Module 3

• SA:V

  • The size, surface area to volume ratio and activity level affect the need for a specialised exchange system. 
  • A useful rule to remember is to always lay out ratios in the format x:1 by dividing the surface area and volume by the value of the volume.
• • Larger organisms have a smaller surface area to volume ratio and most cells are too far from the environment for diffusion alone. 
  • Specialised exchange systems evolved to let deeper tissues exchange with the environment.
• All general exchange surfaces are adapted for efficiency of exchange by having the following:

  • Large surface area - provides more space for molecules to pass, often achieved by folding cell membranes as in microvilli or root hair cells.
  • Thin permeable barrier - Reduces diffusion distance, such as in lung alveoli.
  • Good blood supply - Brings fresh molecules to the surface to maintain concentration gradients, as seen is lung alveoli or fish gills.
• Specific features of the alveoli:

  • Squamous epithelium and capillary endothelium - Both one cell thick (total 1 micrometre), so a short diffusion distance.
  • Numerous alveoli and capillaries - Large surface area to volume ratio and surfactant to prevent sticking.
  • Moving blood and ventilation - Maintains a steep concentration gradient.
  • Narrow capillaries - Pushes erythrocytes against the capillary walls.
• Maintaining the diffusion Gradient

  • Rich Blood Capillary - Network ensures that there’s always a high concentration of carbon dioxide and a low concentration of oxygen as the oxygenated blood is moved out of the lungs via pulmonary veins.
  •  Ventilation - Ensures that there is always a high concentration of oxygen and low concentration of carbon dioxide in the alveoli sacs to maintain the diffusion gradient.
• Ventilation in Mammals

  • Breathing involves inspiration and expiration to move oxygen in and car
  • bon dioxide out.
  • During inspiration - Volume of chest cavity increases, Pressure drops below the atmospheric pressure with air entering the lungs and the diaphragm contracts to move down and the external intercostal muscles contract to raise the ribs.
  • During expiration: - Volume of chest cavity decreases, Pressure rises below the atmospheric pressure with air leaving the lungs and the diaphragm relaxes but in forced expiration, internal intercostal muscles contract which lowers the ribs.
• Alveoli:

  • Lined with thin squamous epithelium
  • Have a good blood supply and elastic fibres
• Trachea and bronchi:

  • Inner lining is a ciliated epithelium with goblet cells
  • Consist of glandular tissues and C-shaped cartilage rings
  • Inside surface contains elastic fibres, smooth muscle, and blood vessels
• Bronchioles:

  • Cartilage present in upper and larger tubes but not in the smaller ones
  • Contain smooth muscle and elastic fibres
• Lungs are a large pair of inflatable structures protected by the ribs
• Plasma membranes surround the thin cytoplasm of cells in the lungs, creating a barrier permeable to oxygen and carbon dioxide
• Lungs produce a surfactant that reduces the cohesive (sticky) forces between water molecules, preventing the collapse of the alveolus
• Cartilage supports the trachea and bronchi, preventing them from collapsing during low air pressure, with its C-shape allowing flexibility for neck movement and oesophagus expansion during swallowing
• Smooth muscle can constrict the lumen to restrict air flow to and from alveoli, with involuntary contraction being important in the presence of harmful substances in the air
• Elastic fibers contract, constricting the lumen irreversibly by deforming elastic fibers in the loose tissue; when smooth muscles relax, the elastic fibers recoil to their original size and shape, dilating the lumen
• Goblet cells secrete mucus to trap tiny particles from the air, reducing the risk of infection
• Ciliated epithelium contains cilia that move in a synchronized pattern to waft mucus up the airway to the back of the throat for swallowing, where stomach acidity kills any bacteria
• Spirometers measure the gas entering and leaving the lungs, producing a graph from which several measurements can be made:

  • Residual volume - Volume of remaining air after forced expiration, usually 1.4 decimetre cubed.
  • Vital capacity - Total air that is moved by the lungs in one breath. Depends on size, gender, age and activity. Usually around 2.5 to 5 decimetre cubed.
  • Tidal volume - moved air in one inhalation or exhalation, usually around 0.5 decimetre cubed.
• Process of spirometry

  1. Medical grade oxygen is inhaled by a patient and the spirometer lid drops down.
  2. Exhaling back into the spirometer causes the lid to rise.
  3. Movements of the lid are recorded by a data logger and indicate the volume of air ventilated by the patient's lungs. Carbon dioxide-rich air is exhaled by the patient and absorbed by soda-lime.
• Precautions of spirometry

  • Soda-lime should be fresh to avoid carbon dioxide intake which would cause blood pH to drop which is toxic.
  • Apparatus should be tested for air leaks as this would invalidate the test.
  • Ensure the patient does not have any cardiac issues e.g. asthma.
• Oxygen Uptake

  Oxygen uptake is measured using a spirometer, estimated using carbon dioxide exhalation. The reduction in volume in the spirometer is due to carbon dioxide being absorbed by soda-lime. This is assumed to equal the oxygen uptake:

  1. Take two points on the graph and measure the time taken and volume reduction.
  2. Divide the volume by time, expressing oxygen uptake in decimetre cubed per second.
• Oxygen intake (dm3 min-1)=change in volume / time taken (s) * 60s 
• Bony fish gas exchange

  Fish use gills in order to exchange oxygen and carbon dioxide in water. 

