Transport in humans
The human circulatory system is a network of blood vessels connecting the heart to all the other organs of our body, in order to supply tissues with the crucial molecules that our cells need to respire. Our heart pumps an impressive 7000 litres of blood around our bodies each day to ensure a constant supply of glucose and oxygen and efficient removal of carbon dioxide.
Composition of blood
Our blood contains four main components: red blood cells, white blood cells, platelets and plasma.
Red blood cells: the haemoglobin inside red blood cells binds to and transports oxygen
White blood cells: detect any pathogens which are circulating in the blood and initiate an immune response
Platelets: form a blood clot at the site of a wound to prevent excessive blood loss and entry of microorganisms through the skin.
Plasma: a liquid containing carbon dioxide, digested food molecules, urea, hormones and heat energy
Adaptations of red blood cells
Red blood cells are highly specialised for transporting oxygen and are adapted in the following ways:
They do not have a nucleus - this creates more room inside the cell to pack in oxygen molecules
They contain haemoglobin - the haemoglobin molecule is great at bonding with oxygen, enabling the red blood cell to easily snatch up any oxygen molecules as they diffuse into the blood from the lungs
They have a biconcave shape - this maximises their surface area for oxygen absorption.
They are small and flexible - allowing them to squeeze through tiny capillaries
Action of white blood cells
The white blood cells hanging around in our blood can destroy pathogens in one of two ways:
Phagocytosis - a special type of white blood cell, called a phagocyte, essentially eats the pathogen alive by engulfing the pathogen and digesting the organism using digestive enzymes found inside the phagocyte. Fragments of the digested pathogen are then released from the white blood cell.
Antibody production - a different type of white blood cell releases protein molecules called antibodies which have a complementary shape to the pathogen. Antibodies bind to the pathogen, rendering it harmless.
Vaccination stimulates the production of antibodies against a particular pathogen. Usually a dead or weakened form of the pathogen is used, which is sufficient to stimulate antibody production without the person becoming ill. During activation of the immune response, memory cells are generated. These are a type of antibody-producing white blood cell which will ‘remember’ the pathogen if re-infection occurs. If the same pathogen is detected in the body, the memory cells already have the appropriate antibodies ready to enable a quick, effective immune response.
Structure of the heart
The heart is composed of a left side and a right side. The left side of the heart has a much thicker wall, as it needs to pump more strongly to deliver blood all around the body (whereas the right side just needs to send the blood to the lungs). We can divide the heart into four chambers - the ones on top are called atria and these chambers receive the blood from the veins supplying the heart. Blood flows from the atria to the ventricles, which are separated from the atria by valves to prevent blood flowing in the opposite direction. Movement of the ventricles forces blood into the arteries leaving the blood, to travel either to the lungs or to the other organs of the body.
Structure of blood vessels
Arteries carry blood away from the heart. The aorta is the largest artery in our body and delivers blood from the heart to all of our tissues. The pulmonary artery takes deoxygenated blood from the heart to pick up more oxygen in the lungs. Arteries pump blood under high pressure so they have thick muscular walls to withstand the high pressure without becoming damaged.
Veins carry blood towards the heart. The vena cava brings blood from the body to the heart and the pulmonary vein carries oxygenated blood between the lungs and the heart. Veins carry blood much more slowly at lower pressure, so they have thinner walls and a large lumen, with valves to prevent the backflow of blood. The low pressure of blood, large lumen and the fact that we can find veins closer to the surface of our skin makes them much easier to obtain a blood sample from compared to arteries.
Capillaries are tiny blood vessels which connect veins and arteries. They are extremely small (just one cell thick) and contain small holes called pores which allows substances to pass through them easily.
Changes to heart rate
Under stressful situations, our body releases adrenaline which increases our heart rate.
Our heart rate changes during exercise and when adrenaline is released.
Exercise: during exercise our cells are respiring more and using oxygen faster. In order to replace this oxygen and remove the increasing amounts of carbon dioxide, the heart needs to pump faster.
Adrenaline: in scary or exciting situations (for example, public speaking or on a roller-coaster), our bodies prepare ourselves for immediate action by releasing the hormone adrenaline. Adrenaline increases our heart rate to provide enough oxygen and glucose to our cells in preparation for running away. This process is called the ‘fight or flight response’.
Coronary heart disease
Coronary heart disease is caused by a blockage inside a coronary artery (a blood vessel which supplies blood to the heart muscle). If a blockage occurs, blood is not delivered to the heart muscle which will not receive oxygen for aerobic respiration, preventing contraction of the heart muscle.
The risk of developing coronary heart disease is increased by a number of factors including:
Smoking
Diet high in saturated fat and salt
High blood pressure
Did you know..
The same chemical reaction which is used by crime scene analysts to detect the presence of cleaned-up blood is the same chemical reaction used by fireflies when they glow. Their light is generated by a process called bio-luminescence as is used as a danger signal or to attract mates.
Download worksheet: Transport in Humans / Answers Next Page: Excretion
