Biorhythms (cont.)
High blood pressure (hypertension)
Hypertension or high blood pressure is a silent killer. Early high blood pressure produces no symptoms or
discomfort. However, untreated long-term high blood pressure can lead to
strokes, heart diseases, kidney disease, and eye damage. Medical scientists now know that a person's
blood pressure varies according to the time of day (circadian rhythm). Such
variations may have important implications in the diagnosis, treatment, and
monitoring of patients with high blood pressure.
It is important to understand a critical distinction:
Circadian rhythms do not cause clinical hypertension in normotensives (people
with normal blood pressure). Circadian rhythms do, however, result in a cyclic
worsening of hypertension in hypertensive patients.
In the majority of individuals - normotensive and hypertensive - blood
pressure rises rapidly in the early morning hours, the time when most
individuals wake up and begin their day. This morning rise in blood pressure is
not caused by the act of getting out of bed and beginning activity. It is a
genetically preset system that automatically increases a person's blood pressure
at that time.
For example, if a person goes to sleep at around 10 p.m., beginning in the
very early morning - sometime between 3 a.m. and 5 a.m. – the blood pressure
begins to rise and continues to rise through the morning.
This increase, which is accompanied by an increase in heart rate, corresponds
to the following biologic changes, which are also governed by circadian rhythms:
- Increased secretion of catecholamines, particularly norepinephrine, into the
bloodstream.
- Increased plasma renin activity.
Catecholamines, norepinephrine, and renin are natural
hormones produced by the kidneys and adrenal glands (small glands located near
the top of both kidneys). These hormones cause tightening of blood vessels in
the body (vasoconstriction). Vasoconstriction causes resistance to blood flow and
elevates blood pressure.
The vasoconstricting effects of catecholamines may also be enhanced in the
morning by the high levels of certain hormones which interact with
catecholamines and increase their effects. Many high blood pressure medications
are designed to counteract the vasoconstrictive effects of these hormones.
In the late morning or early afternoon, the natural rise in blood pressure
reaches its peak. After that, blood pressure declines, falling 15 to 20 mmHg
between about 8 p.m. and 2 a.m., the time at which blood pressure is normally at
its lowest point.
The blood pressure pattern just described is typical of some individuals,
whether the individual is hypertensive or normotensive. These people are
referred to as "dippers." In other individuals (called "nondippers"), there is
no nightly decline in blood pressure. The blood pressure in these people remains
high, even during sleep.
Disease of the heart and blood vessels (cardiovascular
disease), including hypertension and the syndromes of ischemic heart disease
(angina, heart attack, sudden death), also follow circadian rhythms.
This pattern makes sense when you consider that heart rate and blood levels of
enzymes and
catecholamines that strongly influence blood pressure and heart function follow
circadian rhythms.
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