  HeartArteriesGillsVeinsBody TissuesVeinsHeart

  Gills are covered by an operculum (a bony plate as the gills are delicate). In a fish’s gills, it is consisted of:

  • Gill arches 
  • 2 stacks of gill filaments containing protruding rows of very thin lamellae.
  • Lamellae consists of a network of capillaries covered by a single layer of epithelial cells.

  When the fish open their buccal cavity, they close their opercular vents to draw freshwater in to be pumped over the gills again.
• Adaptations of a Bony fish:

  • Lamellae increase surface area to volume ratio
  • Countercurrent flow maintains concentration gradient across lamellae, allowing efficient oxygen diffusion from water into the blood
  • Capillaries are one cell thick to reduce diffusion distance
  • Buccal cavity changes volume to pump water over the gills
• Insect gas exchange

  Insects have a waxy exoskeleton that helps them with protection and water retention which doesn’t allow for effective gas exchange. Hence, insects have the tracheal system that delivers oxygen directly to every tissue:

  • Spiracles open and close to allow air into the tracheal system
  • Gas exchange mainly via tracheal fluid at the end of the tracheoles.
  • Air sacs can be squeezed by flight muscles to push air in and out.
  • Flight muscles can alter the volume of the insect thorax to ventilate the tracheal system.
• Tracheal fluid in Insects

  • Tracheal fluid: At rest, liquid seeps into tracheoles from the surrounding cells but when active, the muscles draw up the tracheal fluid providing them with oxygen containing fluid for respiration.
  • When tracheal fluid is drawn into the cells), it lowers the pressure in the tracheoles, drawing more air in through the spiracles. Decrease of tracheal fluid increases the surface area available for oxygen to diffuse through tracheal walls directly.
• Specialised breathing mechanism - Some animals have them; as they expand their abdomen they close spiracles at the back end of their body. When they contract their abdomen, they open up the spiracles at their rear and close those at the front.
• Larger organisms need transport systems as they have smaller surface area to volume ratios. Cells are further from the surface, meaning diffusion alone is not enough to supply nutrients and oxygen.

  System features and its functions:

  • Fluid or medium - Blood that transports substances
  • Pump - Heart creates pressure to push fluid
  • Exchange surfaces - cells have access to blood
  • Vessels - Contains blood
• Single circulatory loops deliver blood more slowly and at low pressure. Double circulatory loops allow replenished blood from the lungs to be pumped at higher pressure to the body, speeding up nutrient delivery and waste removal.
• There are 3 main types of blood vessels: the artery and the vein (where both vessels consist of the collagen fibres, the smooth muscle layer, elastic tissue layer, the endothelium and the lumen) and the capillary (which only contains the endothelium and the lumen.

  There are also 2 connective blood vessels: the arteriole (Smooth muscle, endothelium) connecting arteries to capillaries and venule (Thin collagen, smooth muscle,elastic fibres, endothelium) connecting from capillary bed to veins.
• Features of blood vessels

  • Smooth muscle - contracts to control blood flow. Abundant in arterioles for directional control.
  • Elastic tissue fibres - Maintains pressure by stretching and recoiling. 
  • Outer layer - Resists pressure changes.
  • Endothelium - Smooth to reduce friction flow. One cell thick for quick exchange.
  • Lumen - Central cavity, allows flow
• Tissue fluid, made of nutrients dissolved in water, bathes respiring cells and tissues and removes wastes:

  1. A high hydrostatic pressure at the arteriole end, caused by ventricular systole, forces fluid out of the capillary and into the tissues.
  2. Large plasma proteins remain behind in the capillaries, lowering the water potential and helping to bring the fluid back in. At the venous end, water potential is greater than the hydrostatic pressure, so there is a net inward movement of fluid.
  3. Tissue fluid not reabsorbed into the blood drains into the lymphatic vessels.
• Formation of Lymph

  Some tissue fluid is drained into the lymphatic system (consisting of a number of vessels similar to capillaries) which drain the excess fluid into larger vessels which eventually rejoin the blood system in the chest cavity.
• Pulmonary vessels carry blood from and to the lungs
• Atria receive blood from vena cava and pulmonary vein
• Ventricles contract and pump blood through the aorta and pulmonary artery
• Coronary arteries supply oxygenated blood to the heart as it needs energy to pump blood
• Atrioventricular valves ensure that when the ventricles contract, blood flows upwards into the major arteries instead into the atria; tendinous cords attach the valves to the walls of the ventricles to prevent them from turning inside out
• Septum is the wall of muscles which separates the ventricles so oxygenated blood in the left side and deoxygenated blood in the right side are kept separate
• The right side of the heart pumps blood to the lungs and the left pumps it to the rest of the body