Bovine biology series
Part - 11 Lungs
The lungs
We begin a discussion of the respiratory system this month, specifically the lungs. Here in these soft tissues the outside elements of the world, namely oxygen in the air and other airborne molecules, are drawn into the body cavity by the involuntary movement of the diaphragm. If the ration for the cow is fuel, a collection of organic molecules that ultimately furnish glucose sugar, then it is oxygen that transforms this chemical energy into movement.....the contraction of muscles, the signals of the brain across nerve cells, the beating heart, and the maintenance of the body proper itself.....Homeostasis.
We may liken the body in this way.....we require, as does the cow, two sources of fuel for survival. One is the fuel of organic sustenance, the carbon and hydrogen and oxygen and nitrogen that is food and the other is oxygen found in the air we and the cow breathe. In order for the food itself to be utilized by the body as fuel, it must be burned or oxidized. Oxygen does that. Think about this.....a pile of wood and discarded newspaper is sitting in a burn pile. It is chemical fuel but is not burned until lit with a match. Once the spark of the match is involved, the burn pile roars off, and two things happen. One is the consumption of oxygen in order to oxidize the chemical energy in the wood and newspaper, and two, the transformation of chemical energy (as stored or potential energy) into heat that rises into the sky. Thus with oxidation, or combustion, there is heat generated.
In the body when this occurs through primarily the Kreb's Cycle, there is heat generated; muscles warm up the more they are used. So one price we pay for exercise in which muscles are moving more rapidly instead of in a resting state is heat.
What price is this heat? Well, when I run a marathon the price can be significant if I am running on a very warm day. The body is obligated to rid itself of this heat of oxidation, the result of many, many muscle contractions over a long period of time. This is done, of course, through the use of body fluids transporting this heat to the skin surface through sweat glands and then evaporated away from the skin. A cow does this instead by breathing more rapidly, the lungs serving an additional purpose of thermoregulation. That is why cows have been breathing so rapidly this week of triple digit temperatures.
So we have a biological system in which food is fuel, and oxygen is required to burn that fuel so that work may be done. Now remember this as well.....the cow will partition this work in this way: first is homeostasis, or maintenance of the body proper. Then the demands of exercise and movement are taken care of, then a pregnancy will be cared for and lastly there is milk production. There is no mystery why on these very warm days cows drop of a gallon or ten pounds or some significant figure in terms of milk synthesis. That is because the cow finds thermoregulation difficult, especially if she is presented with a relatively hot ration. She backs off on this ration, spends more time resting, breathing out from the lungs the heat of oxidation, and thus allocating a higher fraction of energy for maintenance and pregnancy and perhaps growth. Milk production is reduced because it is the last in the line of allocatable energy.
Certainly this subject bares this comment: the quality of not only the feed but of the air affects the dynamics of cow metabolism and resultant energy allocation. It is this reason that two days ago while driving by a 300-cow dairy in Yamhill county I observed a whole side of tin removed from a free stall building. This building is a box with inadequate ventilation. And so on these abnormally warm days, the homeostatic trigger of thermoregulation took its toll. The dairyman, observing rapidly breathing cows and depressed milk production, helped the cow herd by ventilating the barn which increased air flow. Not only does this improve the volume of air inside this barn, but also the air quality improved as the nitrogen gasses and airborne particles of bio and inorganic substances escaped from this box.
Let's look at the lungs more closely.
The muscular diaphragm is of course at the bottom of the lungs. This muscle tissue is controlled largely by the involuntary action of the brain; the cow and we do not think about breathing because it is automatic. However we can usurp this involuntary action by changing the rate of breathing, called respiratory rate. We do this by signaling the brain to contract the diaphragm, drawing air into the lungs, and relaxing, discharging from the lungs.
The lungs themselves are bathed in a liquid mucosa. This liquid captures the air volume and prepares it for exchange with the very small blood capillaries of the venous and arterial circulatory system. It is this gas exchange that is central to our very existence. Simply, within these capillaries that are directed from the heart to the lungs are waste products of metabolism, namely carbon dioxide. The blood volume is bluish at this point, for the hemoglobin molecule in the circulating blood is devoid of oxygen. At the lungs, this carbon dioxide is diffused through the ends of the capillaries, into the mucosa, and then when the diaphragm is relaxed, these molecules are breathed out and discharged from the body cavity. Then when the diaphragm is contracted, a vacuum is created, drawing into this cavity a volume of air. Thus oxygen is drawn into the mucosa of the lung tissue, and the capillaries of the blood accept this oxygen. Within the blood volumes are the hemoglobin molecules which bond with the oxygen. This freshly oxygenated blood is transported to the heart and from here, sent to the extremities of the entire body.
What a remarkable exchange system this is! We learned how some metabolites are discharged through the kidney/urine system. Here is another system in which the body can rid itself of toxins, carbon dioxide included.
So we and the cow breath in oxygen which replenishes the blood, turning it red and preparing the body for an oxygen source when called upon. And at the same time the byproduct of oxidation and metabolism, carbon dioxide, is discharged via the bloodstream through the lungs and out of the body.
