B5- the human body-staying alive

• Exchange of Materials


  All organisms need to exchange things with their environment
• Humans exchange substances with their environment
• Humans and other animals must take in the substances they need from the environment and get rid of any waste products
• Diffusion and osmosis

  1. Cells need oxygen for aerobic respiration, which produces carbon dioxide as a waste product. These two gases move between cells and the environment by diffusion.

  2. Water is needed for many chemical reactions. It's taken up by cells by osmosis. Dissolved food molecules (the products of digestion, e.g. glucose and amino acids) diffuse along with it. These are then used in synthesis reactions.
• CO2 and urea need to be removed before they reach toxic levels
• Urea removal

  Urea (a waste product from the breakdown of proteins in the liver) diffuses from cells into the blood plasma. It is then filtered out of the blood by the kidneys and excreted as urine.
• Surface area to volume ratio (SA:V)

  A ratio that shows how big one value is compared to another. The larger an organism is, the smaller its surface area is compared to its volume.
• Multicellular organisms

  • They have relatively small surface area to volume ratios, which makes diffusion to and from cells deep within their bodies slow.
  • They usually have specialised exchange surfaces to increase their SA:V and therefore the rate at which substances are able to diffuse.
  • They tend to have a mass transport system (e.g. a circulatory system) to move substances around the body--this shortens the distance these substances have to diffuse to and from cells.
• All of the movement into, out of and around the human body is dependent on the gaseous exchange, circulatory, digestive and excretory systems all working together.
• Main types of blood vessels

  • Arteries
  • Capillaries
  • Veins
• Arteries

  • Carry blood away from the heart
  • Walls are strong and elastic
  • Walls are thick compared to lumen size
  • Contain thick layers of muscle and elastic fibres
• Lumen

  The hole down the middle
• Capillaries

  • Really small
  • Form networks called capillary beds
  • Carry blood close to every cell
  • Have permeable walls for diffusion
  • Supply food, oxygen, remove waste
  • Walls are usually one cell thick
• Veins

  • Take blood back to the heart
  • Walls don't need to be as thick as arteries
  • Have a bigger lumen than arteries
  • Have valves to help blood flow in the right direction
• How exchange surfaces in humans are adapted so that substances can move through them

  1. Diffusion

  2. Osmosis

  3. Active transport
• The job of the lungs is to transfer oxygen to the blood and to remove waste carbon dioxide from it
• Alveoli

  • Millions of little air sacs where gas exchange takes place
  • Surrounded by a network of tiny blood vessels called capillaries
• Gas exchange in the alveoli

  1. Oxygen (O₂) diffuses from the air in the alveoli into the blood in the capillaries

  2. Carbon dioxide (CO₂) diffuses in the opposite direction
• Alveoli

  • Specialised to maximise the rate of diffusion of O₂ and CO₂
  • Have an enormous surface area (about 75 m² in humans)
  • Have a moist lining for dissolving gases
  • Have very thin walls (consisting of cells with partially permeable cell membranes)
  • Have a good blood supply
• Absorption of digested food in the small intestine

  1. Digested food is absorbed in the small intestine

  2. The inside of the small intestine is covered in millions and millions of tiny little projections called villi

  3. Villi increase the surface area so that digested food is quickly absorbed into the blood by active transport and diffusion
• Absorption of glucose

  • When there's a higher concentration of glucose in the intestine, it diffuses naturally into the blood
  • When there's a lower concentration of glucose in the intestine, it is actively transported into the blood
• Villi

  • Have a single layer of surface cells to assist quick absorption
  • Have partially permeable cell membranes, which regulate the movement of substances across them
  • Have a very good blood supply to assist quick absorption
• Water absorption in the large intestine

  Water is absorbed into the blood from the large intestine by osmosis
• Circulatory system

  Transport system that moves substances around effectively in multicellular organisms
• Circulatory system in humans

  • Made up of the heart, blood vessels and blood
  • Has a double circulatory system - two circuits joined together
• Double circulatory system

  1. Heart pumps deoxygenated blood to lungs to take in oxygen, oxygenated blood returns to heart

  2. Heart pumps oxygenated blood around body, blood gives up oxygen at body cells, deoxygenated blood returns to heart to be pumped to lungs
• As blood is pumped around body, it also travels through blood vessels near exchange surfaces including villi and kidneys
• Heart

  • Uses four chambers (right and left atria and ventricles) to pump blood
  • Has valves to ensure blood flows in the right direction
• Blood flow through the heart

  1. Blood flows into two atria from vena cava and pulmonary vein

  2. Atria contract, pushing blood into ventricles

  3. Ventricles contract, forcing blood into pulmonary artery and aorta, out of heart

  4. Blood flows to organs through arteries, returns through veins

  5. Atria fill again, cycle repeats
• Left ventricle

  • Has thicker wall than right ventricle to generate greater pressure to pump blood around whole body
• Cardiac muscle

  • Contains many mitochondria to provide ATP for muscle contraction
  • Supplied by two coronary arteries branching from aorta to provide oxygen and glucose for respiration
• The diagram shows the right and left side of the person, not the viewer's right and left
• The Circulatory System

  Multicellular organisms need transport systems to move substances around
• Circulatory system

  Made up of the heart, blood vessels and blood
• Humans and other mammals have a double circulatory system


  • Two circuits joined together
  • Heart pumps deoxygenated blood to the alveoli in the lungs to take in oxygen, then oxygenated blood returns to the heart
  • Heart pumps oxygenated blood around all the other organs of the body, blood gives up its oxygen at the body cells, deoxygenated blood then returns to the heart to be pumped out to the lungs again
  • As it is pumped around the body, the blood also travels through blood vessels near exchange surfaces - including the villi (where it picks up food molecules and water) and the kidneys (where it is filtered and urea is removed)
• Blood

  A tissue, consisting of many similar cells working together
• Components of blood

  • Red blood cells
  • White blood cells
  • Platelets
• Plasma


  The liquid bit of blood, a pale yellow liquid which carries various substances around the body
• What plasma carries

  • Red blood cells
  • White blood cells
  • Platelets
  • Water
  • Digested food products
  • Carbon dioxide
  • Urea
  • Hormones
  • Antibodies
• Red blood cells

  • Small and have a biconcave disc shape to give a large surface area to volume ratio
  • Packed with haemoglobin, which combines with oxygen in the lungs and releases it in body tissues
  • Don't have a nucleus, freeing up space for more haemoglobin
  • Small and very flexible, can easily pass through tiny capillaries