EXCRETORY SYSTEM
At the end of this chapter, students should be able to:
Identify and describe different types of excretory systems in plants and animals;
Explain the mechanisms of some of the excretory organs and relate structure to functions.

Definition of excretion
Excretion is the removal of waste products of metabolism which are harmful or would in time interfere with normal functions of organisms.
Note: excretion is not the same as egestion and secretion. Egestion is the removal of solid undigested food substances which are not by-products of metabolism, e.g. The removal of faeces from anus. Secretion on the other hand is the production of useful substances in the body by some special cells, e.g. Production of enzymes and hormones.

Importance of Excretion
We need to eliminate waste metabolic substances because of the following reasons:
The excretory products are harmful/toxic to the body hence, they need to be removed;
Some metabolic substances are poisonous whenever they accumulate in the body;
Excretion maintains water balance in the body;
Excretion maintains salt balance in the body;
Waste products when not removed can interfere with normal metabolic activities of the body.




MECHANISM OF EXCRETION IN SOME ORGANISMS

FLATWORMS










The excretory system consists of two longitudinal canals with network of ducts (tubules). The ducts branch to all parts of the body and end up in flame like cells.
The waste products e.g. water, ammonia and Carbon dioxide diffuse from the surrounding cells into the flame cells. With the aid of the flagella/ or cilia, which suck the excretory products into the funnel, the fluid containing the waste products is propelled/driven into the tubules. And finally they are eventually removed at the exterior.
Note: another major function of the flame cells is the regulation of the water content of the animal i.e. Osmoregulation.

EARTHWORM










Nephridium is the excretory organ in earthworm. Each Nephridium consists of a flattened funnel-like structure with two lips called the nephrostome. The earthworm excretes urea and guanin. The beating of the cilia of the nephrostome draws coelomic fluid. During this process of drawing fluid, salts and other useful substances are reabsorbed through the walls of the tubes. The unabsorbed substances, including water, are collected in the muscular tubes as urine. The excretory pore relaxes to allow the urine to escape to the exterior. While Carbon dioxide are removed through the moist body surface during gaseous exchange.

CRUSTACEANS











The excretory organ in crustacean are the green glands. Each gland consists of five parts: a small end sac, the green gland, flattened disc called the labyrinth, the Nephridium canal, and the urinary bladder.
How green glands perform excretion in crustacean

The green gland extracts metabolic wastes from the blood. During this process of extraction, water and dissolved salts pass into the end sac by filtration. Nitrogenous waste materials pass into the labyrinth. Selective reabsorption of salts and glucose takes place at the nephridial canal according to the needs of the animal. Urine is formed. The urine formed is hypotonic to the blood, and is collected in the urinary bladder and removed by the muscular contraction of the walls of the bladder, and discharged to the outside through the pair of excretory pores located at the base of the antennae.

Note: some of the crustacean's waste Carbon dioxide is excreted through the gills and some, together with calcium, is known to be used by crayfish to strengthen its exoskeleton.

INSECTS












The excretory organs of insects are the Malpighian tubules. They are found between the midgut and the rectum; each tubule consists of two parts:  the distal end (free and closed end, floating in the haemocoel) and the proximal end (opens into the gut).

MECHANISM OF EXCRETION IN INSECTS.
When nitrogenous waste products and water are liberated into the haemocoel, they are absorbed at the distal end of the malpighian tubules. At the distal end, urates of potassium and sodium are extracted from the blood. The alkaline contents at the proximal end become acidic due to the addition of carbon dioxide, hence uric acid is precipitated and the potassium and sodium ions are re-absorbed as bicarbonate. The uric acid is passed into the ileum, from where it is passed out mixed with faeces. More water is re-absorbed at the rectum. The uric acid, mixed with faeces, is thus expelled as more or less dry material.
Note:
1. Excretion by storage: This is an uncommon form of excretion in insects, which involves storing up of uric acid in special fat bodies within the body of the insect from where it does no harm.
Why would insect use "excretion by storage"?
2. Insects do not drink!
How do they (insects) get their water?
They get all the water they need from the food they eat and from the chemical breakdown of carbohydrate in which water is a by-product.
3. Reasons why insects are very successful group of animals: they can conserve water, hence, they are found everywhere even in the hottest and driest places on earth.
What are those factors enabling insects to conserve water?
Presence of wax on the outer layer which makes their outer surface waterproof;
Presence of spiracles to prevent water loss from the gaseous exchange surface which is inside the body;
Possession of efficient excretory system.

MAMMALS
The excretory system of a mammal consists of:
the kidneys;
the ureters;
the bladder;
the urethra.













                                                                      Excretory system of man
The longitudinal section of mammalian kidney


Structure of the kidney

A kidney is a reddish organ. It is bean-shaped with fibrous capsule. It is located at the dorsal wall of the abdomen. It is made up of three regions; the cortex (outer region), the medulla (inner region) and the pelvis (the funnel-shaped space). It is highly vascularised with many capillaries which are all branches of renal artery and renal vein. At the top/apex of each kidney lies the adrenal gland.

