Bovine biology series
Part - 12 Heart
The heart
Few organs of the body receive more attention than the heart. Perhaps the brain does, but it is far too complex of an organ for us to fully understand. The brain serves many purposes. But the heart, well, it is different. There are really two primary purposes of the heart. But what significant purposes they are! Indeed, even the brain itself must rely on the heart function for survival. It is in this context that I will write about this wonderful organ, simply adding text to the reams of text already written about this handful of muscle.
I remember some early post-mortems done on cows at the farm. The veterinary would slice open the belly, exposing the internal organs contained within the pericardial sac. One of them was the heart, an organ about the size of grapefruit. I remember as well the thoughts had about what made this thing work, what control was it under, and how could this organ push a volume of blood through the udder of cow and the brain simultaneously. Later on I would learn about this heart, not only in biological terms but in other ways too. But it is the biological part that is the focus of this lesson. For the other part, well, you only have to look in the mirror and you will find a reflection of your own very unique story.
So as I said the heart has two primary functions. The heart really has two parts as well. One side, the right side, I visited last month when I wrote about the lungs. Here is a more complete description of the right side of the heart. Since the end result is the lungs, then obviously blood that has traveled throughout the body giving up its oxygen and other nutrients to cells needs to be oxygenated. This oxygen-deficient blood arrives at the heart muscle in two veins. The superior one or the one that sits on top of the heart is called the superior vena cava. Blood from the upper extremities of the body is feed into this very large blood vessel and enters the right atrium. The other blood vessel feeding this part of the heart is the inferior vena cava, the vessel that is fed from the lower extremities of the body. Both of the vena cavas feed oxygen-deficient blood into the right atrium. The heart squeezes blood from this cavity into the right ventricle and out towards the lungs via the pulmonary artery. It is as simple as that.
This all occurs on the right side of the heart. The atrium portion of the heart is more of a receiving chamber or cavity. Once in the right ventricle, however, the heart muscle thickens because at this point it must be a pump, and a strong one at that. Keep in mind that both lungs are located just inches from the heart, so the overall distance from the right heart to the lungs is relatively small.
Now then the left side of the heart is quite a different story. For once blood has been oxygenated in the microscopic sacs of the lung tissue, and then the now bright red blood volume is pumped back towards the heart through two pulmonary veins. There are two, obviously, each originating from the left or right lung. This newly oxygenated blood flows into the left atrium, which serves as a receiving chamber or cavity. From here, blood now flows into the capacious left ventricle. This left side is quite different than its right counterpart. The right one simply pumps blood volume the relatively short distance to lung tissues; the left ventricle must do the heavy work of pumping this blood volume to the tips of the nose, the soft tissue of the hoof and all parts in between!
As you might expect, this means some very heavy pumping, and for those of us who know a bit about pumps, we know that if we increase the distance fluid must be moved, and given the physics of gravity and friction loss, then we will put in a bigger pump as distance is increased per unit volume. So it is with the left ventricle, a pump of very thick musculature ensuring a long life with no breakdowns! Thus the left ventricle contains the thickest and strongest muscle of any of the two ventricles and atriums. This left ventricle pumps the blood volume through the large artery called the aorta, which branches into smaller and smaller arteries feeding the tissues of the body. The aorta sits primarily on top or superior to the heart itself.
We really have two completely different hearts muscles. One side, the right side, feeds the lungs via the pulmonary artery with oxygen-deficient blood. The other side, the left side, feeds the body via the aorta with oxygen-rich blood. And after the tissues have used this blood, so to speak, then it is pumped back to the heart once more.
Consider this. The blood volume through the udder tissue is 500 parts per one part of milk synthesized. Yes, you read that right. So, if a cow is producing eight gallons of milk every day, then 4,000 gallons of blood traveled through the udder! Taken one further step, this 4,000 gallons of blood will weigh 32,000 pounds or 18 tons! All of which is pumped by the muscular left ventricle.
Without a doubt the heart muscle, especially the left ventricle is a masterpiece!
The second primary function of the heart is a bit more complicated. But simply said, it is to keep going. That is, when ol' bossy is standing in the milking parlor, resting in a cow pasture, or eating in a lockup the heart must keep ticking. How does this happen?
Well certainly these two muscular pumps are working involuntarily. We do not have to think about them, as they must keep pumping. There must be then a pacemaker of some sort. So there is, and it is called the sinoatrinodede or SA for short. The SA is a very special bundle of tissue situated near the top of the right atrium. It is the SA that controls the pumping action of the heart muscles, adjusting them according to tissue demand for more oxygen in times of heavy exercise or when a cow is under stress such as calving. It is the SA node that slows the heart during rest, allowing a comfortable respiration rate while the action of digestion is occurring, and the flow of nutrients from the digestive system into the liver or from the rumen directly into the bloodstream.
The SA node is a coordinator. You all know that I like to use the concept of homeostasis. Well, at the very foundation of homeostatic integrity is the SA node.
The SA node is driven by electrical conductivity, which is derived from the proper mix of ions in the blood. This is extremely important, for when one ion, say, potassium as a positively charged cation, is depleted, the heart can stop. For it is potassium and other ions, such as chloride as a negatively charged anion, that work in concert to provide the proper electrical charge that controls the wondrous SA node.
I mention potassium for a special reason. We Marathon runners make sure there is plenty of potassium in our diets (bananas). We do so because during the course of these 26.2 miles the body loses salt, of course, but along with the sodium and chloride ions there is a loss of potassium too. And if our blood potassium level drops too far for too long, then the SA node is impaired. The heart stops.
From the SA node there are bundles of nerve fibers that encircle the atriums and the ventricles. In these nerve bundles are the message to pump....pump....pump. Now in a very large extent the rate of pumping is controlled by exterior factors, away from the SA node. That is, the brain can control the rate of heart pumping when nerve cells or blood parameters dictate a certain change.
For instance when a meal is consumed a cow prefers to lay down in a clean, dry spot and begin the digestion of this meal. During this time blood volume is directed away from the large movement types of muscle and towards the rumen and intestine area where the end products of digestion enter. During this time the mind will slow the heart rate so the body can rest. Similar to you taking a nap after brunch on Sunday afternoon! Then there is a case where the cow interprets something out of the ordinary. Heart rate is increased, increasing the potential blood volume to the flight muscles, the muscles that will carry the body as fast as possible when the gate is left open and the corn field beckons. At this point, blood flow near the digestion portion of the body is significantly reduced. So the rate can be altered, either slowing or increasing the heart muscles, but it is the SA node that really controls the beating itself.
We know that all muscles must have a plentiful supply of blood to fuel them. The heart muscles, the myocardium, are fed by the coronary arteries. They encircle the heart with bright red blood vessels, full of oxygen-rich blood that is required for muscle movement. The blood inside the heart is merely passing through; the coronary arteries are exterior to the muscle. It is these coronary arteries that if plugged or thickened or impaired, can cause heart disease and death in mammals. This condition is known as arteriosclerosis.
This, then, is a very brief discussion of the heart. A biological discussion. As we can fully recognize, centuries ago men of medicine were awestruck at this organ and its ability to pump blood into the far reaches of the body proper. For this reason, the heart simply is a favorite metaphor for poets and storytellers when matters of a persons' being were written about. What we call "matters of the heart".
Next month we will examine arteries and veins, and then in the month after an in-depth examination of blood itself, completing the part of the body known as the respiratory and circulatory systems.
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