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Biology · Exchange and Transport in Animals

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Biology · OCR Paper 1

Exchange and Transport in Animals

Surface Area to Volume Ratio and Exchange Surfaces

A big organism has a small surface area compared with its volume, so it needs exchange surfaces and a transport system.

  • Surface area to volume ratio The surface area of an object divided by its volume. It gets smaller as the object gets bigger.
  • Surface area The total area of the outside of an object.
  • Volume The amount of space inside an object.
Three cubes with sides 1 cm, 2 cm and 3 cm, with their surface areas, volumes and surface area to volume ratios of 6 to 1, 3 to 1 and 2 to 1.
As the cube gets bigger, the ratio gets smaller.

Single-celled and multicellular organisms

  • Single-celled organism Large surface area to volume ratio; Every part of the cell is close to the outside; No transport system or special exchange surface needed
  • Multicellular organism Small surface area to volume ratio; The body surface alone could not supply the cells deep inside; Diffusion over such long distances is far too slow

The Heart and Double Circulation

The right ventricle pumps blood to the lungs. The left ventricle pumps blood to the rest of the body.

  • Double circulatory system A circulation in which blood passes through the heart twice on each trip round the body, once to the lungs and once to the body.
  • Atrium An upper chamber of the heart that receives blood.
  • Ventricle A lower chamber of the heart that pumps blood out.
A labelled diagram of the heart showing its four chambers, the vena cava, aorta, pulmonary artery, pulmonary vein and a coronary artery.
Blue shows deoxygenated blood and red shows oxygenated blood. The right side of the body is on the left of the picture.

The two sides of the heart

  • Right side Receives deoxygenated blood from the body; Pumps blood a short distance to the lungs; Thinner muscle wall
  • Left side Receives oxygenated blood from the lungs; Pumps blood to every organ of the body; Thicker muscle wall

Following the blood round the double circulation

  • Body to heart Deoxygenated blood returns from the body in the vena cava and…
  • Heart to lungs The right ventricle pumps the blood along the pulmonary artery…
  • Gas exchange In the lungs the blood picks up oxygen and gives up carbon dioxide.
  • Lungs to heart Oxygenated blood returns along the pulmonary vein to the left atrium.

Blood Vessels

Arteries carry blood away from the heart. Veins carry blood to the heart. Capillaries link the two.

  • Artery A blood vessel that carries blood away from the heart at high pressure.
  • Vein A blood vessel that carries blood to the heart at low pressure.
  • Capillary A tiny blood vessel with a wall one cell thick, where substances are exchanged with the cells.
Cross-sections of an artery with a thick muscular wall and narrow lumen, a vein with a thin wall, wide lumen and valve, and a capillary with a wall one cell thick.
The structure of each vessel suits its job.
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Biology · OCR Paper 1

Exchange and Transport in Animals continued

Artery and vein compared

  • Artery Carries blood away from the heart; High blood pressure; Thick wall with muscle and elastic fibres
  • Vein Carries blood towards the heart; Low blood pressure; Thinner wall

The Lungs and Gas Exchange

The alveoli are one cell thick, packed with capillaries and ventilated by breathing, which makes them a very effective exchange surface.

  • Trachea The windpipe, the tube that carries air from the mouth and nose towards the lungs.
  • Bronchus One of the two tubes that branch from the trachea, one into each lung.
  • Bronchiole A smaller tube that branches from a bronchus and ends in alveoli.
A labelled diagram of the lungs showing the trachea, bronchi, bronchioles, alveoli and the capillary network around the alveoli.
Air travels down the trachea and the bronchi to the alveoli.

Gas exchange in the alveoli

  • Breathe in Fresh air fills the alveoli.
  • Oxygen diffuses in The blood arriving is low in oxygen.
  • Carbon dioxide diffuses out The blood arriving is high in carbon dioxide.
  • Breathe out The air that leaves has less oxygen and more carbon dioxide than…

Blood

Red blood cells carry oxygen, white blood cells defend, platelets clot the blood and plasma transports everything else.

  • Plasma The pale yellow liquid part of the blood that transports dissolved substances.
  • Red blood cell A cell with no nucleus that carries oxygen around the body.
  • Haemoglobin The red pigment in red blood cells that joins with oxygen.
A magnified drop of blood showing plasma, red blood cells, white blood cells with nuclei and small platelets.
Red blood cells are the most numerous. White blood cells are larger and have a nucleus.

Red and white blood cells compared

  • Red blood cell Carries oxygen; Contains haemoglobin; Biconcave disc with no nucleus
  • White blood cell Defends against pathogens; Makes antibodies, antitoxins or engulfs pathogens; Has a nucleus