Excretion

Life Process of Class 10

The process of removal of toxic wastes from the body of an organism is called excretion. The process of maintaining the right amount of water and proper ionic balance in the body is called osmoregulation.

The mode of excretion is different in different organisms. Many unicellular organisms remove these wastes by simple diffusion from the body surface into the surrounding water, while complex multicellular organisms use specialized organs for excretion. The organs that are involved in this process constitute the excretory system

EXCRETION IN ANIMALS

In different animals, different organs of excretion are present. These are as follows:

  • In Amoeba and other single celled animals, contractile vacuole serves the function of excretion and osmoregulation.
  • In sponges, the waste materials diffuse out of the body through the osculum.
  • In animals like Hydra (belonging to Cnidaria), the oral opening serves as the excretory organ.
  • In flatworms, excretory structures are single cells called flame cells.
  • In higher animals, excretory organs consist of tubular structures called nephridiae.g., earthworm.
  • In human beings, kidneys are the excretory organs. Each kidney consists of microscopic thin tubules which form nephrons. Each kidney contains about 1 million nephrons.

EXCRETION IN AMOEBA

”In organisms like Amoeba and Paramecium, most of the metabolic wastes are removed through the general surface of the body by the simple process of diffusion. Specialized organs of excretion are lacking.

”In Amoeba, waste materials and excess water are removed by the process of diffusion with the help of a contractile vacuole. The contractile vacuole thus increases in size. When full, it moves closer to the plasma membrane and finally releases the contents in water after coming in contact with the plasma membrane. Later, a new contractile vacuole appears.

EXCRETION IN HUMAN BEINGS

Various organs are present in human to get rid of wastes.

Lungs - For excretion of carbon dioxide and water vapor produced during respiration; carbon dioxide produced during respiration in cells is carried by

  • Haemoglobin in the blood.
  • Water in which it gets dissolved.

CO2 from the surface of the lungs is removed by exhalation during breathing.

  • ” Skin -For excretion of urea, salts and excess water by means of sweat glands present in the skin.
  • ” Kidneys - For excretion of liquid wastes, such as urine; the kidneys are part of a set of organs known as the urinary system.
  • ” Liver - For removing urea, bile pigments and toxins, liver is the largest gland in the body and is a detoxifying gland.

EXCRETORY SYSTEM IN HUMANS

”Excretory system consists of the following organs

  • A pair of kidneys
  • Ureters

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· Urinary bladder

Urethra

Kidneys are bean- shaped structures, about 10 centimetres long. They are located just above the waist on either side of the backbone. The right kidney is placed slightly above the left.It measures about 10 cm long, 5 cm wide and 4cm thick, weighing about 135 to 150 gms., constituting 1% of body weight.Its inner concavity called hilum.Ureter, Blood vessels, nerve etc. enter or leave the kidney through hilus (hilum).Remains covered by peritoneium only on the ventral or front side hence termed as retroperitoneal. A narrow tube called the ureter runs from the inner side of each kidney.

The ureters, in turn, are connected to a large sac called the urinary bladder. Urine is collected and stored in the urinary bladder.

Leading from the bladder is another muscular tube called the urethra, which works as the outlet passage.

The end of the urethra is normally held closed by means of a ring of muscle (a sphincter), which controls the release of urine from the bladder.

Urine drains continuously out of the kidney into the ureters where it is forced downwards into the bladder by wave-like contractions of the ureter walls. The bladder stretches and expands in volume as it fills with urine, and when it is nearly full the stretching stimulates sensory nerve endings in its walls so that nerve impulses are sent to the brain. This is how a person knows when his bladder must be emptied. The sphincter muscle around the urethra is then voluntarily (i.e., consciously) relaxed to let urine drain from the bladder, through the urethra, and out of the body. This is called urination.

The waste material is separated by the kidneys from the blood which is fed into it through the renal artery. The cleaner blood after the removal of wastes is sent back through the renal vein.

Structure of Kidneys

A kidney in section shows a dark-coloured outer zone called the cortex, and a pale-coloured inner zone called the medulla. Each kidney is made of a large number of excretory units called nephrons.Each nephron consists of two parts - a rounded, cup-shaped body called the Bowman’s capsule, and the tubular part or the collecting tubule. Bowman’s capsule contains a ball of finely-divided and intertwined blood capillaries which are formed by the repeated branching of the renal artery. These structures are called glomeruli (singular glomerulus). The blood capillaries bring waste and excess water from body to the kidneys.The medulla is distinct by the presence of 15-16 conical pyramids with broader end towards cortex. Each pyramid ends as renal papilla where many ducts of Bellini converge to open. The renal papilla fits over small funnel-like minor calyx, 2 to 3 minor calyces together open in still wider funnel called major calyx. Bowman’s capsule is made of two layered squamous epithelium, glomerulus and together called as Malpighian body or renal corpuscle. Podocytes are the cells of inner membrane with long process closely applied to the walls of capillaries (squamous cell layer with basement membrane). These 3 layers make the sieve like separation wall. Filtration slits are formed by the association of podocytes and capillaries wall. Blood enters glomerulus through afferent arteriole (of larger diameter) and leaves through via efferent arteriole (smaller diameter). Difference of diameter of both the arteriole generates the glomerular pressure. The tubular part is highly coiled. The collecting tubules of many nephrons join to form the collecting duct. Collecting ducts collect urine from the nephrons and transport in the medulla of the kidney.

