Life Processes and Living Things

Circulation (Higher)

Blood One function of the circulation system is to transport substances around the body in the blood. Blood consists of a fluid called plasma containing white blood cells, red blood cells and platelets. Many substances are transported dissolved in the plasma.  These include: Carbon dioxide from the organs to the lungs Soluble products of digestion from the small intestine to other organs Urea from the liver to the kidneys White blood cells form part of the body’s defence system against micro-organisms.  They have a nucleus. Platelets are fragments of cells.  Platelets help blood to clot at the site of a wound.  They do not have a nucleus. Red blood cells transport oxygen from the lungs to the organs.  They do not have a nucleus.  They are packed with a red pigment called haemoglobin.  In the lungs, oxygen combines with haemoglobin to form oxyhaemoglobin.  In other organs the oxygen is released as the oxyhaemoglobin splits up into oxygen and haemoglobin.

The Heart The heart pumps blood around the body.  The wall of the heart contains muscle fibres. The diagram below shows the structure of the heart. Blood returns to the heart from the body in veins.  It enters the right atrium, which contracts forcing the blood into the right ventricle.  When this contracts it forces blood out of the heart through the pulmonary artery to the lungs, where carbon dioxide is removed and oxygen is absorbed. Blood from the lungs enters the left atrium of the heart and is forced into the left ventricle.  This chamber has a very thick muscular wall to push blood out of the heart into the main artery, theaorta, and around the body.  There are valves in the heart to make sure that blood flows in the correct direction. The heart is said to be a double pump, because the right side of the heart pumps blood to the lungs, and the left side pumps it around the body.  This is called a double circulation.

Blood Vessels  Blood travels away from the heart to the organs through arteries, and returns through veins. Arteries have thick walls containing elastic fibres, so they can stretch as blood is forced through, and muscle to pull them back into shape. Veins have thinner walls because the pressure inside them is much lower than in the arteries.  They often have valves to prevent the back-flow of blood. Capillaries join arteries to veins.  They are very narrow, thin-walled blood vessels.  Substances, like oxygen and food needed by the cells in body tissues, pass out of the blood through the capillary walls.  Substances produced by the cells, for example carbon dioxide, diffuse in the opposite direction. Breathing (Higher) Breathing System | Breathing   Breathing System The function of the breathing system is to take in oxygen from the air and get rid of carbon dioxide.  This is called gas exchange.  The diagram below shows the structure of the breathing system. The lungs are found in the upper part of the body called the thorax. They are protected by the ribcage and separated from the abdomen below by a sheet of muscle called the diaphragm. Gas exchange occurs across the walls of the alveoli.  They are adapted for this function in several ways: There are many of them, so they provide a very large surface area They have a moist surface, so oxygen can dissolve before diffusing into the blood They are surrounded by capillaries, so have a good blood supply to carry the gases Oxygen diffuses into the blood from the air in the alveoli and attaches to red blood cells to be transported around the body. Carbon dioxide diffuses out of the blood plasma into the air in the lungs to be breathed out.     Breathing Breathing is the movement of the diaphragm and muscles between the ribs which results in air moving into or out of the lungs.  This movement of air is called ventilation. Breathing in is called inhalation. The muscles between the ribs contract pulling the ribcage up and outwards At the same time the diaphragm contracts and flattens These movements increase the volume of the thorax The pressure inside the thorax decreases so air is forced into the lungs Breathing out is called exhalation. The muscles between the ribs relax so the ribcage moves down and in At the same time the diaphragm relaxes and becomes dome-shaped. These movements decrease the volume of the thorax The pressure inside the thorax increases so air is forced out of the lungs