Biology Notes for IGCSE 2014
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    • 01 Classification
    • 02 Cells
    • 03 Enzymes
    • 04 Plant Nutrition
    • 05 Animal Nutrition
    • 06 Plant transport
    • 07 Human transport
    • 08 Respiration
    • 09 Coordination
    • 10 Excretion
    • 11 Homeostasis
    • 12 Reproduction
    • 13 Inheritance
    • 14 Ecosystem
  • Classification
    • Summary
    • Characteristics of Living organisms
    • Classification of Living organisms
    • Kingdoms of Living organisms
    • Animal Kingdom >
      • Classification
      • Phylum Vetebrates
      • Phylum Arthropods
      • Annelids, Nematodes, Molluscs
    • Plant Kingdom
    • Virus
    • Bacteria Kingdom
    • Fungi Kingdom
    • Dichotomous keys
  • Cells
    • Summary
    • Structure
    • Functions
    • Tissues, Organs, Systems
    • Diffusion
    • Osmosis
    • Active transport
  • Enzymes
    • Summary
    • Enzymes and reactions
    • 'Lock and Key' model
    • Role in germinating seeds
    • Use in biological washing powders
    • Use in the food industry
    • Use of microorganisms and fermenter to manufacture enzymes
    • Use of microorganisms to manufacture antibiotic penicillin
  • Nutrition
    • Animal >
      • Summary
      • 1 >
        • Main nutrients: carbohydrates, fats and proteins
        • Food tests >
          • Food test 1 - Starch test
          • Food test 2 - Benedict's test for Reducing Sugars
          • Food test 3 - Emulsion (ethanol) test for Fats
          • Food test 4 - Biuret test for Proteins
        • Other Nutrients: Vitamins, Minerals, Fiber and Water
        • Using yeast to make bread and beer
        • Using microorganism in making yoghurt and single cell protein
        • Food additives - uses, benefits and health hazards
        • Use of modern technology for increasing food production
      • 2 >
        • Problems of world food supplies and the causes of famine
        • Diet
        • Human alimentary canal
        • Human teeth and dental decay
        • Adding fluoride to public water supplies
        • Mechanical and chemical digestion
        • Absorption – function of the small intestine and significance of villi
        • Assimilation and role of the liver
    • Plant >
      • Summary
      • The equation for photosynthesis
      • Photosynthesis investigations - Principles and Starch test
      • Effect of Light intensity on the rate of Photosynthesis
      • Effect of Temperature on the Rate of Photosynthesis
      • Effect of Carbon Dioxide on the Rate of Photosynthesis
      • Limiting factors in photosynthesis
      • Optimum conditions for photosynthesis in Green house
      • Leaf structure
      • Plant's mineral requirements and fertilisers
  • Transport
    • Plant >
      • Summary
      • Functions of xylem and phloem
      • Distribution of Xylem and Phloem in roots, stems and leaves
      • Root hairs and water uptake by plants
      • Passage of water through root, stem and leaf
      • Transpiration in plants and factors affecting tranpiration rate
      • Adaptations of the leaf, stem and root to different environments
      • Translocation of organic foods in plants
      • Translocation of applied chemicals throughout the plant
      • Transport of materials from sources to sinks at different seasons
    • Human >
      • Summary
      • 1 >
        • The circulatory system
        • Structure and function of the heart
        • Effect of exercise on heartbeat and causes of a coronary heart disease
        • Arteries, veins and capillaries - structure and functions
        • Blood composition and Plasma
        • Blood cells - structure and functions
      • 2 >
        • Blood clotting
        • Immune system - antibody, tissue rejection
        • Functions of lymphatic system
        • Respiration
        • Aerobic and anaerobic respiration
        • Gaseous exchange
  • Respiration
    • Summary
    • Definition
    • Aerobic, anaerobic
    • Gaseous exchange
    • Inspired, expired air
    • Tobacco smoke
  • Reproduction
    • Summary
    • Types
    • Plant >
      • Pollination
      • Flower structure
      • Formation of seed
      • Seed dispersal
      • Self-pollination, cross-pollination
      • Growth and development
    • Human >
      • Reproductive system
      • Menstrual cycle
      • Fertilisation
      • Fetus development
      • Birth
      • Sex hormones
      • Birth control
      • Artificial insemination
      • HIV/AIDS
      • Gonorrhoea
      • Breast feeding
  • Coordination
    • Coordination >
      • Summary
      • Hormones, adrenaline
      • Hormones in food production
      • Human nervous system
      • Simple reflex arc
      • Effectors, biceps and triceps
      • Voluntary, involuntary actions
      • The eye, rods and cones
      • Accommodation
      • Tropism in plants
      • Auxins
    • Homeostasis >
      • Summary
      • Homeostasis
      • Blood glucose
      • Negative feedback
      • Kidney structure
      • Kidney function
      • Formation of urea
      • Dialysis
      • Drugs
  • Inheritance
    • Summary
    • Key definitions
    • Chromosomes, DNA
    • Mitosis and Meiosis
    • Monohybrid cross
    • Codominance
    • Continuous, discontinous
    • Mutation, radiation
    • Sickle cell anaemia
    • Selection
    • Antibiotic-resistance bacteria
    • Genetic engineering
  • Ecosystem
    • Summary
    • Human & ecosystem >
      • Human influences
      • Deforestation
      • Water pollution
      • Air pollution
      • Greenhouse gases
      • Pesticides, nuclear fallout
      • Conservation, recycling sewage
    • Energy flow
    • Food chain
    • Food web
    • Food pyramids
    • Energy efficiency
    • Carbon & water cycles
    • Nitrogen cycle
    • Combustion of fossil fuels
    • Population size