Structure of Nephron

  • Each kidney is made up of a large number of excretory filtration units called nephrons or uriniferous tubules.
  • These are considered as functional unit of kidney.
  •  Each nephron has a cup shaped upper end, called Bowman’scapsule containing a bundle of capillaries called glomerulus. These blood capillaries arise from the artery, which brings waste and excess water from body to kidney. Bowman’s capsule leads into tubular structures. The tubular parts of many nephrons drain into a common collecting duct.
  • The function of glomerulus is to filter the blood passing through it.

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Structure of a nephron

· The function of tubular part of nephron is to allow the selective reabsorption of useful substances into the blood capillaries.

Blood having Metabolic Waste → Afferent Arteriole → Glomerulus → Bowman’s Capsule → PCT (Proximal Convoluted Tubule) → Loop of Henle → DCT (Distal Convoluted Tubule) → Collecting Duct → Ureter → Urinary bladder → Urethra → Urine excreted out

Formation of urine: The purpose of excretion is to filter out waste products from the blood.

1. The waste material is brought to kidneys by the renal arteries.

2. Blood is filtered, from the blood capillaries into Bowman’s capsule.

3. As this filtrate passes through the tubular parts of nephron, some useful products, such as glucose, amino acids, salts and major amount of water are selectively reabsorbed by blood capillaries surrounding the nephron.

4. The nephrons drain the remaining liquid waste (urine) into the collecting duct which eventually enters a long tube, the ureter. Human urine contains water and nitrogenous substances, most of which is urea.

From the ureter, urine passes into the urinary bladder. Urine is stored in the urinary bladder until the pressure of the expanded bladder leads to the urge to pass it out through the urethra. The bladder is muscular, so it is under nervous control. As a result, we can usually control the urge to urinate.

 

Related Topics

EXCRETION IN PLANTS

Plants use completely different strategies for excretion than those of animals. Some of the important plant wastes and the methods by which they are removed are as follows :

1. The gaseous waste products are CO2 and O2 produced as a result of respiration and photosynthesis respectively. These gaseous wastes are removed through stomata in leaves and lenticels in stem and released in air.

2. Plants get rid of excess water by the process of transpiration.

For other solid and liquid waste, plants use the fact that many of their tissues consist of dead cells and that they can even lose some parts such as leaves.

3. Many plants waste products are stored in cellular vacuoles.

4. Some waste products are removed along with falling leaves and other plant parts.

5. Other waste products are stored as resins and gums in old Xylem tissue.

6. Plants also excrete some waste substances into the soil around them.

USEFUL WASTE PRODUCTS

Important waste products of plants which are useful to humans are

Natural rubber

Rubber is a significant article of plant origin which has unusual properties of elasticity, plasticity, resistance to electric currents, impermeability to water and adhesiveness. Most of the natural rubber is obtained from the latex of Heveabrasiliensis(Hevea or Para Rubber, Fam, Euphorbiaceae), Partheniumargeritatum(Guayule Fam. compositae), Manihotglaziorii(Leara Rubber) and the inferior quality rubber is produced by Ficuselastica(Assam or India Rubber, Fam. Moraceae).

Gum

Acacia nilotica(vern. Babool), Boswelliaserrata(vern. Salai), Anogeissuslatifolia(vern. Dhaora) Pterocarpus marsupium (Kino Gum), Buteamonosperma(Bengal Kino Gum), Stercuriaurens (vernGulu, Karrya or Kuteera Gum), milky latex of Achrassapota forms chickle gum from which chewing gum is produced.

RESIN

Two common plant resins are pine resin and damar.

Pine resin is obtained from species of Pinus. It is distilled to yield turpentine (25%) and resin (75%). Turpentine is an antiseptic solvent from paints and polishes. It is also used medicinally as anthelmintic. Resin is used in preparing writing paper, water proofing, sealing joints of pipes, soap, wheel greese, etc.

Damar (from Shorearobustaor Sal and related trees) is employed in the manufacture of varnishes and spirit polishes.

Essential oils (Sandalwood oil, eucalyptus oil)

Excretion

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