Week 3 Blood Pressure Regulation

Week 3 Blood Pressure Regulation

November 7, 2019 0 By Bertrand Dibbert


Blood pressure is mainly affected by cardiac
output and total peripheral resistance. Cardiac output is the amount of blood pumped from
the heart per minute. Cardiac output is the amount of blood ejected by the ventricle times
(which is the stroke volume) multiplied times the heart rate (contractions per minute).
An increase in HR will increase CO if the SV remains the same. This is the same for
stroke volume. If the SV increases, the CO increases. Total peripheral resistance is
the resistance created by the blood vessels throughout the body. An increase in blood
volume will increase cardiac output. Anything that causes an increases in preload, which
is the end diastolic volume, will increase stroke volume and therefore increase cardiac
output. An easy way to think about this is that is there is more blood going into the
heart, there will be more blood being forced out.  If the contractility of the heart increases,
there will be a greater cardiac output. This means that if the heart has a greater force
of contraction, it will eject a greater volume of blood. The other factor affecting blood
pressure is total peripheral resistance. This is the pressure in the blood vessels that
the heart must push blood out against. So, you can imagine that if the total peripheral
resistance increases, this would decrease the stroke volume decreases and this decreases
the cardiac output. If the blood vessels open, we call this dilation or vasodilation. Vasodilation
results in decreased peripheral resistance and this will increases cardiac output. The
opposite is also true for vasoconstriction, which is the tightening of the blood vessels.
If the blood vessels, specifically the arteries, constrict, the peripheral resistance increases
and the cardiac output decreases. Think of this like trying to blow into a balloon. When
there is very little air in the balloon, it’s easy to blow in more air, there is very little
resistance. Once the balloon is full of air, the balloon pushes back at you, this is like
vasoconstriction, it’s more difficult to put air into the balloon. It’s important to understand
how the body controls blood pressure. When the blood pressure drops, multiple organs
will respond to bring the blood pressure back up. Why? Well, if you don’t get blood to your
brain, your brain can’t function, so your body works very hard at getting blood to your
brain. Let’s look at how your heart responds. When your blood pressure drops, the heart
immediately responds by increasing the HR and increasing the force of contraction. 
The heart rate and contractility will increase, increasing cardiac output and therefore increasing
blood pressure. If you increase contractility, we know that this will increase stroke volume
and therefore increase CO. By increasing CO, your body increases blood pressure. By increasing
heart rate, CO increases and therefore blood pressure. In response to a drop in blood pressure,
the arterioles will vasocontrict. The arterioles will vasoconstrict and when they vasoconstrictor,
this increases total peripheral resistance, therefore increasing blood pressure. The kidney
is also important in blood pressure regulation and this is where the Renin Angiotensin System
is important. In general, the kidney will respond to changes in blood pressure by increasing
or decreasing the amount of urine excreted. The greater the fluid output from the body
as urine, the less blood volume and therefore, the lower the blood pressure. A drop in blood
pressure will stimulate renin. Renin stimulates a cascade of effects. Renin stimulates the
conversion of angiotensinogen to angiotensin I. Angiotensin I is converted to angiotension
II by angiotensin converting enzyme. Angiotensin II has multiple effects:
It increases the secretion of anti-diuretic hormone (ADH) also known as vasopressin. Remember
that ADH’s role is to conserve water. ADH stimulates water absorption in the collecting
duct. ADH is the hormone inhibited by both caffeine and alcohol (to what extent we’ll
talk about later this semester). This is why you may find yourself making multiple trips
to the bathroom when drinking a lot of coffee or alcohol. But, if there is more ADH, then
the body will reabsorb more water. Angiotensin II also stimulates vasoconstriction
and remember that this will increase peripheral vascular resistance and by increasing PVR,
blood pressure will go up. Angiotensin II also stimulates aldosterone
secretion which increases tubular sodium and chloride reabsorption. Where sodium goes,
water follows. If sodium is reabsorbed, so is water, which increases blood volume and
blood pressure.   Angiotensin II also stimulates sympathetic
activity which also causes vasoconstriction and results in an increase in blood pressure.