Chapter 21, Lecture 5 is Venus Return, Blood Velocity, Syncope, and Control of Blood Pressure. Venus Return is the volume of blood flowing back through the heart through the systemic veins due to pressure generated by contraction of the left ventricle. The pressure difference, remember, things are going to go down their pressure gradient the same way. Blood is going to move down its pressure gradient the same way that solutes. ions and things like that move down their concentration gradient.
So the venules, by the time the blood gets to the venules, it's only at about 16 millimeters of mercury pressure. The right ventricle is usually about zero. And that's a small gradient, but it's enough to cause venous return to the heart.
If the pressure increases in the right atrium of ventricle, then venous return decreases. So this could be due to leaky tricuspid valve. that lets blood regurgitate and causes a buildup of blood on the venous circulation side.
It can also be due to congestive heart failure. When you stand, it's difficult for blood to pump against gravity to the heart, and so there's two pumps, other than the heart, that help the blood move back to the heart. One is the skeletal muscle pump, and the other is the respiratory pump.
Both of these are dependent upon valves in the veins. So when you stand, both valves in the vein are open and blood flows through the heart. When you contract your leg muscles, it squeezes the vein and that pushes blood through the proximal valve and closes the distal valve. And then the muscles relax and the pressure falls. And it closes the proximal valve and opens the distal valve and the vein fills with blood from the foot.
And so it ratchets it up. chamber by chamber by chamber up through your leg or through your arm until it reaches your torso. The respiratory pump is based on alternating compression and decompression of veins caused by the diaphragm.
So the diaphragm looks like a dome when you exhale. When you inhale it moves downward and flattens out and that decreases pressure in the thorax and increases pressure in the abdomen. So then Blood is going to move from the compressed abdomen to the decompressed thoracic veins because there's more pressure then in the abdomen than there is in the thorax. And it's going to allow blood to flow into the thoracic region. When pressure reverses during exhalation, the valves in the veins close and prevent backflow.
So different things that increase blood pressure. We have things that are going to... going to increase cardiac output or increase systemic vascular resistance. Those that increase cardiac output increased heart rate possibly because of increased sympathetic impulses or decreased parasympathetic.
Remember sympathetic is his fight-or-flight, parasympathetic is rest and digest. Also an increased stroke volume. and that can be because you have increased venous return. If you have more blood coming back to your heart, you're going to have an increased volume of blood that's flowing through it. This can be caused by venal constriction, respiratory pumps, skeletal muscle pump, and increased blood volume.
In terms of increased systemic vascular resistance, you can have an increase in blood viscosity because you've increased in blood cells or because you're dehydrated. You can have an increase in total blood vessel length. and that's because your body size is increased.
People that are very large are going to have an increase in increased total blood vessel length as well as people that are obese. You can also have vasoconstriction and vasoconstriction can happen because of a number of different reasons. Velocity of blood flow is the speed and it's inversely related to cross-sectional areas.
So again we're looking at the all of the blood vessels in an area the area each one added together so the aorta is very big has very big lumen has big cross-sectional area but the aorta then is going to when you consider how many blood vessels are coming off of it if you look at all those individual blood vessels all those arteries coming off of it and take their lumen size their lumen and add them together it's going to be greater. So here in blue, it's showing the cross-sectional area. So as we go from arteries and arterioles to capillaries, there are lots and lots and lots and lots of capillaries. And each one has very, very small lumen. But when you add them all together...
you have a huge cross-sectional area and the same with in the beginning with venules then you get down to veins and the vena cavi and you're going to decrease again so this is showing the cross-sectional area the velocity is going to be fastest here because this is where the blood pressure is the highest so the higher the blood pressure the higher the speed of movement And as we go down here, and the cross-sectional area increases, we are going to decrease the velocity. Now it's going to speed up a little bit again when you get to these larger, these vessels that have a larger lumen. The circulation time is the amount of time necessary for a drop of blood to leave the right atrium and return to it again.
Syncope is fainting. And this loss of consciousness, not due to head trauma, followed by spontaneous recovery, it's usually due to insufficient blood flow to the brain. And it can occur for a couple of different reasons.
You can have vasodepressor syncope, in which your body overreacts to certain triggers. It can be seeing the sight of blood. It can be seeing a bad injury or a fantasized injury.
It can be emotional stress. Somebody tells you, oh, so-and-so just died. Those types of things.
