Blood is a tissue in the body that plays a variety of roles in transport and in defending the body against disease. It is an unusual tissue since it is a liquid, with many different kinds of cells suspended in a watery solution called plasma.
Plasma makes up 55% of the volume of blood and is a solution of many different chemicals in water. For example, the plasma contains dissolved glucose, amino acids and other products of digestion from the intestines. It also transports the waste molecule urea from the liver where it is made to the kidney where it is excreted. Blood plasma contains dissolved carbon dioxide, mostly in the form of hydrogencarbonate ions. Many hormones (for example testosterone, ADH, adrenalin) are transported in the blood plasma and because the plasma is mostly water, it provides a good way of moving heat around the body from respiring muscles to the skin where it can be lost.
The most common cell in blood are the red blood cells (or erythrocytes). These tiny cells are adapted for the transport of oxygen. Each red blood cell contains around 270 million molecules of a transport protein, haemoglobin. Each molecule of haemoglobin can bind up to four molecules of oxygen in the lungs and then unload the oxygen when the red blood cell passes through a capillary in an actively respiring tissue.
(Don’t worry too much about the structure of the protein – this is A level stuff really…. Just remember haemoglobin is a transport protein for oxygen found in red blood cells)
As well as being packed full of haemoglobin molecules, red blood cells have other adaptations for transporting oxygen. Red blood cells lose their nucleus during their development as this allows more haemoglobin to be packed into each cell. Having no nucleus means the red blood cell cannot divide nor repair damage to its structure. This is why each red blood cell only lives for 100-120 days in the body.
Red blood cells have a characteristic shape. It is called a biconcave disc and they have an especially flexible shape. Remember that a capillary is actually smaller in diameter than a red blood cell, so the cells have to squeeze through capillaries in single file…..
I have had a request from a student to write about the level of details needed in the section of the specification on human transport. Here are the relevant bullet points from the specification, together with a very brief outline of the kinds of details to learn:
- Blood composition 55% plasma, 45% cells (red blood cells, white blood cells and platelets)
- Plasma functions – transport of dissolved carbon dioxide, dissolved glucose, urea, salts etc.and transport of heat around body
- Red Blood cells – no nucleus, each cell packed full of 250,000 molecules of haemoglobin, biconcave disc shape to squeeze through narrow capillaries
- Phagocytes/Lymphocytes – two types of white blood cell, phagocytes engulf foreign organisms in blood by phagocytosis, lymphocytes do many functions in defending the body against disease but many produce antibodies
- Vaccination with reference to memory cells and primary v secondary response (see below)
- Functions of clotting and role of platelets (prevent infection, stop blood loss – platelets play central role in clotting as they produce chemicals that are needed for clotting cascade
- Structure and function of the heart (learn names of chambers, blood vessels, names of four sets of valves and what they do)
- Role of adrenaline in changing heart rate during exercise (speeds it up to maximise cardiac output to muscles)
- Structure and functions of arteries/veins/capillaries (simple bookwork)
- General plan of circulation including heart, lungs, liver and kidneys (see below)
The two sections that are perhaps hardest to interpret are the ones on vaccination and the general plan of the circulation.
1) Key terms in vaccination to understand:
- Clonal Selection theory
- Memory cells
- Effector cells (plasma cells)
- Primary response
- Secondary response
At the end of the process, you should be able to provide a clear concise answer to the following question?
Why is it that the first time your body encounters measles virus, you suffer from the disease measles? Why will someone who has had measles as a baby (or been immunised against it) never contract the disease measles even though the virus might get into their body many subsequent times?
2) The blood vessels involved in the four organs mentioned are described below.
Heart – receives blood from the coronary arteries which branch off the aorta before it has even left the heart: Why doesn’t the cardiac muscle in the heart just get the oxygen and nutrients it needs from the blood in the chambers?
Lungs – pulmonary artery takes blood from right ventricle to the lungs, pulmonary vein return oxygenated blood to the heart and empty it into the left atrium. What is unique about the composition of the blood in the pulmonary artery?
Liver – has a most unusual blood supply. There is a hepatic artery that branches off the aorta and brings oxygenated blood to the liver. Blood also goes to the liver in the hepatic portal vein which brings blood from the small intestine. Blood in the hepatic portal vein will contain lots of dissolved glucose and amino acids, both of which are processed in the liver. Deoxygenated blood leaves the liver in the hepatic vein. Find a diagram to show the arrangement of these three blood vessels.
Kidney – straightforward blood supply in that there is a renal artery and a renal vein. (important idea is that the renal artery is much much bigger than you would expect from the size of the organs: 25% of the cardiac output of blood flows through the kidneys on each circuit) Why do you think this is?
I hope this helps – more to follow when I get home from my holidays tomorrow afternoon…..