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O3. Exchange of materials may take place at special structures or organs.


Student Outcome: O3.1

Explain why exchange surfaces in the body must be thin, moist, and have a large surface area.


  • Each cell of the body needs nutrients from outside the body eg oxygen, glucose, amino acids, nucleic acids, vitamins and minerals
  • The cells uses them up and so they must be replaced
  • As a result of using these nutrients, wastes are produced, which must be removed eg carbon dioxide, urea, lactic acid.
  • The exchange of these materials cannot happen with the external environment by diffusion
    • The SA/V ratio is too low so that most of the body's cells would die before any materials can get to them.
    • The body has an impermeable skin that is designed to stop the loss of material eg water.
  • As a result, the body has developed specialised surfaces to help with the exchange of these materials for all cells in the body.
  • They include the alveoli in the lungs, the nephron in the kidney and the villi of the small intestine.
  • Four features of these specialised surfaces include:
  1. They are made up of a single layer of epithelial cells. These cells are very thin ie two membranes and a small amount of cytoplasm. Materials can pass through easily by diffusion or active transport.
  2. They are made of livings cells that must stay alive and moist to be able to function properly. A dead layer would restrict movement of molecules. Also most substances must be dissolved in water before they can diffuse.
  3. They must have a large surface area to enable enough material to diffuse across to service the body. The larger the surface area, the larger the organism can grow.
  4. They must have surfaces with an extensive blood supply in the form of capillaries. These are a little bigger than red blood cells and also have a thin membrane.


Source: Notes from Organisms Theme notes


  • Systems that increase the rate of exchange


Fick's law showed that for a fast rate of diffusion you must have a large surface area, a small distance between the source and the destination, and maintain a high concentration gradient. All large organisms have developed systems that are well-adapted to achieving these goals, as this table shows. For comparison, a tennis court has an area of about 260 m² and a football pitch has an area of about 5000 m².


System Large surface area Small distance High concentration gradient
human circulatory system 100m of capillaries with a surface area of 6000m² capillary walls are only one cell thick constant blood flow replenishes the blood
human lungs 600 million alveoli with a total area of 100m² each alveolus is only one cell thick constant ventilation replaces the air
human small intestine 7m long, folds, villi and microvilli give surface area of 2000m² blood capillaries close to surface of villus stirred by peristalsis and by microvilli


Source: http://www.biologymad.com/master.html?http://www.biologymad.com/GaseousExchange/GaseousExchange.htm


Student Outcome: O3.2

Explain the role of blood capillaries and lymph capillaries in the exchange of materials.


This video gives some background to how the circulatory system works.


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The Capillaries - a very short video on how real capillaries work.

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Capillaries make possible the exchange of molecules between blood and interstitial fluid. Sphincters open and close to regulate the flow of blood through capillary beds. The crucial exchange of substances required for cellular metabolism takes place through the capillary endothelium, which allows cells of the various tissues to take up oxygen and nutrients and to expel carbon dioxide and wastes.


  • The Lymph

The lymphatic system has two primary functions. First of all, it returns excess interstitial fluid to the blood. Of the fluid that leaves the capillary, about 90 percent is returned. The 10 percent that does not return becomes part of the interstitial fluid that surrounds the tissue cells. Small protein molecules may "leak" through the capillary wall and increase the osmotic pressure of the interstitial fluid. This further inhibits the return of fluid into the capillaries, and fluid tends to accumulate in the tissue spaces. If this continues, blood volume and blood pressure decrease significantly and the volume of tissue fluid increases, which results in edema (swelling). Lymph capillaries pick up the excess interstitial fluid and proteins and return them to the venous blood. After the fluid enters the lymph capillaries, it is called lymph.


The second function of the lymphatic system is the absorption of fats and fat-soluble vitamins from the digestive system and the subsequent transport of these substances to the venous circulation. The mucosa that lines the small intestine is covered with fingerlike projections called villi. There are blood capillaries and special lymph capillaries, called lacteals, in the center of each villus. The blood capillaries absorb most nutrients, but the fats and fat-soluble vitamins are absorbed by the lacteals. The lymph in the lacteals has a milky appearance due to its high fat content and is called chyle.


