Cells in mammals require a constant supply of nutrients and oxygen, and a way to remove waste products. Blood is great, as it does all that. Blood needs a way of getting to all cells of the body, a way to… circulate. Without that, blood would just get pulled by gravity towards the centre of the earth. Not a pretty sight I’m afraid.
There are two circulations in the body:
1. The pulmonary circulation takes blood from the heart, pumping it to the lungs in order to oxygenate it.
2. The systemic circulation takes blood from the heart to everywhere else. Eyes, legs, hand, bum, you name it.
Key point: the oxygen-rich blood vessels entering an organ are called arteries, while the oxygen-depleted blood vessels leaving an organ are called veins.
So a blood vessel entering the liver or kidneys would be an artery. A blood vessel leaving the liver or kidneys would be a vein.
The liver attribute is hepatic ( for example, the working cell unit in the liver is the hepatic cell), while the kidney attribute is renal (for example, renal failure). So what would the blood vessel entering the liver be called?
…the hepatic artery! Same principle applies to the rest: the hepatic vein, the renal artery and the renal vein.
There’s a catch (welcome to biology). In the case of the blood vessels leaving or entering the lungs, the rules are reversed. The pulmonary vein carries oxygenated blood to the heart, while the pulmonary artery carries deoxygenated blood into the lungs.
You also need to learn the blood vessels entering and leaving the heart.
1. The aorta is the main artery which carries oxygen-rich blood to the rest of the body.
2. The coronary arteries supply blood to the heart itself (and they are the affected arteries in coronary heart disease).
3. The superior vena cava and the inferior vena cava bring deoxygenated blood from the upper half of the body, and the lower part of the body respectively.
It’s all really logical… apart from the bit on the lungs.
There are 4 types of blood vessels: arteries, arterioles, capillaries and veins. Each type has a different function, and therefore a different structure. Here is a diagram of how arteries branch off into arterioles, then into capillaries, and eventually into veins as the blood becomes deoxygenated.
So what do they do?
Arteries must be able to counteract the pressure created by every heart beat by recoiling, so that the stream of blood is smoothened.
Arterioles are able to direct blood supply to certain parts of the body, so must be able to constrict or dilate.
Capillaries are the site of substance exchange as well as diffusion, so their walls must be thin enough for this to happen quickly.
Veins are unique as they contain valves which prevent backflow of blood.
Arteries and veins contain squamous endothelium (flat, single-celled layer), smooth muscle tissue alongside elastic and fibrous tissue.
From the diagram it is clear that there are important structural differences between arteries and veins, which reflect their different functions. Firstly, veins have valves while arteries do not*. Secondly, arteries have a narrower lumen (hollow diameter) than veins. Thirdly, arteries have a thicker wall of muscle and elastic tissue.
Arteries and arterioles are similar. The key difference is that arteries have more elastic tissue than muscle, while arterioles have more muscle than elastic tissue.
Capillaries are 1-cell thick squamous endothelium, making them very thin and permeable.
*except for the pulmonary artery and the aorta
Blood pressure is measured using a sphygmomanometer. These can be manual or electronic and provide the systolic and dyastolic blood pressure readings (SBP and DBP).
Manual sphygmomanometers must be used alongside a stethoscope to establish the starting point of the blood movement and its free-flow point. Sphygmomanometers work by constricting an artery and then releasing it in a controlled manner. Pressure readings are then taken, all via the sphygmomanometer’s three central components: the inflatable cuff, the mechanism for inflating it (manual or electronic), and measuring unit (mercury column or aneroid gauges; aneroid gauges don’t use fluid, instead they use metallic elements that can sense pressure).
The SBP is higher than the DBP, and both are used to determine whether blood pressure is in the optimal range. Low blood pressure is hypotension while high blood pressure is hypertension.
Low blood pressure doesn’t necessarily present any symptoms, and is otherwise a healthy state to be in. If it’s low enough to be an issue, it can have consequences including dizziness and fainting.
High blood pressure also doesn’t necessarily present itself with symptoms, but can have significant health consequences over time, as it puts extra strain on organs such as the heart, brain, eyes and kidneys. High blood pressure over time increases the risk of developing heart disease, stroke, kidney disease and other illness.
Lymph is formed from the interstitial fluid found throughout tissue and is similar in composition to blood plasma, lacking red blood cells. The lymphatic system plays a role in circulating blood back to the heart as well as passing it though the lymph nodes to clear it of any infectious agents.
Tissue fluid is found between cells and its role is to transport oxygen and nutrients from blood to cells, and waste products and carbon dioxide back to blood. It is similar to blood plasma, lacking blood cells and proteins.
Hydrostatic pressure determines the flow of these fluids between the different systems in a tissue. The oxygenated blood travelling through capillaries from arterioles has a high hydrostatic pressure, pushing the liquid outwards through small pores. This contains all the nutrients and gases cells need, which cross plasma membranes via diffusion or facilitated diffusion. Big components of blood such as cells and plasma proteins cannot escape the capillary, so stay in the blood.
The tissue fluid is then collected back via a water potential difference between itself and blood. Water moves by osmosis back into the blood in capillaries due to blood having more solutes and hence a lower water potential. This is a special subtype of osmosis called oncotic pressure and is derived from the presence of proteins such as albumin in blood that has the effect of pulling water into the capillary.
Some tissue fluid, such as excess tissue fluid, doesn’t return to blood and instead is drained into the lymphatic system. This forms lymph which is poorer in oxygen and nutrients, and returns to blood later on.
Blood carries nutrients such as glucose, hormones such as antidiuretic hormone, excretory products such as urea, and heat around the body.