Gas Exchange

• Single celled organisms

  • Need substances from the environment for processes in the cell (e.g. oxygen for respiration)
  • Need to remove waste substances (e.g. carbon dioxide) to avoid harming the cell
• Single celled organisms

  • Can exchange gases & other substances using the cell membrane
  • Have a larger surface area to volume ratio (adaptations include being wide, flat or with folds)
• Single celled organisms: Diffusion rate

  Rapid for single celled organisms as they only have to move across 1 cell membrane
• Single celled organisms: Cell wall puts no additional barrier to diffusion
• INSECTS:

  • TRACHEAL SYSTEM: A gas exchange surface that insects use. 
• INSECTS:

  • TRACHEAE: It consists of a network of trachea supported by strengthened rings (to prevent them from collapsing) that divide into tracheoles (smaller tubes) that carry oxygen around the body
• INSECTS:

  • SPIRACLES: tiny pores/ openings along the thorax & abdomen. They can be opened & closed by valves, but are mainly kept closed to prevent water loss. Gases diffuse through the spiracles & into the tracheoles (eg oxygen) & diffuse out of the spiracles (eg carbon dioxide)
• INSECTS:

  • EFFICIENCY: Gas exchange systems have evolved to maximise the rate of gas exchange & to prevent evaporation
• INSECTS:

  LIMITS: Relies on diffusion for exchanging substances-> diffusion pathway=short -> limits the size of insects
• XEROPHYTIC PLANTS (specialised to dry habitats)

  • Fewer stomata to avoid dehydration (stomata usually open to allow gas exchange)
  • Sunken stomata to trap moist air and decrease water potential gradient (lower w pt -> slower rate of diffusion)
  • Fine hairs covering epidermis to trap moist air and decrease rate of water loss
  • Curled leaves to shelter stomata from wind and trap air with high water potential (trapped air becomes saturated with water vapour -> high w pt)
  • Thick waxy cuticle to reduce rate of evaporation
  • Needle leaves with reduced surface area to volume ratio to slow rate of diffusion
• Lungs


  • Specialised organ for human gas exchange
  • Inside the body as they are delicate and would dry out if outside
• Trachea

  • Flexible airway/entrance to the human gas exchange system
  • Made of muscle lined with ciliated epithelium + goblet cells
  • Ridges of cartilage surround the front for protection & structure (no cartilage at the back so the oesophagus is not constricted)
• Bronchi

  • Trachea divide into 2 bronchi
  • Supported by cartilage and smooth muscle
  • Air flows along each bronchus to a lung
  • Produce mucus to trap dirt particles & have cilia that move them towards the throat
• Bronchioles

  • Bronchi divide into bronchioles
  • Made of muscle lined with epithelial cells
  • Further branch into alveoli (small air sacs)
• Alveoli

  • Sacs that fill with air when you inhale
  • Oxygen in the alveoli diffuses into the bloodstream & carbon dioxide in the bloodstream diffuses into the alveoli
  • Provide a large SA area for gas exchange
  • Lined with epithelium & have collagen + elastic fibres between them
• Control of ventilation

  Inhale: the ribcage, intercostal muscles, & diaphragm move to allow air to enter the lungs

  Exhale: the ribcage, intercostal muscles, & diaphragm move to allow air to leave out of the lungs
• Alveoli

  • Each alveoli is surrounded by a network of capillaries that provide a large SA for gas exchange between the alveoli & the bloodstream
  • The epithelium is made of a single layer of epithelial cells that line the walls of the alveoli to provide a short diffusion distance from the alveoli to the capillaries (-> maximises rate of gas exchange)
• ALVEOLI: Constant & concentration gradient

  • Constant breathing movement + heart constantly circulating blood around the alveoli -> ensure a steep conc gradient is maintained
  • Concentration gradient: The capillaries supply carbon dioxide to the alveoli + oxygen is rapidly carried away from the alveoli. The quick transport of gases in the bloodstream maintains a steep conc gradient of oxygen & carbon dioxide. The steep conc gradient -> quick diffusion of gases into/ out the bloodstream
• Alveoli: Red blood cells

  • Are slowed as they pass through the pulmonary capillaries -> more time for diffusion. There is also a shorter diffusion distance as the red blood cells are flattened against the capillary walls
• LUNGS & MUSCLES

  • MUSCLES AROUND LUNGS: cause breathing by changing pressure in the lungs by moving
  • DIAPHRAGM: sheet of muscle that separates the thorax from the abdomen
  • INTERCOSTAL MUSCLES: between the ribs. There are 2 sets . Internal intercostal muscles (contract for expiration) & external intercostal muscles (contract for inspiration)
• BREATHING IN (inspiration):

  1)MUSCLE CONTRACTION: external im contract. Internal im relax. Energy is needed to power the muscle contraction.

