CIRCULATORY SYSTEM
(TRANSPORT SYSTEM)
All living organisms need to exchange materials between themselves and their environment e.g. oxygen and carbon dioxide. The cells of organisms need a constant supply of nutrients and oxygen. Waste products like carbon dioxide need to be removed from these cells as well. The function of a transport is to transport material from one place to another (e.g nutrients need to be moved from the digestive system to all the cells of the body).
Revision
Diffusion, osmosis and active transport are three very important processes that involve the movement of particles of a substance.
Diffusion is the movement of particles from an area of high concentration to an area of low concentration.
Osmosis is a special form of diffusion where water molecules move from a region of high concentration to an area of low concentration through a partially permeable (or selectively permeable) membrane.
Active transport involves the movement of particles against a concentration gradient, through a selectively permeable membrane using energy.
With active transport particles move against a concentration gradient which means movement in the direction opposite that of diffusion – or – movement from an area of lower concentration to an area of higher concentration. Hence, this process will require expenditure of energy.
Movement of Substances in Small Organisms
In small organisms (e.g. one-celled (unicellular) organisms) substances like food and gases can move around the cell easily. Water and pass in and out of the cell membrane by osmosis. Gases like oxygen and carbon dioxide can enter and leave the cell by osmosis.
In the case of these small organisms the surface area of the cell membrane is large enough to allow enough substances to pass in and out so that the organism can live.
Small organisms can live because the Surface area is large compared to the Volume of the cell. This is called the Surface area to volume ratio.
Amoeba
The area of the cell membrane determines how much oxygen can diffuse into the cell
For small organisms diffusion and osmosis is sufficient to transport substances around the cell, as well as in and out the cell.
Movement of substances in Large Organisms
Large organisms like plants and animals are made up of many cells (multicellular). The surface area is much smaller than the volume in these organisms (their SA/ Vol. ratio is small). These organisms need a transport system to carry substances throughout the body. Simple diffusion alone would take too long to transport materials from one place to another.
Functions of the transport system within the body of multicellular organisms include
v Transporting digested food to the cells of the body
v Removal of waste products from the body (e.g. urea)
v Transporting oxygen from the lungs to the cells
v Transporting carbon dioxide from the cells to the lungs
v Transporting hormones to the sites where they are needed
v Transporting water, mineral salts and vitamins through out the body
v Protecting against disease (white blood cells and other molecules circulate in the blood to fight disease causing micro-organisms)
v Regulating heat in the body by dilation and constriction of blood vessels
v Assisting in repair of the body by quickly repairing cuts and bruises by forming clots
Plants have transport systems that carry
e.g water and minerals – xylem
sugars and other substances – phloem
Animals have transport systems that carry
e.g. blood – circulatory system
oxygen and carbon dioxide – respiratory system
The Human Circulatory System
In humans (and in many animals) there is a system that carries blood through the body. The function of the circulatory system is to transport substances through out the body. Substances that are transported include food, gases, waste and cells.
Structure of the circulatory system
The human circulatory system is made up of
1) blood vessels which carry blood around the body
2) blood which flows through the blood vessels and contain materials to be transported
3) the heart which is a pump to keep blood flowing through the blood vessels
Blood Vessels
Blood vessels are tubes through which blood travels around the body.
There are 3 types of blood vessels
1) arteries- these carry blood away from the heart
2) veins- these carry blood back to the heart
3) capillaries- these are tiny vessels that pass close to all cells and link arteries and veins
Vein Artery Capillary
Vein Artery
Arteries
Arteries are thick muscular vessels which are elastic. These blood vessels carry oxygen-rich (oxygenated) blood (arterial blood) from the heart to the rest of the body. The arteries branch repeatedly forming smaller blood vessels called arterioles which end in tiny capillaries.
The arterial blood is pumped out of the heart at high pressure and this puts a lot of pressure on the walls of the arteries. The thick muscular walls of the arteries must be able to stretch under pressure as the blood passes through and recoil (pull back) to push the blood through. The elastic walls of the arteries prevent them from bursting as the blood rushes through.
Veins
Veins are wider tubes than arteries and have thinner walls with less muscle and elasticity. They carry oxygen-poor (deoxygenated) blood from organs back to the heart. The pressure of this venous blood is much lower than that in the arteries and therefore puts very little pressure on the walls of the veins. Veins must be able
Ø to allow blood to pass through easily and
Ø to prevent blood from flowing back.
Veins branch into smaller vessels called venules which are connected to capillaries.
Blood moves through veins with the help of
1) muscle movement which helps push the blood back towards the heart
2) valves (flaps of tissue) along the walls which can move in one direction only. These one-way valves prevent the backflow of blood in the veins.
valves open valves closed
Capillaries
Capillaries are very small blood vessels that are one cell thick and are found close to every cell of the body. These blood vessels allow for fast exchange of materials between the blood and the cells of the body. Nutrients and oxygen reach each cell and take away waste produced by the cell (e.g. carbon dioxide). The thin walls of capillaries are permeable, to allow water and dissolved substances such as oxygen, carbon dioxide and dissolved food to pass in and out.
Capillary with red blood cells Exchange between cells and capillary
Capillaries connect arteries to veins. The arteries branch into smaller vessels called arterioles which connect to capillaries and the capillaries connect to small veins called venules. These in turn connect to larger vessels called veins.
Arterioles connect to capillaries which connect to venules
Blood
At any time there is about 5 L of blood in your body.
Blood is considered a liquid tissue that is made up of
(1) plasma (90% water and 10% dissolved substances)
(2) red blood cells which carry oxygen from the lungs to the rest of the body.
