Respiratory surfaces

• These are surfaces on which gases are diffused in and out of an organism, such as alveoli in lungs. These are efficient at gas exchange.

• All respiratory surfaces must have:

  • A high enough surface area to ensure there is enough gas exchanged to satisfy the needs of the organism.

  • Be thin, for shortened diffusion pathways.

  • Be permeable for respiratory gases to diffuse easily.

  • Have a mechanism to produce a steep diffusion gradient across the respiratory surface by bringing in oxygen or removing carbon dioxide rapidly.

• This topic explains the adaptations made by different organisms to provide enough oxygen for organisms.

Unicellular organisms

• An example of these is the amoeba, which are extremely short.

• Respiratory traits include a large surface area: small volume, a thin cell membrane and thin structure due to being single celled.

• This makes it easy for them to absorb enough oxygen for their needs and remove carbon dioxide fast enough.

Multicellular organisms

• Large organisms have a lower surface area: volume ratio, meaning they cannot diffuse through their surfaces.

• Multicellular organisms often have:

  • A higher metabolic rate, meaning more oxygen is required and more carbon dioxide needs to be removed.

  • Cells, tissues and organs become more interdependent.

  • They need a steep concentration gradient across their respiratory surfaces by moving air, water or blood against their respiratory surface. This is known as a respiratory medium.

  • Respiratory surfaces are protected inside the organism due to their thinness, such as in lungs and gills.

• Examples are:

  • Flatworms:

    • Have a large surface area due to their flatness. This allows all cells to be close to the surface, shortening diffusion paths.

  • Earthworm:

    • Cylindrical shape to give more surface area.

    • Keeps the respiratory surface (it’s skin) moist by secreting mucus. This restricts earthworms to damp soil.

    • It is slow moving to keep it’s oxygen requirement low, which gives it a low metabolic rate.

    • Haemoglobin moves oxygen around the body, maintaining a diffusion gradient at the surface.

    • Carbon dioxide is also carried in the blood and diffuses out the skin.

Gas exchange in vertebrate groups

• Amphibians, such as frogs, toads and newts:

  • Must live in moist environments to keep skin permeable.

  • Fertilisation and eggs develop in water, and larvae live in water and have gills.

  • During metamorphosis for larvae to become adults, developing lungs, limbs and most species losing gills.

  • Adults can respire through skin when underwater or inactive and lungs when active.

  • Amphibians lungs are a pair off thin-walled sacs connected to the mouth through an opening known as the glottis.

    • They breath in by filling their mouth with air, closing their mouth and nose, opening it’s glottis and raising the floor of its mouth to force air into the lungs.

  • Some amphibians only breathe through their skin, having no lungs.

• Reptiles, such as crocodiles, lizards and turtles:

  • Their skin cannot be used for gas exchange as it is dry and scaly, so they rely entirely on their lungs.

    • Their lungs have a greater surface area than amphibians.

    • They don’t have diaphragms, so their lungs are inflated and deflated by the expansion and contraction of the rib cage and contraction of intercostal muscles.

  • In turtles, specific muscle contraction is responsible for ventilation.

• Birds, such as doves:

  • They use air sacs, ribs and flight muscles to ventilate their lungs.

  • Lungs maintain a fixed volume with fresh air constantly flowing through them.

Gas exchange in fish

Breathing mechanism

• Gills are used by fish in order to breathe.

  • Each gill is made up of four bony gill arches.

  • These are lined with hundreds of gill filaments that are thin and flat.

    • These clump together when a fish is taken out of water, which causes it to die.

  • These filaments have folds containing lamallae that contain a network of capillaries, blood flowing through these to pick up oxygen.

    • The capillary and lamallae walls are one cell thick, meaning there is only two cell layers between the water and the blood, which means there is a short diffusion path.

  • There is also gill rakers at the top of gill arches, which filter debris and particles from the water.

• This is needed as water has only 1% oxygen saturation, while air has 21% saturation.

Ventilation

• There is also a ventilation system, which is needed to move the respiratory medium across the respiratory surface.

• The laws of ventilation - and science in general - are as follows:

  • When volume increases, pressure decreases.

  • When pressure increases, volume decreases.

  • Pressure moves from a high to low area.

• In fish, pressure is manipulated using the mouth, the floor of their mouth and the opercular valve.

