Exchange surfaces

The need for specialised exchange surfaces

Single-celled organisms only need to use diffusion alone to supply the needs-

• The metabolic activity is usually low, so the oxygen demands and carbon dioxide production of the cell are relatively low.
• The SA:V ratio of an organism is large.

As organisms get larger they can be made up of millions or even billions of cells arranged in tissues, organs and organ systems.

• Their metabolic rate is usually higher
• The bigger the organism, the smaller the SA:V ratio, so gases can't be exchanged fast enough or in lage enough amounts for the organism to survive.
• Larger volume=increase in metabolic processes.
• SA needs to increase to allow a greater intake of raw materials and removal of waste.
• Small=Large SA:V RATIO
• Shape affects ratio
• Large and round=small SA:V ratio
• Small and pointy=large SA:V ratio

Large, multicellular organisms have evolved specialised systems for the exchange of substances the need and the substances they must remove.

• Increased surface area- provides the area needed for exchange and overcomes the limitations of the SA:V ratio of larger organisms. E.g. root hair cells
• Thin layers- these mean the distances that substances have to diffuse are short, making the process fast and efficient. E.g. alveoli in the lungs
• Good blood supply- the steeper the con centration gradient, the faster diffusion takes place. Ensures substances are constantly delivered to and removed from the exchange surface. This maintains a steep concentration gradient for diffusion. E.g. alveoli in the lungs, gills in a fish
• Ventilation to maintain diffusion gradient- for gases, a ventilation system also helps maintain concentration gradients and makes the process more efficient. E.g. alveoli and the gills in a fish where ventilation means a flao of water carrying dissolved gases.

Mammals are relatively big, they have a small SA:V ratio and a very large volume of cells.

High metabolic rate because they are active and maintain their body temperature independent of environment-they need lots of oxygen for cellular respiration, and produce carbon dioxide, which needs to be removed.

 This exchange takes place in the lungs, which get oxygen into our bloodstream from the air around us, and remove carbon dioxide from our blood.

Key structures-

Nasal cavity

• Large SA with a good blood supply, which warms the air to body temp
• Hairy lining, secretes mucus to trap dust & bacteria, protecting delicate lung tissue from irritation and infection.
• Moist surfaces, increase the humidity of the incoming air, reducing evaporation from the exchange surfaces
• After passing the naval cavity, air entering the lungs is a similar temp and humidity to the already there.

Trachea

• Main airway carrying clean, moist, warm air from the nose to the chest. Wide tube supported by strong, flexible cartilage, which stops it collapsing.
• Rings incomplete so food can flow easily down oesophagus behind trachea. Trahcea and its branches are lined with ciliated epithelium with goblet cells between and below epithelial cells.
• Goblet cells seperate mucus onto lining of trachea to trap dust & microorganisms that have escaped nose lining.
• Cilia beat and move mucus, along with trapped dirt and microorganisms, away from lungs. Most goes into throat and swallowed and digested.

Bronchus

• In chest cavity, trachea divides to form left bronchus (plu bronchi), leading to left lung, and right bronchus leading to right lung. Similar structure of trachea, with supporting rings of cartilage, but smaller.

Bronchioles

• The bronchi divide to form many small bronchioles. They have no cartilage rings.
• Walls of bronchioles contain smooth muscle. When it contracts, they constrict, and when relaxes they dilate.

Alveoli

• Are tiny air sacs, which are the main gas exchange surfaces of the body. They are unique to mammalian lungs.
• Each alveolus has a diameter of around 200-300 nanometers, consists of a layer of thin, flattened epithelial cells, along with some collagen and elastic fibres (composed of elastin). These allow the alveoli to stretch as air is drawn in. When they return to their resting size, they help squeeze the air out-elastic recoil.

Adaptions for effective gaseous exchange

• Large SA
• Thin layers-have walls that are only a single epithelium cell thick, so the diffusion distances between the air in the alveolus.
• Good blood supply-the millions of alveoli in each lung are supplied by a network of around 280 million capillaries. The constant flow of blood through these brings carbon dioxide and carries off oxygen, maintaining a steep concentration gradient for both carbon dioxide and oxygen between the air in the alveoli and the blood in the capillaries.
• Good ventilation- breathing air in and out of the alveoli, helping maintain steep diffusion gradients for oxygen and carbon dioxide between the blood and the air in the lungs.

