from emptying

Symptoms of kidney failure

i. Oedema (swelling of the legs, ankles, feet, face or hands due to excess fluids)

ii. High levels of urea in blood leading to vomiting, nausea, weight loss, blood in urine or difficulty in urinating.

iii. Loss of appetite

iv. Shortness of breath

Effects of kidney failure
i. Bone damage

ii. Muscle paralysis

iii. Abnormal heart rhythm

iv. Loss of memory

v Pain in the back or side

vi. If not treated earlier, can lead to death if it involves both kidneys

Prevention/treatment of kidney failure

i. Avoid potassium-rich foods like citrus fruits, bananas, instant coffee, peanuts and chocolate

ii. May require a kidney transplant.

iii. Medications e.g. phosphorus-lowering medications.

iv. Dialysis-The use of semi-permeable membrane to separate large molecules from small ones, used in kidney dialysis machines to remove urea from blood

2. KIDNEY STONES

Are small, hard mineral deposits that form in the pelvis region of the kidneys which can obstruct the flow of urine.

Causes of kidney stones

i. Inadequate intake of water

ii. Certain types of drugs

iii. Lack of vitamins

iv. Dehydration from loss of body fluid (for example, vomiting, diarrhoea, sweating, fever).

 Decrease in urine volume and/or an excess of stone-forming substances in the urine.

 Infection in the urinary tract gradually cause the kidneys to stop functioning

 Obstruction to the flow of urine

 Prostate cancer may block the urethra and prevent the bladder from emptying.

Symptoms of kidney stones

Extreme pain and difficulty in urination

Pain in the low back and/or side, groin, or abdomen

 Blood in the urine due to damage of the inside walls of the kidney, ureter or urethra

 Nausea and vomiting

 Chills and fever

Effects of kidney stones

 May lead to kidney failure

 Toxicity due to urine staying in the body for a long time.

 Severe back pain

 Surgery and medications are expensive.

Prevention/treatments of kidney stones

 Taking a balanced diet that is low in protein, nitrogen and sodium

 Drinking plenty of water

 Avoid beverages that contain caffeine like coffee.

 Surgical treatment to remove the stones
 May require kidney transplant
 Medications (painkillers).

3. URINARY TRACT INFECTIONS (UTIs)

Is an infectionof the urinary system. This type of infection can involve urethra, kidney, bladder and ureters.

Most infection involves the lower urinary tract such as bladder and urethra.

Women are at greater risk of developing a UTI than men.

Causes of urinary tract infections

Bacterial infection in the urinary tract

Symptoms of urinary tract infections

i. Frequent feeling to urinate

ii. Pain during urination

iii. Cloudy urine

iv. Passing frequent, small amounts of urine.

v. Strong-smelling urine.

Effects of urinary tract infections
i. Medications are expensive.

ii. Pain and nuisance due to urge to urinate frequently.

Prevention/treatment of urinary tract infections

i. Drinking a lot of fluids

ii. Maintaining toilet hygiene

iii. Complete urination

iv. Cleanse genital area before sex

Urinate after sex to flush away any bacteria that may have entered your urethra

Keep genital area dry by wearing cotton underwear and loose-fitting clothes

4. LIVER CIRRHOSIS

Is a condition in which liver cells degenerate and are replaced by scar tissue, causing the liver to shrink, harden, become fibrous and fail to function normally.

Causes of liver cirrhosis

i. Alcohol and viral hepatitis B and C

ii. Attack by bacteria and viruses

iii. Parasites such as liver flukes and schistosoma

iv. Obstruction of the gall bladder

v Exposure to chemical poisons such as silica and asbestos

Symptoms of liver cirrhosis

i. Loss of weight

ii. Poor appetite

iii. Abdominal pain

iv. Blood stained vomit

Effects of liver cirrhosis

i. Severe cirrhosis is fatal

ii. Easy bruising, yellowing of the skin (jaundice), itching, and fatigue.

iii. Oedema, ascites (accumulation of fluid in the abdominal cavity), and liver cancer

Prevention/treatment of liver cirrhosis

i. Avoiding excess consumption of alcohol

ii. Avoiding fatty food

iii. Low salt intake

iv. Eating varied, easily digestible food

v Plenty of rest

vi. Medical treatment

vi. Liver transplant in case of severe cirrhosis

5. HEPATITIS

Is an inflammation of the liver caused by viruses

Hepatitis is of the three forms, Hepatitis A, Hepatitis B and Hepatitis C

Causes of Hepatitis

It is caused by virus

Mode of transmission of Hepatitis

Viruses are transmitted through body fluids such as saliva, blood and semen.

Symptoms of Hepatitis

i. Nausea

ii. Loss of appetite

iii. Fatigue

iv. Abdominal discomfort

v. Jaundice

vi Dark brown urine and whitish faeces.