• During inspiration:

  • The mouth opens, which increases the volume inside the buccal cavity.

  • The opercular valve closes, which increases volume and prevents water from escaping.

  • The floor of the mouth lowers, increasing volume.

  • All of these mechanisms decrease pressure in the buccal cavity, causing water to flow in.

  • The water moves across the gills which rest in the pharyngeal cavity.

• During expiration:

  • The mouth closes, decreasing the volume in the buccal cavity.

  • The opercular valves open, allowing the water to flow out.

  • The floor of the mouth is raised to decrease volume.

  • The pressure in the buccal cavity is increased, causing the water to move to an area of lower water pressure, which is the opercular cavity and then the outside.

Gill variances

• There are two types of fish, cartilaginous and bony.

• Cartilaginous fish, for example sharks:

  • Have gills in five spaces on each side, called gill pouches, which open at gill slits.

  • They do not have a ventilation system, which means water cannot be forced over their gills. These fish must constantly move in order to keep diffusing oxygen into their blood.

  • Water moves across gills in a parallel flow, meaning the blood and water flow in the same direction. Once equilibrium is reached, which means once there is 50% oxygen in the blood and water, no more diffusion occurs.

    • This means parallel flow is less efficient, as the max saturation of oxygen in the blood is 50%.

• Bony fish, for example catfish:

  • These have a operculum, and therefore a ventilation system.

  • They are the most common aquatic vertebrates.

  • Water moves across gills in a countercurrent flow, meaning the blood and water flow in opposite directions. Equilibrium can never be reached, as every point along the gill lamellae the water has a higher concentration rate, meaning there is a constant diffusion of oxygen to the blood.

    • This means countercurrent flow is more efficient, as it removes around 80% of the oxygen from the water.

Insect respiration

• Insects are terrestrial and live in arid environments, meaning that they risk dehydration and therefore cannot have a large, open gas exchange surface. They also have a small SA:V ratio, meaning they couldn’t use their skin even without this risk.

  • They protect from dehydration using their chitin exoskeleton, which is waterproof.

• Instead they use paired holes that run along the side of the body, known as spiracles.

  • Some spiracles can open and close using valves to allow for gas exchange and prevent excess water loss. They have hairs surrounding them to prevent water loss and prevent solid particles from entering.

  • These lead into air-tubes called tracheae, which have rings of chitin. This keeps them fairly rigid and protects them from compression by other tissues.

  • These branch into smaller tubes called tracheoles. The ends are fluid filled and extend into muscle fibres, and this interface is where the gas exchange takes place. Oxygen dissolves into the fluid and then directly into muscle cells.

• There are two types of respiration, resting (aerobic) and active (anaerobic):

  • Aerobic respiration takes in oxygen less efficiently. It moves along a diffusion gradient, as the pressure in the tracheoles is higher than in the cells.

    • There is low lactic acid in the cells, and more fluid in the cells. This prevents the entry of lots of oxygen.

  • Anaerobic respiration diffuses more efficiently, as there is more lactic acid in the cells. This lowers the osmotic potential and makes the solution around the cell hypertonic. This causes fluid to leave the tracheoles via osmosis.

    • More air can then move in to replace the fluid, so gases can move through more rapidly.

    • Additionally, movements of the abdomen ventilate the tracheae during activity.

Human breathing system

• Lung tissue is elastic-like, and regains its original shape when not being expanded.

• Lungs are enclosed in an airtight compartment, the thorax.

• Surrounding the lungs and lining the thorax is the pleural membrane, which is a double membrane with the outer lining attached to the rib cage and inner attached to the lungs.

  • In between these membrane is a pleural cavity, containing pleural fluid. This prevents friction between the lungs and inner wall of the thorax during ventilation.

• At the base of the thorax is a dome-shaped sheet of muscle called the diaphragm, separating the thorax from the abdomen.

• Ribs surround the thorax.

• Intercostal muscles are in between the ribs.

• Trachea is a flexible airway that brings air into the lungs.

• It splits into two bronchus (bronchi plural) which lead respectively into the right and left lung.

• These bronchi futher branch into bronchioles, which are a network of many tubes.

• At the ends of these bronchioles are air sacs called alveoli.

  • These are the gas exchange surfaces, where deoxygenated blood enters through the pulmonary artery and oxygenated blood leaves through the pulmonary vein.

