Unit 1.2.1: Exchange Surfaces and breathing - SICM

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Biology AS Level: OCR Board. 1. Unit 1.2.1: Exchange Surfaces and breathing. Exchange surfaces. We have done a lot at GCSE, and I keep going back to ...
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Biology AS Level: OCR Board

Unit 1.2.1: Exchange Surfaces and breathing Exchange surfaces We have done a lot at GCSE, and I keep going back to it…which is great fun because I get to see how much (/little) you know…so let’s try this again. Name three ways in which exchange surfaces are adapted to diffuision. (Yes, this is part of Module 1 too…so you better be able to do this!!) Oh, and while you’re at it…name the Law that this refers to. Fick’s Law thickness of membrane / length of pathway surface area × concentration difference surface area Rate of diffusion α thickness of membrane concentration gradient There are many places in humans in which diffusion is vital. Some examples include: lungs (where the alveoli are extremely well adapted) kidney (exretory products…urine, uric acid etc.) intestine (remember things called villi?) It goes without saying (but I’ll say it anyway…) that the body is well adapted in certain places for diffusion. The three examples we looked at above are prime examples and we will look at these in detail later. Exchange processes We have already mentioned that surface area is important and this, combined with the volume of an organism very much determines the way in which an organism exchanges gases. Let us think about the way in which size affects the surface area:volume ratio. 2μ 5μ

Surface area: 24μ2 Volume: 8μ3 Surface area:volume ratio: 3:1

Surface area: 150μ2 Volume: 125μ3 Surface area:volume ratio: 1.2 : 1

As you can see, there is a huge difference. The smaller cube has a huge surface area:volume ratio. This has many affects on organisms of small size. Think about a small organism such as an amoeba or protozoa. These organisms are so small that they don’t need things such as lungs. They are able to carry out all of their gas exchange directly with the surroundings because of their vast surface area and small volume. However, this also causes problems with water loss which can lead to dehydration very easily.

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Biology AS Level: OCR Board

Humans, as with all larger organisms, require ventilation systems to perfuse all cells or a circulatory system to take the gases round: as all cells require gases such as oxygen for respiration (yes…respiration IS still important…). Of course, this is because we are larger than a couple of cells thick…however much we diet (so girls, you may as well just give up!). Gas Exchange in Humans There’s no need to tell you that the lungs are the organs by which humans take in and expel gases…so let’s go straight into the detail. The thorax (chest), is bound by the ribs and the muscular sheet known as the diaphragm. Within this space, the lungs are present. Air passes into a series of tubes which are designed to not only moisten and warm the air that comes in, but also protect the body by filtering out some dust etc. There are three subsections of these tubes: -

Trachea

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Bronchi

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Bronchioles

The walls of the trachea and bronchi are composed of an epithelial layer as well as connective tissue, smooth muscle and cartilage along with a tough outer coat. Bronchioles differ in the fact that they lack cartilage. Cartilage supports the trachea and bronchi and stops them from collapsing. In the trachea, the cartilage forms C-shaped rings that keep the tube open during inspiration. Smooth muscle lies at the back of the tube. The reason for the cartilage not being all the way around is that the oesophagus (gullet / food pipe!) is at the back of the trachea and the smooth muscle allows expansion of the oesophagus as food travels down. In the bronchi, the cartilage is arranged in plates within smooth muscle. The smooth muscle also helps keep the tubes open. The diameter of the tubes is regulated by the automatic part of the nervous system. This ensures adequate diameter for suitable gas flow. The trachea, bronchi and bronchioles are lined with epithelial cells. These epithelial cells are made up of different types: ciliated and goblet cells. Goblet cells produce and secrete mucus into the respiratory tract and ensures that there is a layer of mucus along the trachea to trap dust and bacteria. Ciliated cells have a protective function. They sweep the mucus along so that it does not block the trachea and so that it can be renewed. Generally, the mucus moves along to the oesophagus where it is swallowed and destroyed by the acidity of the stomach. 2

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Biology AS Level: OCR Board

Alveoli are air sacs and they make up th mass of the lungs. They are attached to the end of the bronchioles via alveolar ducts. Although each alvelous is very small, there are loooaaaaaads of them (about 700 MILLION!!) which provides a massive surface area of about 70-90m2! They are covered in blood vessels that bring in deoxygenated blood via the pulmoary artery. The vast blood supply helps to provide a sufficient concentration gradient for the gases which need to be exchaged. For example, the partial pressure of oxygen in the air in the alveolus will be higher than the pO2 in the blood and so oxygen will move into the blood via diffusion. If you exercise more, you use up more oxygen and so there is less oxygen in the blood. This means that there is a steeper concentration gradient in the lungs: more diffusion into blood!

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Ventilation -

Inspiration -

Biology AS Level: OCR Board

main muscles to be considered include the intercostal muscles and the diaphragm external intercostal muscles contract ribs move upwards and outwards diaphragm contracts: moving downwards incrased thorax volume reduced pressure (below outside air pressure) air enters - inflating the lung

Expiration (do this one yourself!) no active muscle contractions external intercostal muscles relax ribs move downwards and inwards diaphragm relaxes: moving upwards decrased thorax volume increased pressure (above outside air pressure) air forced out - deflating the lung -

in forced breathing, expiration is active and internal intercostal muscles and abdominal muscles contract – incresing the pressure in the thorax.

Friction between the lings and the inner surace of the wall of the thorax is reduced by the pleural fluid which lies between the two pleural membranes that surround the lungs (see diagram on previous page). Breathing control Normally, respiratory movements are involuntary. They are controlled by the rhythmic discharges of the medulla in the brain. From the inspiratory control centre, impulses travel down the phrenic nerves to the diaphragm and down the intercostal nerves to the external intercostal muscles causing them to contract. When the alveoli are stretched, the stretch receptors send impulses to the inspiratory control centre inhibiting inspiration and causing the muscles to relax. Therefore expiration takes place. Normally, expiration is passive. Sometimes it can be forced – by sending impulses to the internal intercostal muscles rom the expiratory control centre.

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Biology AS Level: OCR Board

Increase in CO2 in blood speeds up frequence of nerve impulses. Decrease in CO2 in blood slows fown frequence of nerve impulses Aorta and carotid arteries contain chemoreceptors which detect changes in CO2 levels. pH levels also change with CO2: as CO2 increases, pH decreases (as carbonic acid is formed).

Ventilation rate = volume of breath × number of breaths per minute (units = litres/min) Spirometer A spirometer is an apparatus for measuring the volume of air inspired and expired by the lungs.The spirometer records the amount of air and the rate of air that is breathed in and out over a specified time. The output produced by a spirometer is called a kymograph trace. From this, vital capacity, tidal volume, breathing rate and ventilation rate (= tidal volume × breathing rate) can be calculated. From the overall decline on the graph, the oxygen uptake can also be measured. Tidal volume (VT) - volume of air inspired during quiet respiration (≈ 0.5dm3) Inspiratory reserve volume (IRV) - volume air inspired from VT to maximal inspiration Expiratory reserve volume (ERV) - Air expelled with forced expiration Vital capacity (VC) - volume air maximal inspiration to maximal expiration Residual volume (RV) - amount air left in lungs which cannot be expelled Functional residual capacity (FRC) - lung volume at end of normal quiet expiration (ERV + RV) Total lung capacity (TLC) - Volume maximal inspiration to residual volume (IRV +VT + ERV + RV)

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