Bovine biology series
Part - 13 Arteries and veins
This is the second series on the cardiovascular system; the first appeared last month when we examined the heart. Next month we examine the fluid itself, the blood. This month we take a closer look at the system of blood vessels that link every cell of the body with the heart pump.
We may liken this blood vessel system to the road system that links all of us with every other part of the country. Using this metaphor, of course, can be taken to the next level. That is, just as the interstate freeway system is as different than the gravel lane at the end of a county road, so are the various blood vessels of the body serving different roles.
We begin with a group of vessels called arteries. The word is derived from the Greek word "windpipe". There are two arteries leaving the heart, fully discussed last month. The first is the pulmonary artery. Actually this artery, rising from the top of the right ventricle, bifurcates or splits into two major branches, each leading to the lungs. These arteries carry unoxygenated blood from the heart to the lungs. Once oxygenated, blood is dumped into the left side of the heart, where the very muscular left ventricle pumps the freshly oxygenated blood through the aorta, also situated on top of the heart. The aorta is a massive blood vessel, highly muscular and elastic, allowing for the high pressure and stretching due to blood volumes. Remember that from this left ventricle, the pumping of this left side of the heart moves blood into every reach of the body through the aorta.
Thus arteries are vessels that must be muscular and be elastic due to the pumping action of the left side of the heart......under high pressure. How much pressure? Well, the left ventricle pumps a velocity of 50-70 milliliters (2-2.3 ounces) of blood volume per stroke at a pressure of 125 milliliters of mercury. This is at rest, standing erect.
In terms of blood volumes, about ten percent of the total volume on the arterial side is in the right side of the heart, called the pulmonary side, being pumped to the lungs. That means ninety percent is on the left side, reaching the rest of the body, and we call that portion the systemic side. Of the total blood volume in the body, only about twenty five percent is found in the arterial side. The heart contains about seven percent of the total volume when both ventricles are fully engorged.
So the arterial system of the cardiovascular system operates under high pressure, low volume, and is responsible for transporting oxygenated blood to organs and tissues throughout the systemic body.
What happens at the organ, tissue and cellular level? Well, just as the massive freeways branch off towards smaller and smaller roadways, so do arteries branch and branch some more. As the artery becomes smaller the volume carried is smaller, and there is some loss of pressure as well. Eventually the arteries become arterioles, a name used to describe the smallest vessel carrying oxygen-rich blood. Arterioles are the final end of the systemic distribution system. It is here that blood is finally delivered to cells, in many cases in an arteriole that is large enough (in diameter) to only handle one red blood cell at a time.
The controlling mechanisms of the arterioles include the nervous control of smooth muscles comprising the lining of the arteriole, and chemical regulators such as catecholamines (a class of hormones including epinephrine, norepinephrine and dopamine that can be administered during times of metabolic shock. They serve the role of opening up the arterioles, increasing blood volume to cells and thus supplying blood nutrients like oxygen and glucose).
At this point, the blood volume is dispersed so finely that all cells are capable of being nourished. Some more than others. Cells found in the muscles, for instance, are characterized by a well-developed network of blood vessels. Without a doubt the most intricate network of vessels occurs in the brain, for the demand of nutrients is highest in this organ. Adipose tissue, is an exception. The fat cells making up ninety percent of adipose tissue require very little blood supply, as these fat cells serve as warehouses of lipid material, that very dense and high caloric substance the body stores away for future needs. Fat cells have a very low metabolic demand.....they just sit there.
At the cellular level, of course, blood volume is gleaned of its nutrients, largely under the guidance of chemical or sensory stimuli. For instance an udder that must begin lactating at calving is turned on by the stimuli of lactogen, a hormone that signals the secretory cells to remove glucose and other metabolites from the blood system forming lactose and other components of milk. Sensory stimuli might be the reaction of fright. A cow senses she is in danger of being shoved into the pathway of a crowdgate, or an electric fence, and so she moves. The nervous system stimulates the movement of muscles; the animal prepares for movement while the blood volume is directed towards the large leg muscles and away from other responsibilities, like digestion or lactation. Along the line of events is the increased level of glucose dumped into muscle cells, ready to fuel the movement of muscles in response to the frightful stimuli.
At the cellular level there are waste products that must be removed out of the body. They are collected by the capillary system, a series of very small blood vessels that are emptied into the larger veins, and eventually into the venous blood supply.
Blood that was pumped from the right ventricle, via the pulmonary artery and oxygenated in the lungs is dumped into the capillaries of the pulmonary arteries (there are two, one from each lung) and is returned to the heart. At this point the blood now enters the left side of the heart, and via the left atrium is dumped into the left ventricle and enters the arterial system, being pumped through the aorta.
