Excretory System SS2 Biology Lesson Note

EXCRETION

Excretion is the process by which metabolic waste products are removed from the body of all living things. Excretion is different from egestion which is the removal of solid waste (undigested food substances i.e.faeces) through the anus. 

Excretion is necessary for the following reasons:

1. To avoid or prevent any harm that would be caused by any excretory product.
2. Some excretory products are poisonous to the body and should be removed.
3. To maintain water balance in the body (homeostasis).
4. To avoid interference of waste products with normal metabolic activities in the body.

EXCRETORY STRUCTURES AND WASTE IN ORGANISMS

TYPES OF EXCRETORY SYSTEMS

1. Contractile Vacuole In Protozoa

The contractile vacuole is a simple structure found in the cell of freshwater protozoa. Water constantly enters the cell of a protozoan through the selectively permeable membrane because the cell is hypertonic to its environment. As water enters the cell, a contractile vacuole is formed which collects the water and expands, when it reaches the maximum size, it contracts and discharges the water through a temporary break in the cell membrane at intervals. Excretion of carbon dioxide and ammonia is by diffusion through the cell membrane

2. Flame Cell In Flatworms

The excretory system consists of two longitudinal canals with branched tubules which end in flame cells. The flame cell has a large hollow called the cell lumen with a bunch of flagella hung on it. The waste product from the surrounding cells enters the flame cells. The flagella helps to propel the fluid into the tubules. The fluid passes into the exterior through a narrow tube called a duct.

3. Nephridium In Earthworm

A pair of nephridia is found on each segment of the earthworm except the three and the last. Each nephridium consists of a ciliated funnel, nephrostome which leads into a long coiled tube (narrow and middle ciliated tubes, wide non-ciliated tubes and muscular tube). The tube opens to the exterior as a nephridiopore (excretory pore). Waste product mainly urea is absorbed from blood capillaries surrounding the nephridia. The fluid containing the waste through the long tube of the nephridia, salt and other useful substances are reabsorbed through the wall of the tube. The unabsorbed substances and water gather in the muscular tube and discharge to the exterior through the excretory pore. 

4. Malpighian Tubule In Insect

Malpighian tubules are found between the midgut (small intestine) and the hindgut (large intestine). One end opens into the gut while the other end freely floats in the haemocoel. Nitrogenous waste and water in the haemocoel are absorbed at the distal close end into the tubule. The waste is converted into uric acid as it passes along the malpighian tubule towards the gut. A lot of water is also reabsorbed so that by the time the uric acid reaches the proximal end, it is changed to solid crystals. More water is reabsorbed in the rectum therefore concentrated urine leaves the body as almost dried solid.

5. Excretory System In Mammals

Mammalian lungs excrete water vapour and C02, the liver excretes bile pigment called bilirubin, the skin excretes water, salt and urea through the sweat, and the kidney excretes water, mineral salt and urea. The excretory system of mammals consists of a pair of kidneys, a ureter, a bladder, a renal artery and a renal vein.

STRUCTURE OF THE KIDNEY

The mammalian kidney is a bean-shaped, reddish-brown organ located in the posterior end of the abdomen. The right kidney is slightly lower in the body than the left. Cutting a kidney longitudinally, two distinct regions are observed; an outer cortex and an inner medulla. Several narrow tubules called urinary tubules (nephrons) pass through the two regions stated above. The tubules open at the tips of triangular-shaped masses of tissues called pyramids. The pyramids open into a funnel-shaped cavity called the pelvis. The kidney has many tiny capillaries which are branches of the renal artery and the renal vein. The pelvis continues as a ureter, a long narrow tube connecting the kidney to the urinary bladder.

diagram of the kidney

Functions Of The Kidney

The three major functions carried out by the kidney are as follows:

1. Osmoregulation: This is the process by which an animal regulates the balance between water and salt in their body fluid. When the concentration of the blood is higher than that of the cell content through the kidney, extract or remove these substances from the blood to maintain normal osmotic balance in the body and vice-versa. 
2. Maintenance of Acid-Base Balance: The kidney excretes excess acids or bases when either of them is more concentrated in the body. If the body’s concentration of base is higher than that of acid, more salt will be excreted with the urine as a result, acid-base balance is maintained in the body. Excretion: 

The kidney removes metabolic waste products from the body in the form of urine. Urine formation is made possible by the numerous urinary tubules which are the functional units of the kidney.

3. Structure Of The Urinary Tubule (Nephron)

The nephron consists of a cup-shaped Bowman’s capsule which opens; into short coiled proximal convoluted tubules. The tubule continues as a U-shaped loop, the Henle’s loop in the medulla, the loop re-enters the cortex as the distal convoluted tubule and widens as it enters the pelvis. The nephron is associated with several networks of capillaries. The renal artery branched in the Bowman’s capsule formed a knot of capillaries called the glomerulus which re-joins to form a blood vessel leading out of the capsule and branches into a capillary network around the urinary tubule before joining the renal vein.

Formation Of Urine

ultra-filtration: Blood is brought to the kidney by the renal artery, which enters the glomeruli (capillaries) in the Bowman’s capsule. Water, mineral salt, sugar and other solutes are filtered from blood into the capsule.

Selective Reabsorption: The glomerular filtrate flows down the tubules at the proximal convoluted tubular loop and the loop of Henle’s where some watery sugar, amino acid and salt useful to the body are reabsorbed into the blood capillary by active transport. This process of reabsorption of useful materials back into the blood is called selective reabsorption.

Hormonal Secretion: As the fluid flows through distal convoluted tubules, more water is reabsorbed by the action of antidiuretic hormones (ADH) and urine is finally formed. The urinary tubules empty their content into the pelvis and from the pelvis urine trickles through the ureter into the urinary bladder, which when full contracts and discharges urine out of the body through the urethra.

EXCRETION IN PLANTS

Plants have no special excretory organs and excretory wastes are minimal. Elimination of waste takes place through the stomata and lenticels. The main waste products of plants are water which is eliminated through transpiration and guttation and carbon dioxide from respiration at night when photosynthesis is not taking place. Other waste products in plants are alkaloids (quinine, nicotine, cocaine, morphine) products are converted into harmless substances, and stored in some parts of the plants as useful commercial products.

ASSIGNMENT

1. Excess water in plants is excreted as water vapour and droplets respectively through A. transpiration and guttation B. respiration and guttation C. photosynthesis and guttation D. guttation and condensation

2. Which of the following is not an excretory organ?  A. kidney B. skin C. leaf D. large intestine

3. Which of the following waste products in plants is excreted through the stomata and lenticels? A. carbon dioxide B. alkaloids C. tannins D. anthocyanins

4. Which of the following is responsible for the removal of excess water in humans? A. small intestine B. kidney C. spleen D. bladder

5. The urinary tubule of the kidney function through A. ultrafiltration and selective reabsorption B. osmosis and diffusion C. active transport and osmosis D. active transport and cytoplasmic strand

6. Differentiate between excretion in plants and animals.

7. Mention five structural parts of the excretory system in mammals and their functions.