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Exchange surface: The Lungs
The lungs are organs located in the chest cavity. They are responsible for removing carbon dioxide and introducing oxygen to the bloodstream. The carbon dioxide is a respiratory product, and oxygen is a respiratory requirement (mitochondria). The lungs are surrounded by the ribcage, which protects the organs as well as assisting in ventilation with the diaphragm and intercostal muscles. The trachea, bronchi and bronchioles aid in getting the air into and out of the lungs.
Structures and functions:
-Cartilage offers structural support to the trachea and stops collapse of the airway due to the large pressure changes caused by ventilation.
-Ciliated epithelial cells are cells with small hairs that waft mucus, helping to prevent infections from occurring.
-Goblet cells are specialised, mucus secreting cells which make the mucus which the cilia move.
-Smooth muscle fibres allow constriction and relaxation of the airway, helping to control the air which reaches the alveoli.
-Elastic fibres help to deal by stretching and recoiling, offering structural support.
-Diaphragm and intercostal muscles are two muscles which contract and relax during breathing.
Exchange surfaces adaptations:
-Surface area e.g. RHC (root hair cells or forms of folded membranes)
-Thin (one cell thick) short diffusion pathway so distance travelled is short.
-Good blood supply in order to maintain a strong concentration gradient (e.g. alveoli in the lungs).
Spirometer practical (measuring volume):
-Vital capacity describes the maximum volume of air that can be inhaled/exhaled in one breath.
-Tidal volume describes the volume of air we breath in and out when we are resting.
-Breathing rate is how many times a breath is taken per unit time (breaths per minute)
-Residual volume is the volume of air always present in the lungs.
*Know how to answer questions which address how each of these change between resting/exercise etc.
Inspiration:
-External intercostal muscles contract
-Internal IC muscles relax.
-Ribs are raised upwards and outwards.
-Diaphragm contracts and flattens.
Expiration:
-Internal IC muscles contract
-External IC muscles
-Rib cage lowers
-Diaphragm relaxes and rises up
-Abdominal muscles contract
Inspiration:
Bony fish rely on a counter current principle in order to exchange gas. They have four pairs of gills and distinct apparatus set up for this gas exchange. The apparatus is held apart by the flow of water, explaining why fish cannot survive out of water for very long.
-Fish opens mouth and buccal cavity lowers, enabling the flow of water into it.
-Fish closes mouth and buccal cavity rises, increasing pressure and forcing water over the gills.
-The operculum acts as a valve and pump, letting water out as well as pumping it in.
-The blood meeting the water will always have a gradient for exchange.
Insects and breathing
-Insects have pores in their skin which open. They are called spiracles. The pores lead to a system of tracheoles which control the exchange of gases through diffusion (constant gradient maintained). The end of the tracheoles contain water.
Tracheoles and water:
-The ends of the tracheoles are filled with water. This means when cells are anaerobically respiring, they will produce lactic acid. The lactic acid draws in the water from the tracheole ends by osmosis. This creates a pressure difference which helps to bring in more oxygen through the spiracles.
OD curves and ppO2 further:
-The partial pressure at which oxygen binds to haemoglobin is also dependent on other factors, such as the concentration of carbon dioxide or whether the haemoglobin is fetal haemoglobin.
The Bohr effect: When cells respire they release carbon dioxide, which dissolves in solution to form carbonic acid. This causes a low pH to arise, changing the shape of the haemoglobin protein, decreasing the Hb affinity for CO2.
Fetal Hb: Has a higher affinity for oxygen as it must be able to cope with surviving at a low ppO2, so O2 moves across placenta.
Decreased affinity: OD curve shifts right.
Increased affinity: OD curve shifts left.
Practical element:
-You will be expected to outline a practical measuring the breathing rate of insects.
-This practical involves gently sealing a locust in a syringe then counting how many times its abdomen moves.
-You then calculate spiracle activity based on how active the abdomen is.
Haemoglobin structure/function:
-Quaternary structure of 4 polypeptide chains (2 x alpha chains and 2 x beta chains. Forms a globular tertiary + quaternary structure.
-Haem group contains a coordinated Fe2+ ion using in binding oxygen and carbon dioxide.
-Hb combines with oxygen to form oxyhaemoglobin.
-Carbon monoxide is poisonous as it binds irreversibly with the haemoglobin.
-The way in which oxygen molecules bind is known as positive cooperativity- the idea that by binding more oxygen molecules, Hb undergoes a conformational change to make the next binding easier.
-This type of behaviour is explained in an oxygen dissociation curve, which shows how the partial pressure of oxygen affects the binding affinity of oxygen to haemoglobin.
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