(3) white blood cells which protects the body from disease by killing germs
(4) platelets which are cells that help stop bleeding by forming scabs (clots).
Plasma
Plasma is a straw coloured liquid that consists of water and dissolved substances. That is :
1) water (90%)
2) blood proteins (e.g. antibodies)
3) soluble foods from digestion
4) mineral salts (e.g. Ca2+,Na+,Cl- etc.)
5) waste products (e.g. carbon dioxide and urea)
6) hormones
7) gases (oxygen and carbon dioxide)
The main function of the plasma is to transport substances around the body. Plasma makes up 55% of the blood. The other 45% is blood cells.
Functions of Plasma. To transport
Ø dissolved foods from the ileum to tissues
Ø carbon dioxide as HCO3- (hydrogen carbonate ion) from tissues to the lungs
Ø other wastes from tissues to excretory organs
Ø hormones from glands to target organs
Ø heat from muscles and liver to all parts of the body
Ø ions and water to maintain correct concentration of body fluids
Ø white blood cells and antibodies to sites of infection
Ø platelets and plasma protein to cuts
Blood Cells
Blood contains different types of cells-red blood cells, white blood cells and platelets
Each type of cell has a different function.
Red Blood Cells (RBC)
These are also called erythrocytes. Red blood cells live for about 120 days. RBC contain a protein pigment called haemoglobin which contains iron and this gives them the red colour. Haemoglobin is responsible for transporting oxygen in the RBC from lungs to cells in the body.
Oxygen binds reversibly to haemoglobin to form oxyhaemoglobin. As blood passes through tissues with little oxygen, the oxygen from the red blood cell is given up for the respiring cells to use. Each molecule of haemoglobin can combine with up to 4 molecules of oxygen.
Haemoglobin + oxygen oxyhaemoglobin
oxyhaemoglobin Haemoglobin + oxygen
RBC have several special features that make them efficient at carrying oxygen:
Ø they are shaped like bi concave discs. They are much thinner in the middle than at the end. This makes the surface area to volume ratio greater allowing for rapid diffusion of oxygen into and out of the cell.
Ø They have no nucleus and mitochondria and are also missing other organelles which allow more room for the haemoglobin molecule in the cell and therefore more oxygen to be carried. By the RBC
Ø They are very small and changes shape which allows them to squeeze through capillaries and be flattened against the capillary walls. This brings the red blood cells very close to the tissue cells and allows diffusion to occur rapidly.
White Blood Cells (WBC)
White blood cells are also called leucocytes. There are about 1 WBC to every 700 RBC. WBC have a life span of about one day or less and are also made in the bone marrow.
WBC have several features that makes them different from RBC.
v They all contain a nucleus
v They are spherical or irregular in shape
v Most are larger than RBC
v They can change shape and squeeze through walls of capillaries into the fluid that surrounds tissue cells.
The function of the WBC is to protect the body against infection. There are two main types of white blood cells
ü Phagocytes which remove foreign particles and micro-organisms like bacteria and dead cells. They do this by phagocytosis where the phagocytes engulf foreign particle before ingesting and digesting it.
Phagocytes Lymphocytes
ü Lymphocytes which produce chemical substances called antibodies. These antibodies protect us from disease-causing organisms (pathogens)by making them clump together for easier digestion by phagocytes or by neutralizing their toxins.
Platelets
These are cell fragments which are formed when a small part of a large cell in the bone marrow breaks off. They have a lifespan of about 10 days and are very small. Platelets do not have a nucleus, but contain mitochondria. Platelets are very important in the process of blood clotting which seals off any wound to prevent excessive loss of blood and entry of pathogens into the blood.
Formation of a Blood Clot
When the skin is cut and a small blood vessel is broken, platelets stick to the walls of the damaged blood vessel and swell releasing chemicals that stimulate more platelets forming a platelet plug. If it is a small cut then a platelet plug may be all that is needed.
If the cut is large then a clot is then formed by fibrinogen which is a soluble protein that is always present in the blood. The fibrinogen is converted into an insoluble protein called fibrin. Fibrin forms a mesh of protein threads across the wound and this traps blood cells and more platelets. The whole mass is then called a blood clot.
The bone marrow is a spongy red tissue found in the centre of bones. It contains specialised cells called stem cells which can produce many different types of cells – red blood cells, white blood cells and platelets.
Heart
The function of the heart is to keep blood flowing throughout the body. Each time the heart beats it forces blood into the arteries and into other blood vessels. The blood returns to the heart through the veins. It makes a circle and this is why the system is called a circulatory system.
The heart beats more than 100,000 times a day, circulating about 5 L of blood. The heart weighs less than a pound and is slightly larger than a fist. In humans blood circulates twice –
1) from the heart to the lungs and back to the heart (to collect oxygen and release carbon dioxide)
2) from the heart to the rest of the body (to supply oxygen and other substances to the cells and collect waste)
This type of circulatory system is called a double circulatory system.
The right side of the heart pumps oxygen-poor blood to the lungs (pulmonary circulation) while the left side pumps oxygen-rich blood to the rest of the body (systemic circulation). Hepatic circulation refers to the path of blood from the intestines, gallbladder, pancreas, stomach and spleen through the liver.
The heart Inside the heart (L.S.)
Structure of the Heart
The heart is a large muscle made up of a special kind of muscle called cardiac muscle. It is about the size of your fist. The muscles of the heart contract and relax constantly due to electrical impulses which are sent out by a special group of cells called the Sinus node (or Pacemaker).