A video showing where the lymph nodes are. It also gives details of some of the names - you don't need to know that.

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Not directly relevent but in this video they are looking to make substitutes for blood.


Student Outcome: O3.3

Know the structural features of nephrons in the kidney, and understand the importance of filtration and reabsorption.


  • The glomerulus


The glomerulus is the main filter of the nephron and is located within the Bowman's capsule. The glomerulus resembles a twisted mass of tiny tubes through which the blood passes. The glomerulus is semipermeable, allowing water and soluble wastes to pass through and be excreted out of the Bowman's capsule as urine. The filtered blood passes out of the glomerulus into the efferent arteriole to be returned through the medullary plexus to the intralobular vein.


  • Bowman's Capsule

The Bowman's capsule contains the primary filtering device of the nephron, the glomerulus. Blood is transported into the Bowman's capsule from the afferent arteriole (branching off of the interlobular artery). Within the capsule, the blood is filtered through the glomerulus and then passes out via the efferent arteriole. Meanwhile, the filtered water and aqueous wastes are passed out of the Bowman's capsule into the proximal convoluted tubule.


Source: http://coe.fgcu.edu/faculty/greenep/kidney/Glomerulus.html


Brief introduction to the urinary system - nice graphics though.

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Pretty dry video on the kidneys

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Student Outcome: O3.4

Know the structural features of alveoli in the lungs, and describe how gases are exchanged through this surface.


From Organisms
Alveoli are small, balloon-like sacs at the end of the small air passages in the lungs (the bronchiole). Oxygen is inhaled and absorbed into the bloodstream through the thin walls of each alveolus, by way of the pulmonary veins. Carbon dioxide from the pulmonary artery is exhaled as a waste product of the lungs.


The greater the surface area the lungs have for gas exchange, the greater is their efficiency to absorb oxygen. The 700 million (or more) alveoli found in both lungs, if flattened out, would cover an area of some 50-100 square yards. This is approximately the size of a tennis court, and is all neatly folded and bundled into the chest cavity. Each alveolus has a wall that is only one cell thick. A capillary wall has about the same thickness. The distance between air and blood is about 1/1000th of a millimeter. The oxygen is transported by the red blood cells, which squeeze single file through the pulmonary capillaries. Red cells that are packed with hemoglobin, or red pigment, which attracts the oxygen. Carbon dioxide is diffused in the same way back through the capillaries and alveolar walls to be exhaled.


The enormous surface area of the alveoli and the short diffusion distance between alveolar air and capillary blood quickly allows the blood to achieve an equilibrium with gases of the alveolar air. This function is further increased by the fact that each alveolus is surrounded by a capillary network so extensive that it forms an almost continuous sheet of blood around each alveolus.


Source: http://www.innerbody.com/text/card02.html


This video introduces the respiratory system.                                                       If that doesn't thrill you, try this one on the same subject.

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Student Outcome: O3.5

Know the structural features of villi in the digestive system, and describe how nutrients are absorbed.


Virtually all nutrients, including all amino acids and sugars, enter the body across the epithelium covering small intestinal villi. As shown in the diagram on the right, each villus contains a capillary bed and a blunt-ended lymphatic vessel referred to as the "central lacteal." Source: http://instruct1.cit.cornell.edu/courses/biog105/pages/demos/105/unit6/media/villus.structure.jpg


After crossing the epithelium, most of these molecules diffuse into a capillary network inside the villus, and hence into systemic blood. Some molecules, fats in particular, are transported not into capillaries, but rather into the lymphatic vessel, which drains from the intestine and rapidly flows into blood via the thoracic duct.


Source: http://biology.about.com/library/organs/bldigestsmallint2.htm


Awesome but short introduction to the digestive system.                                              Fairly useful video on digestion - take into account the American accent and pronunciation.

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