Alveoli-The inner surface of the alveoli is covered in a thin layer of a solution of water, salts and lung surfactant- makes it possible for the alveoli to remain inflated. Oxygen dissolves in the water before diffusing into the blood, but water can evaporate into the air in the alveoli.

Ventilating the lungs-Air is moved in and out of the lungs as a result of pressure changes in the thorax (chest cavity) brought about by the breathing movements.

• The rib cage provides a semi-rigid case within which pressure can be lowered with respect to the air outside it.
• The diaphragm is a broad, domed sheet of muscle, forms the floor of the thorax.
• The external and internal intercostal muscles are found within the bribs. 
• The thorax is lined by the pleural membranes-surround the lungs. The space between them, the pleural cavity, is usually filled with a thin layer of lubricating fluid so the membranes slide easily over each other so you can breathe.

Inspiration-(taking air in or inhalation) is an energy-using process

• The diaphragm contracts, flattens, and lowers.
• The external intercostal muscles contract, moving the ribs upwards and outwards. The volume of the thorax increases so the pressure in the thorax is reduced. Now lower than the pressure of the atmospheric air, so the air is drawn throughthe nasal passages, trahcea, bronchi, and bronchioles into the lungs-equalises the pressures inside and outside the chest.

Expiration-normal expiration (breathing out or exhalation) is a passive process

• The muscles of the diaphragm relax so it moves up into its resting domed shape.
• The external intercostal muscles relax so the ribs move down and inwards under gravity.
• The elastic fibres in the alveoli of the lungs return to their normal length-is to decrease the volume of the thorax. Now the pressure inside the thorax is greater than the pressure of the atmospheric air, so air moves out of the lungs until the pressure inside and out is equal again.
• You can exhale forcibly using energy.
• The internal intercostal muscles contract, pulling the ribs down hard and fast, and the abdominal muscles contract forcing the diaphragm up to increase the pressure in the lungs rapidly.

The amount of gaseous exhange that needs to take place in your lungs will vary a lot depending on your size and level of activity.

Measuring the capacity of the lungs

• A peak flow meter- measures the rate at which air can be expelled from the lungs
• Vitalographs-the patient being tested breathes out as quickly as they can through a mouthpiece, and the instrument produces a graph of the amount of air they breathe out and how quickly it is breathed out-forced expiratory volume in 1 sec.
• Spirometer- commonly used to measure different aspects of the lung volume, or to investigate breathing patterns.

Components of the lung volume-

• Tidal volume- volume of air that moves into and out of the lungs with each resting breath.
• Vital capacity-volume of air that can be breathed in when the strongest possible exhalation is followed by the deepest possible intake of breath.
• Ispiratory reserve volume- the maximum volume of air you can breathe in and above a normal inhalation.
• Expiratory reserve volume- the extra amount of air you can force out of your lungs over and above the normal tidal volume of air you breathe out.

Residual volume-the volume of air that is left in your lungs when you have exhaled as hard as possible. This cannot be measured directly.

Total lung capacity- the sum of the vital capacity and the residual volume.

Breathing patterns- The pattern and volume of breathing changes as the demands of the body change.

• The breathing rate is the number of breaths per minute.
• The ventilation rate is the total volume of air inhaled in 1 min
• ventilation rate = tidal volume x breathing rate (per min)
• When the oxygen demands of the body increase, e.g. during exercise, tidal volume of air moved in and out of the lungs with each breath can increase from 15% to 50% of the vital capacity.
• The breathing rate can also increase. In this way the ventilation of the lungs and so the oxygen intake during gaseous exchange can be increased to meet the meet the demands of the tissue.

Normal tidal volume of a male is 500cm3. Ventilation rate=6dm3 per min, Resting breathing rate?

• VR=TV x BR (per min)
• 6dm3=6000cm3
• 6000=500 x BR=6000/500=12 B per minute

Inspiration

Expiration

External intercostal muscles

Contract – pulling ribs upwards and outwards.