Structure of a urinary tubule

The kidney is made up of functional units, each is called a kidney tubule or nephron. A human kidney contains over one million nephrons. At the anterior end, each nephron is made up of cup-shaped and hollow structure called bowman's capsule, and a long tube called convoluted tubule. In the hollow of the bowman's capsule, there is a mass of blood capillaries called glomerulus. These blood capillaries in the glomerulus are branches of the renal artery, which brings blood from the dorsal aorta to the kidney. The glomerulus and the bowman's capsule form the malpighian body/corpuscle.
The capsule (i.e. Bowman's capsule) opens into a short coiled tube, the proximal convoluted tubule, which lies in the cortex and joins the bowman's capsule anteriorly and the loop of Henle posteriorly. Then the tubule goes back, as the ascending tubule, from the medulla to the cortex region, where it is again coiled and is known as the distal convoluted tubule. The distal convoluted tubule joins a collecting tubule, which passes through the medulla region and opens at the pelvis.

The capillaries in the glomerulus rejoin to form a blood vessel leading out of the capsule. This vessel then branches into a capillary network around the urinary tubule before rejoining to form a branch of the renal vein. Through the renal vein, blood from the kidney flows to the posterior vena cava and thence back to the heart.

Note: the proximal convoluted (coiled) tubule forms the Henle loop in the medulla when it descends and form a U-shaped structure.


The kidney nephron

Mechanism of kidney excretion in mammals or how the kidney carries out its function of urine formation and discharge

The processes of urine formation include:
Ultrafiltration;
Selective reabsorption;
Tubular secretion
Blood from the heart flows to the glomerulus under high pressure, resulting in filtration of amino acids, glucose, mineral salts, water and urea out of the thin walled capillaries into the the Bowman's capsule. This process of filtering materials from the glomerulus into the Bowman's capsule is called ultra filtration or pressure filtration.

Note: the liquid that filters from the blood into the space inside the Bowman's capsule is called the glomerular filtrate. It is similar to blood plasma, but does not contain blood corpuscles or blood proteins.

The fluid that filters into the Bowman's capsule flows down the tubule, therefore bringing about selective reabsorption of useful substances back into the blood in the blood capillaries. Hence, at the proximal convoluted tubule, those useful substances such as glucose, amino acids, mineral salts and water are reabsorbed into the blood capillaries against concentration gradient or by active transport.
All the urea, small amounts of mineral salts and water are left in the tubule.

The fluid in the tubule becomes more concentrated as it flows through the distal tubule where more water is reabsorbed by the action of anti-diuretic hormone (ADH) and urine is finally formed.

Note: During ultrafiltration, bigger molecules like plasma proteins and the blood cells cannot pass through the wall of capillaries, hence, they are returned back into the blood.

Note: The filtered blood leaving the kidney by the renal vein contains:
less oxygen and glucose, and more carbon dioxide, as a result of cellular respiration; and
less nitrogenous wastes, salts and water as a result of excretion.

Reasons why blood flowing from the heart to the glomerulus is under high pressure.
The blood capillaries in the glomerulus have narrow diameter relative to the renal artery.
The blood capillaries that leave the glomerulus, are narrower in diameter than those that lead into it.
A large amount of blood flows through the kidney.
Note: It is estimated that about one quarter of the blood that leaves the heart with each beat flows through the kidney. When a large volume of blood flows through narrow capillaries, which offer a resistance to the flow of blood, a high pressure is built up.

How urine formed is expelled:
Urine passes through the ureter to the bladder where it is stored. The bladder has a muscle at its posterior end called a sphincter, which closes the outlet of the bladder. When one is ready to urinate; when the bladder is full, it contracts, while the sphincter relaxes and hence, the urine is discharged/expelled through the urethra.



Other functions of the kidney:
Osmoregulation
Maintenance of acid-base balance of the blood
Note: urinary tubule/kidney nephron is the functional unit of the kidney.

Class work
1. Relate the environment of three types of vertebrates of your choice to their;
type of excretory wastes, and
means of excretion.
2. Assess the reason why blood is not found in the urine, whereas mineral salts are present in it.
3. Suggest three factors that dictate the forms of nitrogenous waste excreted by an animal.
4. Compare excretion in insects and amphibians with respect to the:
structure of their organs of excretion;
mechanisms of excretion, and
forms of excreted wastes.
5. The liver cells make bile, which  is needed for the digestion of fats. Justify with explanatory reason(s) why the bile salt is considered excretory product.
6. Describe the process of excretion through the mammalian skin.
7. How would you explain the pungent odor of the expelled urine in man?


Excretion in Plants

Plants do not have special excretory organs. Gaseous wastes diffuse through the stomata, while insoluble excretory products are stored in some of the cells. Such non-gaseous wastes are excreted in the form of insoluble oil droplets, crystals and granules. The main excretory organs of flowering plants are the stomata in the leaves and lenticels in the stem. The main excretory products formed in plants are water, carbon dioxide and oxygen.
Mechanism of Excretion in Plants
Plants have their waste products eliminated through:
Transpiration;
Respiration;
Photosynthesis;
Guttation.
Note: These processes require diffusion.

Through transpiration, photosynthesis and respiration, excess water is lost in form of vapor i.e. through respiration and transpiration; likewise carbon dioxide i.e. respiration; oxygen i.e. photosynthesis and as droplets (e.g. In tomato, potato, grass) of water i.e. guttation. Other insoluble materials such as tannins, mucilage, gum, crystals, alkaloids and anthocyanin are stored in the bark of stems, leaves and petals, which are shed periodically.
Note: wastes stored in the bark of stem, leaves and petals can be removed from plants by the process of leaf-fall.

Assignment
Provide the economic importance of the following insoluble wastes in plants:
Tannins;
Gums;
Alkaloids