  • Carbon dioxide diffuses into the alveoli air where it is expelled.

  • The insides of the alveoli are coated with pulmonary surfactant, which reduces the effort needed to expand the lungs and reduces the tendency for alveoli to collapse, which happens due to low air pressure. Additionally, it allows gases to dissolve before they diffuse through. It is made of phospholipids and protein.

    • These molecules move apart when the alveoli expand during inspiration, but become concentrated to reduce surface tension when alveoli contract during expiration.

  • Their walls are one cell thick and made of squamous epithelium, coated in moisture, have an extensive capillary network and have a large SA:V ratio.

• Air changes it’s contents during respiration:

Ventilation

There are two types, inspiration and expiration:

• Inspiration, an active process:

  • Intercostal muscles contract.

  • Ribs are pulled upwards and outwards.

  • Diaphragm contracts and flattens.

  • The pleural cavity is pulled downwards by the diaphragm and outwards by the intercostal muscles which expands the lungs, increasing volume inside the lungs/alveoli.

  • This reduces pressure in the lungs, forcing air into the lungs as it is lower pressure than atmospheric pressure.

• Expiration, a passive process:

  • Intercostal muscles relax.

  • Ribs move downwards and inwards.

  • Diaphragm relaxes and rises.

  • Pleural membranes move inwards with the intercostal muscles and upwards with the diaphragm which decreases volume in the lungs/alveoli.

  • This increases air pressure in the lungs, forcing air out of lungs as it is higher pressure than atmospheric pressure.

Issues associated

• Emphysema:

  • A condition in the body in which the alveoli are damaged and are unable to contract properly due to damage to pulmonary surfactant.

  • This makes it harder for air to leave the lungs, causing feelings of breathlessness.

• Asthma:

  • Asthma occurs when smooth muscles are tightened, which can be triggered by certain things.

  • It can be chronic, meaning it has flairs, or acute meaning it is constant. Acute asthma is life threatening.

  • It can also cause a mucus build up, which increases risks of infections.

Measurements

• Spirometer trace is a measure done by a spirometer to calculate maximum lung capacity, including inhalation and exhalation capacity.

• Tidal volume is breathing at rest, and vital capacity is overall capacity.

Plant respiration

• Plants respire at all times but can only photosynthesise throughout the day.

  • Respiration is the process of taking in oxygen and exhaling carbon dioxide.

  • They photosynthesise by taking in carbon dioxide and releasing oxygen, which is the majority of how they respire. Some of this oxygen is used in respiration, but most is diffused out through the leaves.

• Gases diffuse through the stomata (stoma singular, these are holes in the epidermis, which is protected by a waxy cuticle) down a concentration gradient into the sub-stomatal air chamber, then into inter-cellular spaces between the spongy mesophyll cells and then into the cells.

  • There are two types of mesophyll, spongy mesophyll with air spaces and palisade mesophyll which is more elongated.

• Diffusion direction depends on the concentration of gases in the atmosphere and reactions in the plant cells.

• Stoma can open or close, normally opening in the day and closing in the night. They may also close during the day for extraordinary circumstances, such as extreme heat, excess water loss or extreme bright light.

  • The stomata are on the lower surface of leaves, and are the only chloroplast containing epidermal cells. The walls are unevenly thick, with the inner being thicker than the outer wall. The hole is surrounded by two guard cells, which open and close.

• Steps to opening:

  • Photosynthesis occurs, causing respiration and then ATP production.

  • ATP is used in the active transport in K+ (potassium) ions into the guard cells from the epidermis cells.

  • Starch that is stored in the leaf is converted to malate.

  • The malate and potassium ions in the guard cells decreases the osmotic potential of the cells. This causes water to move in from the epidermis cells via osmosis.

  • This causes the guard cells become turgid and swell, thicker inner walls resisting expansion and only the outer walls stretching.

• Steps to closing:

  • Photosynthesis no longer occurs, so ATP is no longer produced.

  • Active transport no longer occurs, so potassium ions move down the concentration gradient to the epidermis cells.

  • Malate production ceases, and in some cases is converted back into starch.

  • The osmotic potential of the cell increases, causing water to leave the cells and enter the epidermal cells surrounding it.

  • Guard cells become flaccid and stomatal pores close.