As the demand for activity increases, running instead of walking for instance, the muscles produce greater volumes of carbon dioxide, temperatures are increased, and there is an increase in hydrogen ion concentration in the blood. All of these drive the involuntary reflex of increasing respiratory rate. This in turn increases the oxygen volume that is available to the blood and thus the tissues. To a point. The limiting factor may be the quality and quantity of hemoglobin in the blood, the quality and quantity of oxygen in the incoming air volume, and the overall health or integrity of the lung tissue. Consider the calf that has had pneumonia as a two-month-old. We know this calf will never reach a full potential because the integrity of the lungs is impaired. In spite of the highest quality air and maximum level of hemoglobin concentrating, the lungs cannot mediate the bridge between the two because the tissue has been damaged for the life of this calf.
The mucosa of the lungs is part of the alveolar of the lung tissue, small sac-like structures that are connected to blood capillaries. It is here in the alveolar sacs that this gas exchange occurs. The lumen or opening of these sacs is inflated by pressure, much like a bubble or a balloon. This pressure originates from air pressure, exerting a positive force within the lumen, keeping the sac inflated and the entire lung tissue inflated. A collapsed lung for instance, either from direct trauma or lack of diaphragm movement, severely restricts gas exchanges that are essential to life. Similarly, drowning involves the flooding of these lumen with water and therefore the gas exchange mechanism cannot occur. For these two reasons, a collapsed lung is inflated with direct positive air pressure in the absence of a functioning diaphragm and water must be coughed up or discharged back through the windpipe or the lungs drown in fluid.
The heart muscle is inexorably tied to the lungs. The heart muscle serves two roles. The first is providing oxygen-deficient blood to the lungs, and this is done through the pulmonary artery. Blood is pumped to the lung tissue, aerated with oxygen, and is picked up by the capillary veins, where this oxygen-rich blood is brought back to the heart via the pulmonary vein. From here, the heart muscle distributes this blood supply via the left atrium and finally channeled through the left ventricle and to body tissues from head to toe. Certainly the blood volume and heart rate are miraculously working in unison with respiration rate and metabolic demand. And how delightful this all works if you consider the fact that the body proper itself is blessed with these systems working automatically.
As if on auto-pilot, the pumping of oxygen-rich blood to tissues converting chemical energy into kinetic or work energy occurs at the same time the same blood supply is carrying away from these tissues carbon dioxide rich blood, along with other buffers and by-products for discharge through alveolar sacs in lung tissue. And in times of accelerated demand, metabolism is increased, oxygen intake is increased as the diaphragm moves faster, up and down, and muscles are fueled with the flame of oxidation of sugar, the fuel of muscular contraction.
The miracle of homeostasis cannot be denied. For instance consider this fact: if water is limiting during times of increasing metabolic demand, blood volume is reduced. And when blood volume is reduced the heart races faster, attempting to compensate for a lack of fluid moving the by-products of this increased metabolism to the lungs. This in turn causes an increased respiratory rate for more oxygen. Finally if water is limited to the point that homeostasis is defeated, then the heart muscle goes into shock and we have heat stroke.
The heart simply could not keep up with a diminishing blood volume, and thus in attempting to overcompensate, gave up in the face of fluid loss. There is no doubt that heat stress in cows in this type of weather is related to a lack of fluid intake, which not only is required for the movement of digesta through the digestive system, but equally important for the maintenance of blood volume. We runners are told time and time again to drink before we feel thirsty, and over the course of a marathon we consume small volumes of water every other mile or so, attempting to not only provide the mechanism for body heat removal through sweat, but the maintenance of blood volume which in turn does not shock the myocardium....the heart muscle. In other words, some pretty serious stuff, for runners and cows alike!
Finally a comment about pneumonia. This general medical term simply refers to infection and inflammation of the lung tissue. And what a better place for an infection than these lungs.....warm, wet, dark (away from light) and at the same time presented with every breath a wide array of toxins, microbes, viral pathogens and so on. Now do you know why we say open these barns up and let these cows breath? Now do you know why given the similar opening to the lungs, the mouth and throat, that water and feed must be not be laden with large volumes of potential pathogenic biota? And now do you know why calves especially are prone to pneumonia, as their immune system is not yet fully developed but the lung tissue is?
The damage in some cases is severe.....the lung tissue is atrophied and thus the whole body suffers. Lung tissue is not regenerative. Once gone it is gone. The tissue here is not that dissimilar from udder tissue; both contain alveolar cells that serve as places of exchange. Udder tissue is not regenerative either.
Can a cow or we survive on just part of the lung tissue? Of course. I have never heard of it in a cow, probably because there would be no purpose, as the cow with only half her lung capacity would not produce enough milk justifying such radical surgery. But in humans this does happen, of course, especially in individuals who have had a portion of lung tissue removed from the ravages of cancer. They function, certainly, but only at reasonable levels of exertion. I would suspect they might not play too many basketball games down at the YMCA unless they learned how to adequately compensate for the reduced level of oxygen intake.
So the lungs are special, irreplaceable, and serve as our connection with the outside world, just as window is in our room. That breath of fresh air when we walk outdoors is more than refreshing......it is absolutely essential to our survival. A cow is no different. She must have this breath of fresh air if she is to remain healthy and her body proper is maintained.....Homeostasis.
Next month.....the muscular valentine. The myocardium. The heart.
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