Effects of Hepatitis

i. May lead to liver failure

ii.  If not treated early may lead to death

Prevention/treatments of Hepatitis

i. Hygienic processing of food

ii. Proper disposal of sewage

iii. Treatment of water

iv. Proper handling of blood

v. Screening blood before transfusion

vi. Using sterilized needles and syringes

6. BLADDER CANCER

Causes

i. Not yet very clear but number of reasons such as

ii. Smoking

iii. Radiation

iv. Parasitic infection

v. Exposure to chemicals (carcinogens) may cause the disease.

Symptoms

i. Blood in urine.

ii. Frequent urinary tract infections, painful urination and urge to urinate without actual flow.

iii. Weight or appetite loss.

iv. Abdominal or back pain, persistent raised temperature or anaemia.

Effects

i. Expenses on medication.

ii. Pain – reduces the quality of life.

Prevention/treatment

i. Stop smoking.

ii. Avoid exposure to industrial chemicals.

iii. Eat healthy foods-choose low-fat, low-cholesterol diet that includes plenty of fruits and vegetables.

iv. Avoid dehydration by increasing your fluid intake, particularly water. Water dilutes cancer-causing chemicals.

7. GOUT

Causes

i. Abnormal metabolism of uric acid, either producing too much or having difficulty excreting it.

ii. Eating too much meat and alcohol worsens it.

Symptoms

i. Tender, swollen joints

ii. Red or purplish skin and warmth around the affected joint.

iv. Pain due to the formation of crystals in the joints.

v. Difficulty in walking.

Effects

i. Inability to walk

ii. May lead to arthritis hence bone erosion

iii.  May lead to kidney stones

Prevention/treatment

i. Diet low in protein, especially avoiding red meat.

ii. Drinking plenty of water.

iii. Treatment using prescribed drugs.

EXCRETION IN PLANTS

In plants, breakdown of substances is much slower than in animals. Plants do not have specialized excretory organs for the removal of metabolic wastes.

QUESTION: Why plants do not have specialized excretorysystem like in animals?

ANSWER

Plants do not need a specialized excretory system like in animals because:-

(i) The rate of producing waste products in plants is much lower.

(ii) Most excretory products from plants are removed by diffusion through the stomata or lenticels.

(iii)Some major excretory products of plants are recycled after being released. E.g. carbon dioxide used in photosynthesis, oxygen used in respiration and water used in water cycle.

(iv) Plants produce less poisonous substances compared to the nitrogenous wastes produced by animals.

(v) Plants have large vacuoles which store waste substances often accumulating at concentrations that lead to crystal formation in form of oil droplets or granules.

(vi) Plants store the waste products in organs that are destined to fall or die off. E.g. leaves Plants eliminate some waste through diffusion.

TYPES OF EXCRETORY PRODUCTS ELIMINATED BY PLANTS

The following are common excretory products eliminated by plants

i. Alkaloids

ii. Resins

iii. Gums

iv. Latex

v. Rubber

vi. Excess oxygen produced by photosynthesis during a day.

vii. Carbon dioxide produced by respiration

viii. Excess water throughtranspiration.

ALKALOIDS

Are nitrogenous excretory products in plants and occur in various forms.

Examples of common alkaloids:

— Caffeine and theophilinet

— Quinine

— Cocaine

— Cannabis

— Opiates(morphine)

— Nicotine

— Colchicine

— Pyrethrins

— Khat (miraa) mirungi)

THE IMPORTANCE OF COMMON EXCRETORY PRODUCTS OF PLANTS

The table below shows functions of some plant waste products.

Urine is a waste product of metabolism formed in the structural and functional units of the kidneys called nephrones. Blood at high pressure travels into these tubules by the tube of blood capillaries and walled glomerulus.

The following steps are involved in the process of urine formation:

Ultra filtration, blood enters the glomerulus through the afferent arterioles, it passes under pressure that result in filtration of blood, water and small molecules are forced out of glomerular capillary walls and Bowman’s capsule. Large molecules remain in the blood of the glomerulus.

Selective reabsorption, some molecules are selectively reabsorbed into the blood. The glomerular filtrate flows through the proximal convoluted tubule, the U-shaped loop of Henle and distal convoluted tubule. The useful substances present such as glucose, amino acids and salts are reabsorbed by active transport. The filtrate now contains more urea, some salts and water. Reabsorption of solutes increases the water concentration of the filtrate. The water is reabsorbed into blood by osmosis.

Tubular secretion, some nitrogenous waste products like creatinine and some other substances like K⁺ ions are removed from blood by distal convoluted tubules are added to urine. The urine formed is collected in the urinary bladder.