EXCRETION

EXCRETION

Excretion is the process by which waste products of metabolism are removed from the cells/body of living things.

Importance of Excretion

1.        Excretory products are harmful or poisonous to the body and must be removed
2.         Excretion helps to maintain water balance in the body
3.         Excretion helps to maintain salt balance (homeostasis) in the body
4.         Waste products may interfere with normal metabolic activities of the body when not removed

Structures of excretion in living organisms

	Organism	Excretory Systems/organs
1	Protozoa e.g. Amoeba, Paramecium	Contractile vacuole
2	Flatworms e.g. Tapeworm	Flame cell
3	Annelids e.g. earthworm	Nephridia  (singular- nephridium)
4	Insects	Malpighian Tubule
5	Crustaceans e.g. Prawn	Green glands
6	Fishes, Amphibians (e.g. toad, frog), Reptiles	Kidneys
7	Birds	Kidney and lungs
8	Mammals	Kidneys, skin, liver and lungs
9	Flowering plants	Stomata and Lenticel

Contractile Vacuole

The contractile vacuole is a membrane-bound organelle found in protozoa in fresh water. As water molecules move into the cell via osmosis, the excess water is collected in the contractile vacuole by diffusion. Ions are pumped back into the cytoplasm by active transport. As the vacuole gets filled with water, it moves to the cell surface where the water is discharged via a pore to the exterior. Contractile vacuole ensures the osmotic equilibrium of the cell is maintained.

Flame cells

Flame cells are found in Platyhelminthes (flatworms) and Nematodes. A flame cell is a hollow, bulb-shaped cell lined by tufts of cilia. As waste materials flow into the flame cell, rapidly beating cilia move the liquid containing waste materials along the tubules which open through the nephropores to the exterior of the body.

Malpighian Tubules

It makes up the excretory system in insects. It consists of tubules located between the midgut and hindgut of the alimentary canal. Each tubule consists of the proximal end which opens into the gut as well as the distal end which is closed and extends to the haemocoel (body cavity with blood). Wastes pass into the tubules through the distal end. As the waste materials pass, some solutes, as well as water, are pumped back into the haemocoel. At the rectum, more water reabsorption takes place and the uric acid becomes very concentrated and precipitated as crystals. Excretion of insoluble uric acid crystals is an adaptation by insects to minimise water loss by excretion.

Nephridia

The nephridia (singular- Nephridium) are excretory tubules present in annelids e.g. earthworm. A pair of nephridia is found on every segment of the worm. One end of the nephridia consists of a ciliated funnel called nephrostome that opens to the body cavity. The other part consists of the coiled and muscular tube. As waste products move along the coiled tube, useful materials such as glucose, salts are reabsorbed back to the blood circulation. Waste is stored temporarily in the muscular tube and then eventually expelled through the nephridiopores.

Kidney

Urinary System and associated blood vessels

The urinary system of mammals consists of a pair of kidneys, a pair of ureters, a urinary bladder and the urethra. The kidneys are bean-shaped organs in the abdominal cavity.

The kidney is supplied with blood vessels- renal artery and renal vein. The renal artery arises directly from the dorsal aorta and brings oxygenated blood containing excretory products. The renal vein drains the filtered deoxygenated blood from the kidney to the posterior vena cava. The ureter connects the kidney to the bladder where urine is stored temporarily. The bladder leads to the urethra which opens to the exterior.

The longitudinal section of the kidney consists of two main parts:

i.                    An outer dark-red region called the cortex.

ii.                  An inner thicker paler-red in colour region known as the medulla.

Nephron

The nephron (urinary tubule) is the functional unit of the kidney. It starts at the cortex as a cup-shaped structure called Bowman’s capsule. In the Bowman’s capsule, the renal artery divides into a mass of blood capillaries called the glomerulus. The capsule with the glomerulus is termed Malpighian corpuscle. The capsule opens to a short coiled tube called proximal convoluted tubule. The proximal convoluted tubule connects to the U-shaped loop called Henle’s loop where selective reabsorption also takes place. Henle’s loop enters the cortex to form another coiled tube called distal convoluted tubule. The tube then completes its course in the medulla where it opens to a collecting duct.

Formation of Urine

The main processes involved in the formation of urine are ultrafiltration, selective reabsorption and hormonal secretion.

1.  Ultrafiltration: blood is brought to the kidneys through the renal artery into the glomerulus of the Bowman’s capsule. As the blood circulates in the glomerulus, water, urea, nitrogenous compounds, mineral salts, sugars, glucose and plasma solutes are filtered into the capsule.
2.  Selective reabsorption: the fluid flows down the capsule to the proximal convoluted tubule and Loop of Henle where sugars, amino acids and salts which are useful to the body are selectively reabsorbed back into the blood capillaries by active transport. Water is reabsorbed by osmosis.
3.  Hormonal secretion: Anti-diuretic Hormone (ADH) controls the concentration of the urine

The average composition of urine is:

Component of Urine	Percentage composition (%)
Water	96
Mineral salts (mainly sodium chloride)	1.8
Urea	2
Other nitrogenous substances	0.2

Function of the Kidney

1. Excretion of toxic metabolic by-products such as urea, ammonia and uric acid
2. Maintenance of acid-base balance in the body (pH)
3. Maintenance of osmotic concentration of extracellular fluid
4. Maintenance of ionic balance in extracellular fluid

Stomata and Lenticels

The main excretory organs of plants are the stomata and lenticels. The stomata are surrounded by guard cells and the waste products are removed by diffusion.

The main waste products from plants are water vapour (from transpiration stream), oxygen (from photosynthesis) and carbon(IV)oxide from respiration. Other waste products from plants are tannins, acids, resins, mucilage, latex, alkaloid, oils, gum and anthocyanin.

Organelles of excretion in a cell include mitochondria (CO₂) and chloroplast (O₂).

Waste products of metabolism

	Excretory organ/system	Waste products excreted
1	Contractile vacuole	Carbon(IV)oxide (CO2), ammonia and water
2	Flame cells	Carbon(IV)oxide (CO2), ammonia and water
3	Nephridia	Water, urea, carbon(IV)oxide (CO2) and uric acid
4	Malpighian tubules	Water, carbon(IV)oxide (CO2) and uric acid
5	Green glands	Water, urea, carbon(IV)oxide (CO2) and ammonia salts
6	Gills	Carbon(IV)oxide (CO2), water and urea
7	Skin	Sweat (containing urea, salts and water)
8	Kidney	Urine containing urea, salts, water and hormone
9	Lungs	Carbon(IV)oxide (CO2) and water vapour
10	Liver	Bile salts, water and urea
11	Stomata	Carbon(IV)oxide (CO2), oxygen and water
12	Bark of trees	Tannins, mucilage, gum, crystals, resin, oil, latex, anthocyanin and alkaloids

Differences between Excretion in Flowering Plants and Humans

	Flowering plans	Humans
1	No definite excretory organs	There is definite excretory organs- kidney, liver, skin
2	Waste products include resins, alkaloids, gum e.t.c.	Waste products include urine, urea, sweat, salts
3	Excretory products are produced in small quantities	Excretory products are produced in large quantities
4	Some excretory products can be reused e.g. oxygen, carbon dioxide, water vapour	Excretory products cannot be reused e.g. urea, uric acid
5	Most excretory products are stored e.g. tannins, resins, alkaloid	Most excretory products are completely removed
6	Excretory products released at a slower rate	Excretory products released at a faster rate