Exchange

• Tissue fluid 


  Environment around the cells of multicellular organisms.
• Passively 


  No metabolic energy is required, by diffusion and osmosis
• Actively 


  Metabolic energy is required by active transport
• Surface area to volume ratio

  As the object becomes larger the SA:V gets smaller
• Exchange surfaces show these characteristics:

  -Large SA relative to the volume of the organism which increases the rate of exchange

  -Very thin so diffusion distance is short

  -Selectively permeable to allow selected materials to cross

  -Movement of the environment medium

  -Transport system
• Gas exchange in single-celled organisms

  Large SA:V. Oxygen is absorbed by diffusion across their body surface. Carbon dioxide from respiration diffuses out across their body surface.
• Gas exchange in insects

  Internal network of tubes called tracheae. These are supported by strengthened rings to prevent collapsing. Tracheae divide into smaller dead end tubes called tracheoles. These extend throughout all the body tissue.
• Gas exchange in insects

  Respiratory gases move in and out of the tracheal system in three ways:

  -Along a diffusion gradient

  -Mass transport

  -Ends of the tracheoles are filled with water
• Gas exchange in fish

  Waterproof, and therefore a gas-tight, outer covering. Small SA:V. They have specialised internal gas exchange surface the gills.
• Structure of the gills 


  Made up of gill filaments. They are stacked up in a pile rather like the pages in a book. At right angles to the filaments are gill lamellae (increases SA). Water is taken in through the mouth and forced over the gills and out through an opening on each side of the body.
• Countercurrent flow 


  The flow of water over the gills lamellae and the flow of blood within them are in opposite directions.
• Countercurrent flow


  The blood and water flow in opposite directions
• Gas exchange in plant leaf

  No living cell is far from the external air, and therefore a source of oxygen and carbon dioxide.

  Diffusion takes place in the gas phase (air), which makes it more rapid than if it were in water
• Gas exchange in plant leaf adaptations

  -Many small pores, called stomata, and so no cell is far from a stoma and short diffusion pathway

  -Numerous interconnecting air-spaces that occur throughout the mesophyll so that gases can readily come in contact with mesophyll cells

  -Large surface area if mesophyll cells for rapid diffusion
• Stomata 


  Minute pores that occur mainly but not exclusively on the leaves especially the underside. Each stoma is surrounded by a pair of guard cells. These cells can open and close the stomatal pore. They control the rate of gaseous exchange.
• Insects have evolved the following adaptations that reduce water loss:

  • Small SA:V
  • Waterproof coverings - rigid outer skeleton of chitin that is covered with a waterproof cuticle
  • Spiracles - openings of the tracheae at the body surface and close to reduce water loss
• Xerophytes

  Plants that are adapted to living in areas where water is in supply.
• Modifications to reduce water loss:

  • Thick cuticle
  • Rolling up of leaves
  • Hairy leaves
  • Stomata in pits or grooves
  • Reduced SA:V of the leaves
• Mammalian lungs 


  Located inside the body because:

  • Air is not dense enough to support and protect these delicate structures.
  • The body as a whole would otherwise lose a great deal of water and dry out.
• Ribcage 


  Lungs are supported and protected by a bony box.
• Lungs 


  Pair of lobed structures made up of a series of highly branched tubules, called bronchioles, which end up in tiny air sacs called alveoli.
• Trachea 


  Flexible airway that is supported by rings of cartilage. The cartilage prevents it collapsing as the air pressure inside falls when breathing in. Tracheal walls are made up of muscle, lined with ciliated epithelium and goblet cells.
• Bronchi 


  Two divisions of the trachea, each leading to one lung. Similar structure to the trachea they also produce mucus to trap dirt particles and have cilia that move the dirt-landen mucus towards the throat. The larger bronchi are supported by cartilage, although the amount of cartilage is reduced as the bronchi gets smaller.
• Bronchioles 


  Series of branching subdivisions of the bronchi. Their walls are made of muscle lined with epithelial cells. This muscle allows them to constrict so that they can control the flow of air in and out of the alveoli.
• Alveoli 


  Minute air sacs with a diameter of 100um and 300um at the end of the bronchioles. Between the alveoli there are some collagen and elastic fibres. Lined with epithelium. Elastic fibres allow the alveoli to stretch as they fill with air when breathing in. They spring back during breathing out in order to expel the carbon dioxide-rich air. Alveolar membrane is the gas exchange surface.
• Ventilation (breathing)


  To maintain diffusion of gases across the alveolar epithelium, air is constantly moved in and out of the lungs.
• Inspiration (inhalation)


  When the air pressure of the atmosphere is greater than the air pressure inside the lungs, air is forced in the lungs.
• Expiration (exhalation) 


  When the air pressure in the lungs is greater than that of the atmosphere, air is forced out of the lungs.
• Pressure changes within the lungs are brought about by ten movement of three sets of muscles:

  • Diaphragm - sheet of muscle that separates the thorax from the abdomen
  • Intercostal muscles - lie between the ribs. Two set - internal intercostal muscles - contraction leads to expiration - external intercostal muscles - contraction leads to inspiration
• Site of gas exchange in mammals

  Epithelium of the alveoli
• Diffusion of gases between the alveoli and the blood will be very rapid because:

  • Red blood cells are slowed as the pass through the pulmonary capillaries, allowing more time for diffusion.
  • Distance between the alveolar air and red blood cells is reduced as the red blood cells are flattened against the capillary walls
  • Walls of both alveoli and capillaries are very thin
• What do glands produce?

  Glands produce enzymes that hydrolyse large molecules into small ones ready for absorption.
• Oesophagus 


  Carries food from the mouth to the stomach.
• Stomach 


  Muscular sac with an inner layer that produces enzymes. Role is to store and digest food, especially proteins. Has glands that produce enzymes which digest protein.
• Ileum 


  Long muscular tube. Enzymes are produced by its walls and by glands that pour their secretions into it. Inner walls of the ileum are fooled into villi. Surface area of the villi are increased by millions of tiny projections, microvilli, on the epithelial cells of each villus. This adapts it for it's purpose of absorbing the products of digestion into the bloodstream.
• Large intestine 


  Absorbs water. Most of the water that is absorbed is water from the secretions of the many digestive glands.
• Rectum 


  Final section of the intestines. The faeces are stored here before periodically being removed via the anus in a process called egestion.
• Salivary glands 


  Situated near the mouth. They pass their secretions via a duct into the mouth. These secretions contain the enzyme amylase which hydrolyses starch into maltose.
• Pancreas 


  Large gland situated below the stomach. It produces a secretion called pancreatic juice. This secretion contains proteases to hydrolyse proteins, lipase to hydrolyse lipids and amylase to hydrolyse starch.
• Human digestion has two stages:

  • Physical breakdown
  • Chemical breakdown