The veins are quite different from the arteries. Their function is different, of course, in that they are returning to the heart a volume of blood that is oxygen-depleted, and is usually very much higher in the principal waste product of cellular metabolism, carbon dioxide.
Remember that one of the end products of catabolism, the conversion of chemical compounds into work in the presence of oxygen, is carbon dioxide. Another is metabolic water. Both are dumped into the venous blood volume for transport......carbon dioxide is exhaled via the lungs and metabolic water is shunted to the kidneys where it accumulates as urine.
Of the total blood volume, seventy five percent is found in the venous system. The blood pressure is very low here.....5-10 millimeters of mercury. Where the arterial system was low volume and high pressure, the venous system is high volume and low pressure. The very large milk vein, located on the surface of the cow's belly, or the large jugular vein, are examples of veins that are thin walled but large in diameter. When treating a cow for hypocalcemia or grass tetany, we may inject a needle into these two locations for the purpose of getting medicine quickly (but not too quickly!) to the heart where it can be pumped via the left ventricle to the systemic body. The advantage here is, of course, a very large diameter vein with relatively thin walls.
We see this same thing on the backs of our hands or arms, those very large blue blood vessels are carrying venous blood back towards the heart, the right side of the heart, in this case, so it can be pumped to the lungs for oxygen and carbon dioxide removal.
In the venous system, the blood enters the right ventricle from two main vessels. The superior vena cava carries blood from the head and shoulder area, whereas the inferior vena cave carries blood from the lower extremities of the body. Both are emptied into the right atrium, then dumped into the right ventricle where the volume enters the arterial system, via the pulmonary artery.....to the lungs.
In summing up the circulatory blood vessel discussion, we really divide the vessels into three parts, the high pressure, low volume, muscular but small diameter vessels of the arterial system; the capillaries, the small vessels at the cellular level that contain the waste products of cellular activity, and the high volume, low pressure, thin walled but spacious vessels of the venous system. All are traced away from the heart or towards it. Thus the two systems, arterial and venous, each serve a different purpose. The heart, of course, works with the lungs to discharge waste products and grasp onto the fuel of oxidation.....oxygen.
The demands of the circulatory system are driven by homeostasis first and then the obligations of fright or fight, pregnancy, growth or lactation. These later ones obligate the circulatory system to carry more metabolites to the organs and tissues per unit blood volume, and similarly, a greater volume for discharge. And along with concentrations of these metabolites per unit volume of blood, the metabolic rate, the respiration rate, and the heart rate itself can all be increased as demands are called for.
What beauty there is here, really, for the body is systemic schematically, each part working with another. And in most cases without our direct control. As if automatic and taken for granted, we are lulled into a sense of complacency, until any one of the parts begins to fail.
When this happens, the body senses an imbalance and alters itself accordingly. For instance, if a cow is undergoing tremendous milk synthesis and she is not eating enough dry matter, we know the cow loses weight. And we know she undergoes some form of subclinical ketosis. That is, the incomplete oxidation of lipid material into sugar.....glucose. This condition of carbohydrate deficiency, the blood volume of glucose is too low and the secretory system of the udder tissue is starved for glucose. Well, ketone bodies, such as beta-hydroxybutyrate, accumulate in the blood faster than they can be metabolized into glucose in the liver. As these ketone bodies accumulate (ketoplasia), the blood volume is increased (ketonemia) and as such, they are discharged into the lung tissue and exhaled or discharged through the urine (ketonuria). We smell ketone-like (acetone) substances on the cow’s breath. The cow, of course, reduces her milk production volume.
Now this is a significant fact. Why does a cow drop in milk production? Simply it is the reason of Homeostasis. It is why the body will shut down or reduce all peripheral obligations to save itself. In the case of ketosis, she is saving herself, guaranteeing enough glucose for brain function, heart movement, respiration as controlled by the muscular diaphragm, and some rumen motility so feed may be consumed and moved through the digestive tract.
This is why getting cows bred is difficult when there is a metabolic problem. The wonderful intrinsic (essential) factor of homeostasis simply shuts down all that is not necessary in order to preserve the core organs......the heart, liver, kidneys, lungs and brain. This is wonderful indeed!
Next month we will examine the blood volume itself. Making up about seven to eight percent of the total weight of an animal, blood is one of the most interesting tissues of the body, and without a doubt, one of the most necessary.
We might even take two months to cover this topic.
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