Excretion

• Excretion


  The process by which living organisms separate and remove/eliminate metabolic waste products from the body cells
• Metabolic waste products

  • Carbon (IV) oxide
  • Nitrogenous wastes
  • Excess water
  • Mineral salts
  • Tannins
  • Excess quinine
  • Resins
• Egestion/excreation


  The removal of indigestible and undigested food materials from the gut/ alimentary canal
• Secretion


  The production and release of useful substances produced by the cells e.g. hormones, enzymes, sebum, oxalates and mucus
• Homeostasis


  The maintenance of internal environment constant despite the changes in the external environment
• Osmoregulation


  The process by which the osmotic pressure of blood and tissue fluids is kept constant
• Thermoregulation


  Maintenance or keeping the body temperature constant
• Plants lack complex excretory organs because their metabolic activities are few and produce/release metabolic wastes slowly, they produce less toxic wastes, some excretory products are reutilized within the plant body, and most products are converted into non-toxic/harmless form/products
• Plants are able to accumulate their wastes for long because most of the products are harmless/less toxic and most products are converted into non-toxic/harmless form/products
• Methods of excretion in plants

  • Transpiration
  • Diffusion
  • Deposition/storage in non toxic form
  • Tissue/organ fall
  • Exudation
  • Guttation
• Unicellular organisms like amoeba and paramecium excrete carbon (IV) oxide, excess water and nitrogenous wastes like ammonia
• Methods of excretion in unicellular organisms

  • Diffusion across cell membrane
  • Use of contractile vacuole
• Animals need elaborate excretory systems because they are more active and produce more metabolic wastes, and they do not recycle their wastes like plants do
• Metabolic wastes in animals


  • Carbon (IV) oxide
  • Ammonia
  • Urea
  • Uric acid
  • Trimethylamine oxide

  
• Homeostasis


  The maintenance of internal conditions constant despite changes in the external environment
• Internal conditions kept constant in homeostasis

  • Osmotic pressure of blood and tissue fluid
  • Temperature
  • pH of tissue fluid and blood
  • Glucose (blood sugar) concentration
  • Ionic concentration
• Negative feedback


  Corrective measure/action taken to restore the condition back to normal
• Positive feedback


  A change below or above the normal is not corrected leading to further rise or drop in the condition from the normal
• Processes involved in homeostasis


  • Osmoregulation/salt and water balance
  • Thermoregulation
  • Regulation of ionic content
  • Blood sugar/glucose regulation
• If a marine amoeba is transferred to a freshwater environment, more water will enter the amoeba through osmosis and more contractile vacuoles are formed to excrete more/excess water

  
• Marine fish regulate their osmotic pressure by swallowing plenty of water, having chloride secretory cells in the gills to remove excess salts, eliminating nitrogenous wastes as Trimethylamine oxide, and having small and few glomeruli to slow down filtration rate in the kidneys
• Organs used for excretion and homeostasis in animals


  • Lungs
  • Skin
  • Kidney
  • Liver
• Lungs


  • They have many small air sacs called alveoli which increase the surface area for diffusion of carbon (IV) oxide, and alveoli have a thin epithelium for faster diffusion of carbon (IV) oxide
• Mechanism of excretion in the lungs


  Carbon (IV) oxide diffuses out of blood into the alveolar space/cavity which is then expelled from the lungs through exhalation, the exhaled air is moist leading to loss of water/moisture
• Functions of the skin

  • Protection of the underlying tissues
  • Regulation of body temperature/thermoregulation
  • Excretion
  • Reception of stimuli
  • Synthesis of vitamin D
  • Storage of fats
• Epidermis


  • The cornified layer provides protection against mechanical/physical damage, entry of pathogens, and reduces loss of water by evaporation, the cells of the cornified layer are continuously lost through friction and replaced by cells from the granular layer
• Adaptation to function of different parts of the skin

  1. Epidermis

  2. Dermis
• Epidermis


  Consists of 3 layers: cornified layer, granular layer and malpighian layer
• Cornified layer


  • Outermost layer of skin made up of dead cells covered with flexible substance called keratin
  • Provides protection against mechanical/physical damage
  • Provides protection against entry of pathogens
  • Reduces loss of water by evaporation
• Cells of cornified layer are continuously lost through friction and replaced by cells from granular layer
• Thickness of cornified layer varies in the body, thickest in areas of high friction like palms and soles, thinnest on lips and eyeballs
• Granular layer


  Middle layer consisting of living cells that give rise to the cornified layer
• Malpighian layer


  • Innermost layer consisting of actively dividing cells that give rise to the new epidermis
  • Contains melanin pigment that gives skin colour and protects against UV rays
• Dermis


  • Sweat/sudorific glands
  • Blood vessels/arterioles
  • Hairs and erector pili muscle
  • Sebaceous glands
  • Nerve endings
  • Subcutaneous layer/adipose tissue
• Sweat glands


  • Secrete sweat containing excretory wastes like excess water, traces of urea, lactic acid, ammonia and excess mineral salts
  • Sweat flows to skin surface and evaporates, cooling the body
  • Also helps excrete wastes and regulate salt and water balance
• When temperature is high


  Arterioles dilate, more blood flows near skin surface, more heat is lost
• When temperature is low


  Arterioles constrict, less blood flows near skin surface, less heat is lost
• Hairs and erector pili muscle

  • Hairs trap air which is a poor conductor of heat, preventing heat loss when cold
  • Hairs lie flat on skin surface when hot, allowing more heat loss
• Sebaceous glands


  • Secrete sebum which keeps hair and epidermis flexible/supple/soft and waterproof
  • Sebum also contains antiseptic substances for protection against bacteria
• Nerve endings


  Detect stimuli of pain, pressure and temperatures of the external environment