# 91 Function of the kidney - filtration and reabsorption

The function of the kidney is to filter blood, removing urea and excess H2O, reabsorbing glucose, some H2O and some mineral salts. 
Picture
Urine is made by filtration and selective reabsorption 
  • As blood passes through the kidneys, it is filtered. This removes most of the urea from it, and also excess H2O and salts. 
  • As this liquid moves through the kidneys, any glucose in it is reabsorbed back into the blood. Most of the H2O is also reabsorbed along with some of the salts. 
Picture
The final liquid produced by the kidneys is a solution of urea  and salts in water. It is called urine, and it flows out of the kidneys, along the ureters and into the bladder. It is stored in the bladder for a while, before being released from the body through the urethra. 

Filtration happens in renal capsules 

Blood is brought to the renal capsule in a branch of the renal artery. Small molecules, including water and most of the things dissolved in its, are squeezed out of the blood into the renal capsule. 

There are thousands of renal capsules in the cortex of each kidney. Each one is shaped like a cup. It has a tangle of blood capillaries, called a glomerulus, in the middle. The blood vessels bringing blood to each glomerulus is quite wide, but the one taking blood away is narrow. This means that the blood in the glomerulus cannot get away easily. Quite a high pressure builds up, squeezing the blood in the glomerulus against the capillary walls.

These walls have small holes in them. So do the walls of the renal capsules. Any molecules small enough to go through these holes will be squeezed through, into the space in the renal capsule.

Only small molecules can go through. These include water, salt, glucose and urea. Most protein molecules are too big, so they stay in the blood, along with the blood cells.
Picture
How urine is made.
Useful substances are reabsorbed 

The fluid in the renal capsule is a solution of glucose, salts and urea dissolved in water. Some of the substances in this fluid are needed by the body. All of the glucose, some of the water ans some of the salts need to be kept in the blood. 

Wrapped around each kidney tubule are blood capillaries. Useful substances from the fluid in the kidney tubule  are reabsorbed, and pass back into the blood in these capillaries.

The remaining fluid continues on its way along the tubule. By the time it gets to the collecting duct, it is mostly water, with urea and salts dissolved in its. It is called urine. The kidneys are extremely efficient at reabsorbing water. Over 99% of the water entering the tubules is reabsorbed.

The relative amount of water reabsorbed depends on the state of hydration of the body (how much water is in the blood), and is controlled by secretion of the hormone ADH.
  • On a hot day: we sweat more to cool down --> the body needs to conserve water --> produce a small amount of concentrated urine.
  • On a cold day: little sweat is being produced --> we tend to produce a larger volume of dilute urine.
Filtered blood returns to the vena cava (main vein) via a renal vein. The urine formed in the kidney passes down a ureter into the bladder, where it is stored. A sphincter muscle controls the release of urine through urethra.

Video: Nephron function

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