Label a diagram of the circulatory system of mammals.
	Describe the structure and function of capillaries.
	Label a diagram of the human respiratory system.
	Explain how gas exchange occurs in the lungs.
	Explain how the lungs are adapted for gas exchange.
	Describe how breathing occurs and how it is controlled.
	Describe the composition of inhaled and exhaled air.

Circulatory

Mammals and birds have the most complex circulatory systems of all animals. These are known as double circulation because two circulatory systems are used to transport blood around the whole of the body.

Systemic circulation carries oxygenated (oxygen-rich) blood from the heart to the cells of the body and deoxygenated back to the heart. The other, the pulmonary circulation, carries deoxygenated blood to the lungs and deoxygenated blood to the heart. Blood has to pass into the heart twice on its journey round the body.

Can you locate the aorta, pulmonary vein and pulmonary artery?

Can you find the carotid artery? Where does it take blood?

The two venae cavae (great veins) transport blood to the heart. What are the two venae cavae called? Where do they bring blood from to the heart?

Linking arteries and veins are capillaries. These are tiny blood vessels which spread throughout the tissues of the body. They are found very close to all cells in the body and provide the cells with all the substances they need. They are also involved in the removal of waste products. Useful substances (e.g. oxygen and food) diffuse from the blood into the cells and waste products (e.g. carbon dioxide and urea) diffuse from the cells to the blood for removal.

1. Capillaries are very tiny so that the capillary system provides a huge surface area for diffusion of substances into and out of the cell to occur.
2. Capillaries are very thin in diameter so that the blood passes slowly through them, thus giving more opportunity for diffusion.
3. The walls of capillaries are made of squamous epithelium which is very thin and so allows diffusion to occur easily.
4. The walls of capillaries have no elastic fibres, smooth muscle or collagen which would interfere with the ability of capillaries to penetrate between individual cells.

Tissue fluid

Capillary walls are permeable to everything in the blood apart from red blood cells and plasma proteins. As blood flows through capillaries under pressure from the arterial system, fluid is squeezed out of the vessels. This fluid fills the spaces between the cells and is known as intercellular fluid or tissue fluid. It is through this fluid that diffusion between the blood and the cells takes place.

Much intercellular fluid is returned to the blood. Some of it returns by osmosis and diffusion at the venous end of the capillaries. Most of it drains into a system of tubes called the lymph capillaries and once in these vessels the fluid is called lymph. The lymph capillaries join up to form larger and larger vessels through which lymph is transported around the body, by using the squeezing effect of muscular movements, until it reaches the neck area where the lymph is returned to the blood via the left and right subclavian veins.

Lung structure and function

The branching structure of the lungs with each bronchus splitting up into many bronchioles and then millions of tiny alveoli (air sacs) gives the lungs a huge surface area for gas exchange to occur. The lungs have a total surface area of 70 square metres which is about the size of two tennis courts. Each alveolus is covered with a network of tiny capillaries and so there is a great supply of blood for gases to be exchanged with.

Gas Exchange

The role of the respiratory system is to provide body cells with oxygen for respiration and to remove carbon dioxide, the waste product of respiration, from the body. Air from outside the body is taken into the lungs and eventually into the alveoli. In the alveoli the oxygen from the air is exchanged for carbon dioxide in the blood. This process is called gas exchange. The exchange happens by diffusion because the concentration of oxygen is greater in the air than in the blood, so oxygen moves into the blood, and the concentration of carbon dioxide is greater in the blood than the air, so carbon dioxide moves from the blood to the air.

The evidence for gas exchange having taken place can be seen in the relative compositions of inhaled and exhaled air.

Why do you think the air in the alveolus has less oxygen and more carbon dioxide than exhaled air?

Fick’s Law

Use the information on Fick’s Law (10.3) and the structure of the alveolus (10.6) to allow you to be able to:

Ventilation

Ventilation is the process by which air is taken into and out of the lungs.

When we breathe in, the intercostal muscles (the muscles between the ribs) contract causing the ribs to move up and outwards, and the diaphragm contracts which causes it to flatten. This causes an increase in the volume of the abdominal cavity and thus a decrease in pressure. As the pressure is lower than the air pressure outside the body, air rushes into the lungs.

When we breathe out, the intercostal muscles relax causing the ribs move down and inwards, and the diaphragm relaxes which causes it to arch. This causes a decrease in the volume of the abdominal cavity and thus an increase in pressure. As the pressure is lower than the air pressure outside the body, air rushes out of the lungs.

The basic stimulus to inhale and exhale is given by an area of the hindbrain, located in the medulla, known as the respiratory centre. This stimulus is carried by phrenic nerves to the diaphragm and intercostal muscles. This gives a basic, slow breathing rhythm. Stretch receptors in the bronchi, other receptors, including ones in the respiratory center, which are sensitive to carbon dioxide levels in the blood and the higher centres of the brain interact to give a finely tuned respiratory response to most situations.

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