Hello, welcome to the Hypertension Medicine Course. I am Jessica.
In the first lecture, we must discuss a fundamental question—what is
the essence of blood pressure? Or, more abstractly, why do humans need
blood pressure?
If we do not clarify this foundational issue, all other discussions are
just castles in the air.
Three Common Misconceptions About Blood Pressure
It sounds simple. We have all measured blood pressure, and doctors
always tell us two precise numbers: systolic and diastolic, such as
145/95 mmHg or 128/75 mmHg.
Seeing that blood pressure is represented by just two numbers, many
people naturally form some opinions: some think human blood pressure is
just two points—systolic and diastolic; others believe blood pressure
is basically constant; or that blood pressure is the same throughout
the body, no matter where it is measured. After all, only then can two
simple numbers represent it!
But in fact, all three views are incorrect.
First, our blood pressure is not two points, but a continuous
waveform.
Most people may never see what real blood pressure looks like. No
problem. Let me take you to my daily workplace—Fuwai Hospital, to the
cardiac ICU to see real blood pressure.
For some critically ill patients, we need to monitor their blood
pressure in real time, not missing a single second. In this case, the
common blood pressure monitor, which measures once per minute, is not
enough. We place a precise pressure sensor into the patient's radial or
femoral artery—near the wrist or groin—to measure their blood pressure
every second.
On the monitor, blood pressure is no longer two numbers, but a
continuous waveform, like a series of small peaks—rising, falling,
rising again. The peak is the pressure when the heart pumps blood
through the vessel—what we call "systolic pressure"; the trough is the
lowest pressure when blood returns to the heart—"diastolic pressure".
Our blood vessels actually experience every blood pressure value
between systolic and diastolic, but we choose the two most
representative points to mark blood pressure, that's all.
Second, human blood pressure is not fixed; it varies greatly at
different times and under different circumstances.
For example, when you are lying down, stable, each peak and trough is
about the same. But if you change position, sit up, your heart rate
increases, you eat, or sleep, blood pressure changes accordingly.
At night, blood pressure can drop to 90/60 mmHg; if a tiger suddenly
appears, systolic pressure can instantly soar to 170 or even 200 mmHg.
Third, even at the same time, blood pressure varies throughout the
body.
Sometimes, blood pressure measured at the radial artery (wrist) is very
low, dangerously so, but at the aorta (near the heart), it may be
normal.
In fact, from the aorta to the capillaries at the fingertips, blood
pressure can decrease by 70%.
In summary, at different times, under different circumstances, and at
different vessel locations, our blood pressure is different.
Now you understand why doctors have so many requirements when measuring
blood pressure. Recall:
When you go to the hospital, you must be calm, seated, and blood
pressure is usually measured at the brachial artery (inside the arm).
The pressure recorded is the pressure of blood flowing through that
artery at that moment, transmitted through soft tissue to the skin
surface.
Notice? This is blood pressure measured at a specific time, under
specific conditions, at a specific artery, using an indirect method. It
does not reflect your overall blood pressure. For convenience, we use
this result to generally represent your blood pressure.
Key Factors Affecting Blood Pressure
You may wonder: Why is our blood pressure like this? High during
exercise, low during sleep, high in the aorta, low in capillaries—how
does it change?
Many people think blood pressure is the pressure of blood pumped by the
heart against the vessel wall, and the heart is the sole source. So,
blood pressure depends on the heart—stronger heart, higher pressure;
weaker heart, lower pressure. Many hypertensive patients ask me, "Dr.
Jessica, can you make my heart beat less forcefully?"
But this is incorrect. As long as the body is basically healthy,
everyone's heart power is similar. The heart is not the trigger for
hypertension.
If the heart is fine, then it must be the vessels. Yes, all blood
pressure problems are actually vascular problems. Hypertension is not a
heart disease, but a vascular disease.
Let's analyze how vessels affect blood pressure.
Blood pressure is the lateral pressure of blood against the vessel
wall, so it follows Ohm's law—pressure equals flow times resistance.
Flow is the amount of blood pumped by the heart. Resistance is the
resistance to blood flow.
Which variable affects blood pressure most? Let's see—
Normally, the heart pumps 4-5L per minute. Unless there is heart
failure or severe dehydration, this amount is basically constant. If
you drink 1L less or more, as long as your kidneys work, your body
adjusts urine output to keep blood volume stable. So, in health, flow
is not the main factor.
What is? The second variable—resistance.
Resistance equals a coefficient divided by the fourth power of vessel
diameter.
> R=8ɳl/πr4
> R: vascular resistance
> l: vessel length
> π: pi
> r: vessel diameter
> ɳ, l are constants; R, r are variables.
If there is no disease, blood composition does not change, so the
coefficient's effect is minimal.
That leaves one variable, which affects resistance—and thus blood
pressure—by the fourth power: vessel diameter. Vessel diameter is not
constant; it is affected by temperature, sleep, emotions, age, plaques,
elasticity, even breathing. This is the biggest variable system for
blood pressure.
For example, when calm, the aorta is about 3.2cm, systolic pressure is
120mmHg. If danger occurs and the aorta contracts to 0.9 times its
usual diameter (2.88cm), blood pressure can quickly rise to 182mmHg.
If vessel elasticity worsens and plaques (fat deposits) narrow the
vessel, even a slight narrowing raises blood pressure. Over time, this
easily leads to hypertension.
## The Essence of Blood Pressure: A Pulse Wave
Now we can answer the fundamental question—why do humans need blood
pressure? What does it do?
Blood pressure is essentially energy, energy emitted from the heart.
Energy changes, so blood pressure varies at different times and under
different conditions. Energy also attenuates, so at different locations
and vessels, resistance varies and blood pressure decreases.
Blood pressure throughout the body is the sum of this energy in
different spatial dimensions. Adding the time dimension, the periodic
changes in this energy are the oscillating energy field of life.
This energy field is the foundation for all organs, tissues, cells, and
even atoms in our body. It carries nutrients and oxygen in the blood to
every cell, sustaining life. Every function depends on the periodic
fluctuation and regulation of blood pressure. When a person dies, the
first sign is blood pressure gradually dropping to zero.
To some extent, the amplitude of this energy field determines the
working limits of all cells and the limits of our body. Life always
exists between the peaks and troughs of blood pressure—between the
highest systolic and the lowest diastolic.
Therefore, focusing only on two numbers from a blood pressure monitor
is meaningless. A transient high blood pressure does not necessarily
mean harm—it shows the momentary limit; low blood pressure is not
necessarily abnormal, as long as it meets the body's needs.
Next Lecture Preview
This lecture discussed how blood flow and vessel diameter affect blood
pressure.
How does the body manage these factors to ensure blood pressure rises
when needed and falls when needed, achieving precise regulation? We
will discuss this next time.
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