The task of the arterial system is actually very simple; transport blood from the heart to the tissues. Because the heart ejects blood at a high pressure, from the ventricles into the main arteries (aorta and pulmonary artery), and because the pressure in the tissue is low, blood flows easily from high to low pressure
The blood flow in the major arteries is pulsatile; that is, the blood flow and its pressure increases with every heartbeat. Between the beats
The highest value is the systolic pressure and the lowest value is the diastolic pressure. Typical values are 120 mmHg for the systolic pressure and 80 mmHg for the diastolic pressure (in the aorta). This is written as 120/80 mmHg.
Remember that in the pulmonary system, the blood pressures are much lower than in the (large) systemic circulation. In the pulmonary system, the blood pressures are typically 25/8 mmHg.
You can feel this pulsatile action of the arteries when you feel the pulse, for example in the wrist. This has been used for centuries!
When one talks about the blood pressure, one really talks about the blood pressures in the systemic circulation, not in the pulmonary system.
By the way, the systolic and the diastolic values, which together are known as the “blood pressure”, are two of the most important vital signs.
What are actually the “Vital Signs”?
1 . consciousness
2 . body temperature
3 . blood pressure
4 . heart rate
5 . respiration rate
How do you measure the blood pressure?
The blood pressure can be measured directly (but this is difficult, messy and bloody as you have to stick a needle inside an artery) or indirectly.
Indirectly, you can measure the blood pressure with a sphygmomanometer (awful word!) and a stethoscope.
Another important value is the Pulse Pressure: this is the difference between the systolic pressure and the diastolic pressure.
Example: if the systolic pressure is 120 mmHg and the diastolic pressure 80 mmHg, then the pulse pressure is 120-80 = 40 mmHg.
Sometimes (not often), instead of the systolic and diastolic blood pressure, an average pressure is used. This is the Mean Arterial Pressure (=MAP) and is equal to the Diastolic Pressure + the Pulse Pressure divided by 3. Example: Systolic P = 120 mm Hg and Diastolic P = 80 mmHg, then the MAP = 80 + (120-80)/3 = 80 + 13 = 93 mmHg
The blood pressure is always given as systolic/diastolic. Example: 120/80 mmHg. Never the other way around! (i.e. 80/120 mmHg). That would be extremely confusing.
Students often think that a blood pressure is always stable and fixed at 120 mmHg systolic and 80 mmHg diastolic pressure. This is not the case at all. The blood pressure varies considerably between every person and from moment to moment.
Students often think that the blood pressure declines a lot from the aorta to the arteries that are far away such as the arteries in the foot. This is not true. The decrease in blood pressure along the arteries is actually very small, only a few mm Hg. A significant decrease in blood pressure occurs at smaller vessel sizes and especially in the arterioles (see next page).
Manual measurements of the blood pressure (with the sphygmomanometer) are not very accurate. Therefore, values are often rounded to the nearest 2-5 mmHg. So a reading of 124.5/76.3 mmHg is nonsense. More realistic in this case would be: 125/75 mmHg.
Some teachers like to talk about the windkessel function of the aorta. The word “windkessel” is from German and means "air-chamber".
The windkessel function relates to the fact that when the blood is ejected from the left ventricle into the aorta, there is then such an increase in blood pressure that the walls of the aorta will expand. After ejection, during diastole, when the aortic valves have closed, the walls of the aorta, because they are elastic, will rebound or constrict and thereby push the blood further down the arteries. This action works like a second ‘pump’ but its magnitude is often exaggerated.
This is not really an important factor.
D. The arteries in the (large) Systemic Circulation:
Although strictly not physiological, but rather anatomical, I have plotted for your convenience the major arteries in the human body in this diagram.
The first artery is the aorta, the largest, biggest and longest artery in the body. It carries blood from the heart (the left ventricle), first along an arch (the aortic arch), then onwards through the thoracic aorta, perforating the diaphragm, into the abdominal cavity where it is called the abdominal aorta.
From the aorta, numerous arteries emerge. From the arch, the common carotid arteries (to the brain), the subclavian arteries (to the shoulders and the two arms). And in the abdominal aorta, numerous arteries branch out to all the abdominal organs including the kidneys (renal arteries).
Finally, low in the abdomen, the aorta splits into two iliac arteries, which, in the upper leg form the femoral artery, which, in the lower legs, splits into the anterior and posterior tibial arteries.
The Systemic Arteries
Note that both in the hands and in the feet, the vessels develop into an arterial arcade to perfuse the muscles of the palms, soles, fingers and toes.
Of course, this is only a very rough overview. There are many, many, more vessels coming out of these vessels. This is only an overview of the major arteries!
And these are the major arteries in the pulmonary circulation. What a contrast with the arteries in the Systemic (=Large) circulation!
This is really a ‘small’ circulation, at least in terms of length of the vessels.
From the right ventricle, blood flows through a short main pulmonary artery before splitting into two major vessels, the right and the left pulmonary arteries. Thereafter, the vessels split into many more and smaller arteries, just as in the systemic circulation.
So, on the one hand, the pulmonary circulation is much ‘smaller’. This is the main reason why the pressure in these arteries is much lower (i.e. approx. 25/8 mmHg) than in the systemic arteries (120/80 mmHg).
However, on the other hand, don’t forget that this circulation also copes with a throughput of 5 liters/min. Not a small thing at all!