  2)THORACIC CAVITY: The ribs are pulled up & out. The diaphragm contracts, moves down & flattens. The vol of the thoracic cavity increases.

  3)LUNG PRESSURE DECREASES: Increasing pressure in the thorax -> pressure in lungs decreases -> pressure gradient between outside & inside the lungs .

  4)AIR FLOW: Air flows in the lungs down the pressure gradient & into the alveoli
• BREATHING OUT /expiration): 

  1)MUSCLE RELAXATION: The internal im contract, external im relax.

  2)THORACIC CAVITY: the ribcage moves down & in. The diaphragm relaxes & moves up. The vol of the thoracic cavity decreases

  3)LUNG PRESSURE INCREASES: The decreasing vol in the thoracic cavity causes the pressure in the lungs to increase. A pressure gradient between outside the lungs & inside the lungs is created

  4)AIR FLOW: Air flows out from the lungs down the pressure gradient. Air flows out of the alveoli & up the trachea
• LUNG DISSECTION:

  1. Dissection tools are sterilised with ethanol or heating, blades are sharp
  2. IDENTIFY THE TRACHEA: The trachea should be visible without cuts. The trachea= tube that runs into the lungs, surrounded by C shaped cartilage rings
  3. OPEN THE TRACHEA: by cutting down the gap in the C shaped cartilage. Cartilage= difficult to cut so cut through the gap (smooth muscle)
  4. CUT OPEN THE BRONCHI;Cut through the trachea & into the bronchi.
  5. FEEL THE TISSUE: The lung tissue should feel spongy. It contains mills of alveoli filled with air
  6. DISINFECT: Disinfect your hands & the surrounding area
• FISH DISSECTION:

  1. USING THE CORRECT EQUIPMENT: Dissection tools are sterilised with ethanol or heating. Blades are sharp
  2. IDENTIFY THE GILLS: under the operculum (a bony flap) either side of the head
  3. REMOVE THE GILLS: by pushing back the operculum & carefully cutting out the gills which are supported by gill arches that can be cut out
  4. LOCATE THE GILL FILAMENTS; the gills are made of gill filaments (many folds) visible along the gills
  5. DISINFECT: Disinfect your hands & the surrounding area
• INSECT DISSECTION:

  1. USING THE CORRECT EQUIPMENT: Dissection tools are sterilised with ethanol or heating. Blades are sharp for clean cuts
  2. FIX THE INSECT: Insects= very small so difficult to identify the exchange system. Fix insect to the dissection board
  3. REMOVE THE EXOSKELETON; by cutting along the abdomen
  4. LOCATE THE TRACHEAE: by filling the insect with a saline solution -> the trachea to appear as small grey tubes
  5. EXAMINE THE TRACHEAE; Using an optical microscope -> allows you to see rings of chitin in the trachea 
  6. DISINFECT: Disinfect your hands & the surrounding area
• FISH:

  • GILLS: Fish use an exchange surface that's specialised for obtaining oxygen from the water. Gills are filaments of thin tissue that are high branched & folded. Gills creates a large SA for gas exchange,
  • GILL LAMELLAE: the gills at right angles are covered in lamellae that increase the SA further
• FISH:

  • DIFFUSION OF OXYGEN: When water flows through the gills. Oxygen in the water diffuses quickly into the bloodstream. Oxygen can diffuse from from the water into the blood because of the counter current system
• FISH:

