The Heart and Blood Vessels

In this chapter you will learn:

  1. The heart and Blood Vessels

  2. Capillaries

  3. Red blood cells

  4. White blood cells

  5. Platelets

  6. Plasma

  7. The human gas exchange systems

  8. transport systems

The heart and Blood Vessels

The heart pumps blood around the body.

The heart is located in the centre of a human’s chest.

The wall of heart is made of a muscle tissue, which continually contracts and relaxes, a special type of muscle called the cardiac muscle.

The cardiac muscle does not get tired and works constantly.

The cardiac muscle required their own blood supply and oxygen so they can work non-stop.

The heart has two pumps to work the double circulation system.

The size of the heart is roughly the size of a fist.

The normal heart rate is typically 60 - 100 times per minute.

The heart has two sides, separated by a thick inner wall called the septum.

The deoxygenated blood is carried to the lungs to be oxygenated by the right side of heart.

The oxygenated blood from the lungs to the rest of the body is pumped by the left side of heart.

The heart has a total of four chambers: right atrium, right ventricle, left atrium and left ventricle to pump the blood.

The heart has valve systems to prevent the blood flowing backwards and ensure the blood only flows in the right direction and one way only.

The valve in the right side of the heart is called the TRICUSPID and the valve in the left side is called the BICUSPID.

Both sides have semilunar valves, they are located between the pulmonary artery and the aorta.

Blood passes through the heart twice in one complete circulation.

Each pump has an upper chamber called atrium that receives blood and a lower chamber called ventricle that pumps blood out.

Arteries have thick muscular walls which contain elastic fibres. The blood flows from the heart to the organs, including the lungs, through arteries

Arteries carry oxygenated blood from the heart to the tissues and organs of the body.

The contracting of the arteries can be felt at your wrist or neck as a pulse.

The arteries carry blood under high pressure so the artery walls need to be thick and strong and the blood vessels must be elastic, that allow artery to stretch under pressure.


Capillaries are very small thin blood vessels.

Capillaries carry blood to and from the body’s cells.

The wall of capillaries are only one cell thick and permeable .

Capillaries have large surface area to make exchange of substances as fast as possible.

Some capillaries are so thin that blood cells are unable to pass through them.

Oxygen, carbon dioxide, sugars, amino acids and other nutrients, and wastes are exchanged through the capillaries.

Blood is a mixture made up of red blood cells, white blood cells and platelets all suspended in a liquid called plasma.

Red blood cells

Red blood cells transport oxygen from lungs to all cells of the body.

Red blood cells have a biconcave shape that provides a large surface area to volume ratio to absorb the maximum amount of oxygen.

Red blood cells are small and flexible so that they can fit through narrow blood capillaries and pass easily through blood vessels.

Red blood cells have no nucleus, this allow more space for haemoglobin molecules.

White blood cells

White blood cells are part of the immune system and defend the body against infection.

White blood cells have nucleus. They are larger than red blood cells.

These are two main types of White blood cells: Lymphocytes and Phagocytes.

Lypmhocytes produce antibodies to attack and destroy virus or bacteria that enters the body.

Phagocytes destroy virus and bacteria by engulfing them in a process called phagocytosis.

About 70 per cent of white blood cells are phagocytes.

About 25 per cent of white blood cells are lymphocytes.

White blood cells are produced by your bone marrow.

A low white blood cell count usually increases the risk of getting infections.

But having a very high white blood cell count means you have an infection or leukaemia.


Platelets are small fragments of cells that help to clot blood.

Platelets protect the body by stopping bleeding, this reduces blood loss and prevents harmful bacteria from getting into your body through the blood system.

Platelets are produced by giant cells in the bone marrow.

Platelets do not have a nucleus.

When your platelet levels are lower than normal, you may suffer from excessive bleeding and bruising.


A clear straw-coloured liquid is called Plasma, makes up more than half the volume of blood.

Plasma carries everything needed for the cells in the body and removes waste products.

Exchange surfaces in cells and organs

All organisms need to take in essential substances such as food, oxygen, water, and other nutrients, from the environment.

All organisms also need to remove waste substances out of their cells.

Small organisms exchange these essential and waste substances between themselves and the environment.

How easily an organism can exchange substances with the environment depends on the organisms surface area to volume ratio.

The single-celled organism has no trouble in exchanging sufficient materials with its environment.

A single cell has a large surface area to volume ratio.

Note: Multicellular organisms have a smaller surface area to volume ratios compared to a single celled organism.

The bigger a multicellular organism, the more difficult it is to exchange substances.

The effectiveness of exchange surfaces in plants and animals is increased by having: a large surface area, thin permeable cell membranes and a moist exchange surface.

Animals have additional adaptations for effective exchange surfaces.

Animals have lots of thin blood vessels to bring in essential nutrient and ions for life and carry waste molecules away e.g. the thin bronchiole tubes in the lungs.

Thin capillaries in each villus in the small intestine which have a particularly large surface to volume ratio - this allows fast diffusion in either direction.

The tiny pockets called alveoli in the lungs greatly increase the gas exchange surface area to volume ratio.

The human gas exchange system

The main role of lungs is to transfer oxygen from air to the blood stream and to remove the waste gas carbon dioxide.

The primary organs of the respiratory system are the lungs, which function to take in oxygen and expel carbon dioxide as we breathe.

The lungs are in the upper part of the body (thorax), protected by the ribcage.

The human respiratory system is adapted to to take in oxygen and eliminate carbon dioxide, and for efficient gas exchange to happen.

Ventilation is the process by which air moves in and out of the lungs.

The windpipe (trachea) is the largest airway.

The trachea divides into two bronchi. One bronchus enters each lung.

Each bronchus spilt into smaller tubes called bronchioles.

Each bronchiole ends in a cluster of microscopic air sacs called alveoli.

Gas exchange takes place between the specialised moist thin membrane surfaces of the alveoli and the fine blood vessels in the capillaries that supply the lungs.

The alveoli are well designed by evolution to make gas exchange in lungs happen easily and efficiently.

The alveoli have a huge surface area because of their tiny spherical sac like structure. The total surface area of the alveoli is around 70 square metres.

The sac walls are very thin, only one cell thick, to reduce diffusion distance and diffusion time and giving a faster rate of gas exchange.

The alveoli are also lined with a thin film of moisture. the moisture in the membranes is good for dissolving gases and increases the rate of gaseous diffusion.

The movement of air into and out of the lungs is known as ventilation.

Fish gills

Gills are the gas exchange system in fishes.

Water, containing dissolved oxygen, enters the fish through its mouth and passes over the gills, and then out under the operculum.

Fish have very thin gills covered in protective muscular flaps.

Exchange of gases in fish is very efficient because of: the large surface area of the gills, the short distance required for diffusion and the efficient ventilation of the gills with water.

The surface area is increased even more by lots of tiny thin structures called lamellae.

The lamellae have a thin layer of surface cells so there is only a short diffusion distance and times.

Transport systems

In multicellular organisms transport systems are needed to supply essential nutrients and oxygen to their cells and remove waste products.

In single-celled organisms, these substances can diffuse directly into or out of the cell membrane, where they're needed.

In multicellular organism tubes or vessels carry materials from one part of the organism to another.

Transport systems make close contact with cells, such as those of exchange surfaces.

Substances are transported in the blood are oxygen, carbon dioxide, glucose and urea.

Other substances transported in the blood include: antibodies and hormones.

Humans have a double circulatory system consisting of two closed circuits,that are joined together. The heart acts as the central pump between the two circuits.