Relax – permitting rib cage to move downwards and outwards.

Internalintercostal muscles

Are relaxed.

Contract – moving the ribs downwards and decreasing the volume of the thorax.

Diaphragm

Contracts – moves downwards and flattens

Relaxes –returns to domed position,

Air pressure in lungs

Decreases

Increases

Air movement along pressure gradient

Into lungs

Out of lungs

Lung volume

Increases

Decreases

Insects have no transport system so gases need to be transported directly to respiring tissues.

• Along the thorax and abdomen of most insects are small openings=spiracles. Air enters and leaves the system through the spiracles, but water is also lost.
• Insects need to maximise the efficiency of gaseous exchange, but minimise the loss of water.
• The spiracles allow oxygen to diffuse in and carbon dioxide to diffuse out of the tracheae.
• The tiny hairs maintain humidity and a low water concentration gradient so reduces water loss.
• In many insects the spiracles can be opened or closed by sphincters. They are kept closed as much as possible to minimise water loss.
• When an insect is inactive and oxygen demands are low, the spiracles will be closed most of the time.
• When the oxygen demand is high or the carbon dioxide levels build up, more of the spiracles open.
• Leading away from the spiracles are tracheae. They carry air into the body. They run both into and along the body of the insect. Lined by chitin, which keep them open if they are bent or pressed. Relatively impermeable to gases and so little gas exchange takes place in the tracheae.
• The finest braches are called tracheoles, these extend to the surface of nearly every cell. They have no chitin so gaseous exchange can happen. They are actually one elongated cell.
• At the cell surface, gas is exchanged by diffusion across the moist epithelium that lines the terminal ends of the tracheal system.

• In most insects, for most of the time, air moves along the tracheae and tracheoles by diffusion alone, reaching all the tissues.
• The vast numbers of tracheoles give a very large SA for gaseous exchange.
• Oxygen dissolves in moisture on the walls of the tracheoles and diffuses into the surrounding cells.
• Towards the end of the tracheoles is tracheal fluid, limits the penetration of air for difusion.
• The tracheal fluid actually reduces the SA for gaseous exchange.
• However, when the oxygen demands build up, a lactic acid build up in the tissues results in water moving out of the tracheoles by osmosis. Exposes more surface area for gaseous exchange.

Some insects, larger insects, have very high energy demands. To supply the extra oxygen they need, these insects have alternative methods of increasing the level of gaseous exchange.

• Mechanical ventilation of the tracheal system- air is actively pumped into the system by muscular pumping movemetns of the thorax and/or the abdomen. Change the volume of the body, changes the pressure in the tracheae and tracheoles. Air is drawn into the tracheae and tracheoles, or forced out as the pressure changes.
• Collapsible enlarged tracheae or air sacs-act as air resevoirs-used to increase the amount of air moved through the gas exchnage system.

• Water is more viscous than air
• Water has less oxygen than air
• Water is denser than air

They need gaseous exchange systems as the are active organsims and need a good supply of oxygen. They have a small SA:V ratio. They are multicellular.

Gills- Because they are active, their cells have a high oxygen demand. Their SA:V ratio means that diffusion would not be enough to supply their inner cells with the oxygen they need.

• They maintain a flow of water in one direction over the gills, which are their organs for gaseous exchange.
• Large SA, good blood supply, and thin layers needed for successful gaseous exchange.
• In bony fish, they are contained in a gill cavity, and covered in the operculum, which is active in maintaining flow of water over the gills.

Gills have a large surface are for diffusion, a rich blood supply to maintain steep concentration gradients for diffusion, and thin layers so that diffusing substacnes have short distances to travel.

Adaptions

• The tips of adjacent gill filaments overlap-increases resistance to the flow of water over the gill surfaces and slows down the movement of water. More time for gaseous exchange to take place.
• Counter-current system-The water moving over the gills and the blood in the gill filaments flow in opposite directions-A steep concentration gradient is needed for fast, efficient diffusion to take place.
• More gaseous exchange can take place,

Bony fish, with their countercurent systmes remove about 80% of oxygen from the water flowing over them.

Cartilaginous fish have parallel systmes-can only extract about 50% of oxygen from the water flowing over them.