  COUNTER CURRENT FLOW: = the circulatory system of fish. Blood flows through the lamellae in the opposite direction of the flow of water through the gills. It ensures there's always a steep conc gradient between the water and the blood. Allows oxygen diffusion between oxygen rich blood (high conc) and water (low conc) then the water (high conc) meets blood with little oxygen (low conc) for diffusion again. Allows 80% oxygen to be absorbed rather than 50% (without the countercurrent flow)
• MEASURING LUNG FUNCTION:

  • SPIROMETER: A spirometer measures gas exchange in the lung by measuring the vol of air that's inspired & expired by an individual (by breathing in & out of the spirometer). A spirometer can measure tidal volume, ventilation rate, forced expiratory volume, & vital capacity
• MEASURING LUNG FUNCTION:

  • TIDAL VOLUME:= vol of air in a normal breath (at rest). Average tidal vol= 0.4-0.5 dm^3. 
  • VENTILATION RATE: = number of breaths in a min (at rest). Average ventilation rate= 15 breaths per minute. 
  • FORCED EXPIRATORY VOLUME: = max vol an individual can expire in 1 sec. The forced expiratory vol cannot be more than the total vol of gas in the lungs, as there is always a small amount of air that cannot be expired (the residual air that ensures the alveoli dont close). 
• MEASURING LUNG FUNCTION:

  • VITAL CAPACITY:= the max vol of air that can be breathed in & out the lungs
  • PULMONARY VENTILATION RATE (PVR): Can be calculated using: tidal volume x breathing rate= PVR
• OVERVIEW OF LUNG DISEASE:

  • LUNG DISEASE: Affects the ability of the lungs to exchange gases over their surface
  • DISEASES: Lung disease can cause issues with gas exchange & ventilation eg asthma, pulmonary fibrosis, tuberculosis, emphysema
  • CAUSES: Gas exchange problems are caused by damage in the exchange system. Causes of these problems: decreased SA, increased diffusion distance, decreased conc gradient
  • SYMPTOMS: Of gas exchange/ ventilation issues: coughing mucus/ blood, faster/ shallow breathing, blood oxygen saturation readings below 90%, fatigue
• EFFECTS OF LUNG DISEASE:

  • EMPHYSEMA: Caused by smoking/ air pollution. Dirt & bacteria become trapped in the alveoli which damages the alveoli walls. The alveoli walls provide a large SA for gas exchange. Damage to the walls -> decreases the SA -> rate of gas exchange declines
• EFFECTS OF LUNG DISEASE:

  • ASTHMA: Increases production of mucus in the epithelial cells -> increases length of diffusion distance -> decreases rate of gas exchange. An asthma attack -> causes the smooth intercostal muscle in the bronchioles to contract -> makes it difficult to breathe
• EFFECTS OF LUNG DISEASE:

  • TUBERCULOSIS: Tuberculosis bacteria can infect the lungs, damaging the lung tissue. The cells of the immune system can also damage the lungs. The damaged lungs -> decreased tidal vol
  • TRANSMISSION; Tuberculosis bacterias is transmitted by droplets, spread by coughs & sneezes
  • SYMPTOMS: Coughing up blood, chest pain, fatigue
  • THREAT: Is one of the top 10 causes of death worldwide, 1.7 mill died from TB in 2016. TB can be treated with antibiotics but antibiotic resistance is a major problem -> new drugs need to be constantly invented
• SMOKING & LUNG CANCER:

  • BRONCHITIS: When smoke is breathed in, it damages the cilia cells of the airways -> the cilia can't waft away the mucus produced by goblet cells -> the mucus builds up -> can lead to bronchitis (where the airways are partly blocked with mucus)
• SMOKING & LUNG CANCER:

  • CANCERS: The carcinogens can cause tumours to form in the airways. Smoking -> moth/ throat/ lung cancers
• SMOKING & LUNG CANCER:

  • EMPHYSEMA: Smoke damages the walls of the alveoli, reducing their surface area ->can cause emphysema (where gas exchange can't happen efficiently -> causing shortness of breath)
• SMOKING & LUNG CANCER:

  • HEART DISEASE & STROKES: Nicotine is very addictive & puts a strain on the heart. Carbon monoxide reduces the ability of red blood cells to carry oxygen -> can also put strain on the heart. Both can cause heart disease & strokes
• MESOPHYLL & STOMATA: