B4

Multicellular organisms need a transport system to move material between exchange surfaces. The human circulatory system consists of the blood, the blood vessels, and the heart.

Blood is a tissue. The liquid plasma contains red blood cells, white blood cells, and platelets.

Blood plasma transports many substances including:

• Carbon dioxide from the organs to the lungs.
• Soluble products of digestion from the small intestine to the other organs.
• Urea from the liver to the kidneys,where urine is made.

Blood clotting is a series of enzyme-controlled reactions.

• The final reaction causes fibrinogen to change into fibrin
• Fibrin forms a network of fibres that trap blood cells and form a clot.
• The clot dries and forms a scab

• Are biconcave discs and do not have a nucleus.
• Contain the red pigment called haemoglobin.
• Use their haemoglobin to combine with oxygen, which forms oxyhaemoglobin in the lungs.
• Carry the oxygen to all the organs, where the oxyhaemoglobin and oxygen.

Your red blood cells contain haemoglobin that binds to oxygen to transport it from the lungs to tissues.

• Have a nucleus.
• Form part of the body's defence system against microorganisms.

Some white blood cells produce antibodies, some produce antitoxins, and others engluf microorganisms.

White blood cells help to protect the body against infection.

• Are small fragments of cells
• Do not have a nucleus
• Help the blood to clot at the site of a wound.

Platelets are cell fragments that start the clotting process at wound sites.

Blood flows around the body in three main types of blood vessel: arteries, veins, and capillaries.

Arteries:

• Carry blood away from the heart
• Have thick walls containing muscle and elastic tissue.

Veins:

• Carry blood towards the heart
• Have thinner walls than arteries
• Often have valves along their length to prevent backflow of blood

Capillaries:

• Are narrow, thin-walled vessels
• Carry the blood through the organs
• Allow the exchange of substances with all the living cells in the body

In human beings and other mammals the blood vessels are arranged into a double circulatory system.

• One transport system carries blood from your heart to your lungs and back again.
• The other transport system carries blood from your heart to all other organs of your body and back again.

The heart is a muscular organ that pumps blood around the body. It is made up of two pumps held together.

• Arteries carry blood away from the heart. Veins carry blood back to the heart.
• The right pump forces deoxygenated blood to the lungs where it picks up oxygen and loses carbon dioxide.
• After returning to the heart, the oxygenated blood is then pumped to the rest of the body by the left pump, which needs a bigger force.
• The heart has four chambers. The upper ones are atria. The right atrium recieves blood from the vena cava. The left atrium recieves blood from the pulmonary vein.
• The atria contract together to move blood into the lower chambers, the ventricles. When the ventricles contract they force blood out of the heart. The right ventricle pushes blood into the pulmonary artery. The left ventricle pushes blood into the aorta.
• The left ventricle has the thicker wall. Valves in the heart prevent the blood from flowing in the wrong direction.

• The heart muscle is supplied with oxygenated blood via the coronary arteries.
• Coronary heart disease occurs when the coronary arteries become blocked narrower due to a buildup of fatty material inside them.
• Doctors can use a stent to open up the arteries, allowing the blood to deliver nutrients and oxygen to the heart muscle again.
• Bypass surgery can also be used to replace damaged coronary arteries with lengths of vein.
• Statins are prescribed to lower cholesterol, which in turn reduces the fatty buildup in arteries.

Doctors, scientists and engineers have developed ways to solve problems with damaged hearts.

• Leaky valves mean the blood could flow in the wrong direction. Artificial or animal valves can be inserted in the heart to replace damaged valves.

Adults have a natural resting heart rate of about 70 beats per minute. The natural resting heart rate is controlled by a group of cells that act as a pacemaker.

The natural pacemaker is located in the right atrium.

Sometimes the rhythm of the heart becomes irregular if the natural pacemaker does not work properly.

• An artifical pacemaker is an electrical device that can be fitted in the chest to correct irrefularities in the heart rate.
• If a person has a very weak or diseased heart they may require a heart transplant. Donors are not always available so artificial hearts are being developed.

Artificial hearts can be used to:

• Keep patients alive while waiting for a heart transplant.
• Allow the heart to rest as an aid to recovery.

One disadvantage of an artificial heart or artificial valve is that the person needs drugs to prevent the blood from clotting.

• Moving air in and out of the lungs is called ventilating the lungs, or breathing.
• The lungs contain the exchange surface of the breathing system.
• The lungs are situated in the thorax, inside the ribcage and above the diaphragm, which seperates the lungs from the abdomen.

Breathing in:

• The intercostal muscles, between the ribs contract moving the ribcage up and out.
• The muscles of the diaphragm contract and the diaphragm flattens.
• The volume of the thorax increases.
• The pressure in the thorax decreases and air is drawn  into your lungs.

Breathing out:

• The intercostal muscles of the ribcage and the diaphragm relax.
• The ribcage moves down and in and the diaphragm becomes domed.
• The volume of the thorax decreases.
• The pressure increases and air is forced out.

Your lungs are adapted to make gas exchange more efficient.

• Oxygen is absorbed from the air into the blood in the lungs. Carbon dioxide is removed from the blood to the air.
• Efficient exchange surfaces have a large surface areaa, thin walls or a short diffusion path, and an efficient transport system.
• The lungs contain the gasaeous exchange surface. The surface area of the lungs is increased by the alveoli.
• The alveoli have a large surface area, thin walls, and a good blood supply.
• The lungs are ventilated to maintain a steep concentration gradient.
• Oxygen diffuses into the many capillaries surrounding the alveoli, and carbom dioxide diffuses back out into the lungs to be breathed out.

Plant tissues are collections of cells that are specialised to carry out specific functions.

Plant tissues include:

• Epidermal tissue, which covers the plant.
• Palisade mesophyll, which has many chloroplasts and can photosynthesise.
• Spongy mesophyll, which has some chloroplasts, many air spaces between the cells, and a large surface area for diffusion of gases.
• Xylem, which transports water and dissolved mineral ions from the root to the rest of the plant.
• Phloem, which transports dissolved food substances from the leaves to the rest of the plant

Plant organs:

• The plant tissues are arranged to form organs.
• Each plant organ has its own functions.
• Stems, roots, and leaves are plant organs.
• The plant organs form a plant organ system to transport substances around the plant.

Flowering plants have seperate transport systems.

• Phloem tissue carries dissolved sugars from the leaves to the rest of the plant including the growing regions and the storage organs. This process is called translocation.
• Xylem tissue transports water and mineral ions from the roots to the stem, leaves, and flowers.

The importance of transport in plants:

• All the cells in the plant need the sugar produced in photosynthesis for respiration.
• The sugar and mineral ins are needed for growth.
• Water is needed for photosynthesis.
• Water is needed to support the cells, particularly in young plants and in the leaves.

Water loss from the  leaves:

• Gases diffuse in and out of leaves through tiny holes called stomata. The size of the stomata is controlled by guard cells that surround them.
• Water is absorbed from the soil by the roots. The water passes through the plant to the cells in the leaves.
• In the leaves, water evaporates from the cells  in the leaf into the air spaces between them. This water vapour diffuses out of the plant through the stomata on the leaf surface when the stomata are open. This is transpiration.
• The movement of the water through the plant is called the transpiration stream
• The guard cells can close the stomata to prevent excessive water loss.

A plant could dehydrate if the rate of evaporation in the leaves is greater than the rate of water uptake by the roots.

Factors that increase the rate of photosynthesis, increasing the opening of stomata to let carbon dioxide into the leaf, will also increase the rate of transpiration. These factors include:

• Temperature - as temperature increases, the molecules move faster so more water evaporates and the rate of diffusion of water from the leaf also increases; as temperature increases, the rate of photosynthesis also increases, so more stomata are open.
• Humidity - the rate of diffusion of water from the leaf is faster in dry air than in damp air.
• Air flow - windy conditions increase the rate of evaporation and keep a steep concentration gradient between the inside and outside of the leaf by blowing away the water vapour
• Light intensity - More light means there will be an increase in the rate of photosynthesis

So transpiration is more rapid in hot, dry, windy, or bright conditions.

Plants can control water loss

• Plants have a waxy, waterproof cuticle on the leaves that can be very thick and shiny in hot environments.
• Most of the stomata are on the underside of the leaf.
• Wilting of the whole plant can also reduce water loss. The leaves collapse and hang down, which reduces the surface area.
• The stomata can close, which stops photosynthesis but prevents more water loss and further wilting.