And so your body is going to overreact to that, slow your heart down, cause the blood vessels to dilate. So less blood is delivered, more goes to the legs, the brain doesn't get enough oxygen, and you faint. Situational syncope is pressure stress due to urination, defecation, or severe coughing. Drug-induced syncope can be caused by antihypertensive drugs.
So antihypertensive drugs are like diuretics. are going to try to decrease the pressure of the blood vessel on the blood by dilating it. So they work as vasodilators. So you have diuretics, you're decreasing the amount of fluid in it. Vasodilators, you're causing dilation so that the lumen gets larger.
In either case, you're not going to have enough blood pressure, and so you can faint. And tranquilizers, excuse me, slow everything down. Orthostatic hypotension is excessive decreases in blood pressure that can occur when standing up.
And this is generally, some people have this, they have to be very careful. If you think about it, when you're lying down, blood pools in your back, in the back of your legs, in your back. When you stand up, all that blood now has to redistribute throughout your body, so that enough blood gets to your brain. And in most of us, this happens fairly quickly.
quickly so that when we get out of bed we can stand right up and not faint. But in people that suffer from orthostatic hypotension, they have a real big problem. They have to be very careful when going from a lying position to a sitting position, from a sitting position to a standing position, because the redistribution of blood throughout their body is not as efficient. Blood pressure and blood flow are controlled by a number of things. Heart rate, stroke volume, systemic vascular resistance, and blood volume.
Some things are rapidly adjusted, and others are slow, long-term regulators. And others are going to adjust the distribution of blood flow. For instance, when you exercise, you need blood going to your skeletal muscle, not to your GI tract.
In the medulla oblongata, which is the lowest part of the brain stem, it's where the... brain stem connects to the spinal cord. The cardiovascular center is located.
Now the brain stem has three parts. It has medulla oblongata, the pons, and the midbrain. And in these three regions of the brain stem there are a lot of control centers.
So the brain stem isn't involved in any higher order thinking, but it does have clusters of cells that are going to serve as centers. that are going to take care of different, what I call housekeeping work. They keep your body alive so that your brain can think. So they're going to control your blood pressure, your heart rate, your breathing, your things like hiccuping, coughing, sneezing.
All of these types of reflexive things are controlled in the brainstem. So the brain stem is going to, the medulla is going to get input from a number of different sensory receptors, and then direct the appropriate output by increasing or decreasing nerve impulses through the sympathetic and parasympathetic branches of the autonomic nervous system. Remember sympathetic is fight-or-flight, parasympathetic is rest and digest, and they alternate throughout the day depending on what you're doing.
If you're relaxing and eating a meal or anything like that, your parasympathetic is dominant. If you're exercising, if you're running, if you're in an emergency situation, the sympathetic is dominant. Before physical activity, there can be an anticipatory increase in heart rate. And as physical activity begins, proprioceptors send information about limb positions to the cardiovascular center. So we have proprioceptors in our joints, in our limbs.
And your brain is aware all the time what you're... arms and legs are up to, what you're doing with them, because these receptors are constantly sending information about limb position. So proprioceptors are important.
Chemoreceptors monitor changes in blood. So they monitor things like hydrogen ion concentration, carbon dioxide concentration, oxygen concentration. Baroreceptors monitor stretching.
that's caused by pressure in the arteries and veins. So if there's a lot of stretching, then maybe you have to have some vasodilation. The cardiovascular center is going to send its output through the sympathetic and parasympathetic neurons.
The sympathetic impulses are going to reach the heart through cardiac accelerator nerves, so they're going to increase. The nerve impulses increase the heart rate, the contractility, but again they don't affect the rhythm. A decrease in impulses causes decreased heart rate and contractility.
Parasympathetic stimulation goes along the vagus nerves and decreases heart rate. And vasomotor nerves conduct impulses to smooth muscle in the blood vessel walls, changing their diameter. The vasomotor tone is the resting level of contraction or vascular resistance. Sympathetic stimulation of veins causes constriction that moves blood and increases blood pressure. So here you can see the cardiovascular center.
We have from the higher brain centers are sending input through the cerebral cortex, the limbic system, and the hypothalamus. From the proprioceptors, they monitor the joint movements. Baroreceptors monitor blood pressure.
And chemoreceptors measure hydrogen ion concentration, carbon dioxide, and oxygen. The output from the medulla is through the vagus nerve, that's parasympathetic, to decrease the heart rate. Cardiac accelerator nerves are sympathetic. They increase heart rate and contractility. And the vasomotor nerves are sympathetic, and they increase vasoconstriction.