Bovine biology series
Part 6 - Throat, pharynx, trachea, esophagus
In this month's biology lesson we will bridge the gap between the mouth cavity, the first place food is dealt with both physically (chewing) and chemically (saliva-containing buffers, electrolytes and protein substances, and the rumen itself) and the throat, pharynx, trachea and esophagus. This is really the portion of the digestive tract that moves food and air into their respective proper locations.
Let's take a look at how this happens.
Uh, oh, you say, did I forget the tonsils? Not at all. In humans and dogs these small protrusions of tissue are really an extension of the lymphatic system. They extend upwards, into the opening directly behind the throat and in the front of the pharynx.
But in cows and some other domestic animals they are hidden beneath a membrane except in one case: the lingual tonsils found at the base of the tongue. These tonsils, as part of the lymphatic system, serve a role in helping protect the body from invasions of bacteria and in the formation of white blood cells. We will deal with the lymphatic system at later time.
But for now, the tonsils are present in cow at the base of the tongue.
The pharynx is really quite an interesting physical structure. Found directly behind the base of the tongue and mouth, the bolus of food and liquid enter this organ. But so does air when a cow breathes. So one primary function of the pharynx is directing food to the esophagus and air into the trachea. Now the trachea is not part of the digestive system; it is part of the respiratory system. The trachea, or windpipe, is nothing more than a tube connecting the pharynx to the first lobes of the lungs. We will cover these respiratory organs later too.
The pharynx is like a traffic light at an intersection. When the cow is breathing in that wonderfully ventilated air that many of you have in your barns, a little flapper known as the epiglottis closes the esophagus from the mouth and nasal cavity. The inhaled air is directed towards the lungs via the trachea.
When bossy reaches through the lockups, eats from that wonderful total mixed ration, she forms a bolus, swallows, and the epiglottis moves the other direction, keeping food particles and this food bolus away from the trachea.....sending it down the esophagus into the rumen, an action we call peristalsis.
How is this traffic light controlled? Well, this is truly remarkable, because the epiglottis is by default opened towards the trachea. Why is this? Because the act of respiration is involuntary. We don't have to think about breathing, it just happens. Respiration rate or the speed at which the diaphragm moves bringing in air, is largely dictated by the activity we are doing. When cows get out, as in someone forgot to plug in the electric fence, my goodness does all madness break out! So does cows' respiration rate. Without thinking about it, she breathes faster and faster as she runs through the flowerbeds and manicured lawns. She is responding to cellular demand for oxygen and the demand of blood pH regulation.
All because the little epiglottis is directing air to the lungs and the discharge of air back out.
The act of eating, however, is voluntary. That is, the cow makes a conscious effort to move her mouth, chew, and swallow.
What is swallowing all about? Well, here is the key. When she swallows, the muscles of the pharynx contract, moving the food into the esophagus, as well as moving the epiglottis. The traffic light is switched. The cow cannot breath during the actions of swallowing. [How well I know this.....when I race along at a good pace and someone hands me a banana, I have to juggle chewing/swallowing and breathing at the same time! That is why we runners drink liquid instead of solid food, usually, because swallowing is quicker, easier, and doesn't take up as much time. We can quickly return to our most immediate need.....the intake of oxygen so we can stay aerobic!]
So, the paths of food and air converge in the pharynx, but this wonderful organ directs them each appropriately. Two side notes here. The Eustachian tubes are connected with the pharynx, and what are these? They are the opening to the ear and help maintain the ear pressure, which controls our equilibrium. When we swallow, we can feel our ears move as adjustments in pressure is made. Also, the pharynx is connected to the nose and nasal passages. In this way, air can come in as we breathe, of course, and this is a preferred method of air intake. Why? Air drawn from the nose is cleaned and warmed. This avoids shock to the respiratory system on cold days from breathing cold air in Chicago or the dust of wheat chaff in Dufer.
Again, the pharynx is like a traffic light at an intersection, directing, controlling and balancing. Remember that swallowing involves the movement of pharyngeal muscles, a voluntary action, and that breathing is more relaxed and is involuntary.
The esophagus is really a muscular tube. In fact the musculature of the esophagus is a continuation of the muscles of the pharynx. This tubular muscle passes through the thorax at the base of the neck, which is the opening of the body cavity next to the spinal column. Through the thorax the trachea and major blood vessels feeding the head and brain go too, such as the jugular veins sitting on both sides of the throat. Next, the esophagus is found slightly to the left of the body center, passing between the trachea (to the lungs) and the major artery, the thoracic aorta.
Now at this point the esophagus enters the part of the body called the diaphragm. Inside this diaphragm are the lobes of the lungs, of course. But into this thin membrane goes the esophagus towards the rumen.
At the point of esophagus - rumen juncture there is a sphincter muscle specifically called the cardiac sphincter. We know what sphincter muscles are...a group of muscles that close an orifice, an opening. The one we are most familiar with is the one at the teat end...keeping bacteria out and certainly for all these very high producing cows in Western Oregon, all that high quality milk in.
Anyway, the cardiac sphincter is so named because it is very close to the heart muscle (although it has nothing to do with the circulatory system!). When you think about it, why does there need to be a muscle like this at the juncture of the esophagus and rumen? Well, common sense tells us that when food is swallowed and peristalsis moves the food towards the rumen, the tube better not have anything back the other direction, as in head-on collision! That's right, because during the cud chewing time of a cow, the mastication period that we talked about last month, a bolus of fiber, usually, is sent back to the mouth for that grinding and physical breakdown. So, the cardiac sphincter is a muscle that opens, allowing food to go both ways, but not at the same time.
What controls this remarkable little muscle? The nervous system does specifically the vagus nerve. The brain controls this nerve primarily, upon sensing the action of eating and swallowing, thus slowing or halting the rumination and subsequent regurgitation of rumen digesta back through the esophagus to the mouth. On the other hand, when bossy is laying in comfortable, clean and dry free stall that are found on dairy farms in Western Oregon, the brain senses, aha! Let's not eat quite yet; instead, I'll just signal the rumen here to send back some of that high fiber feed and get it chewed some more! And in this way, this little cardiac sphincter opens and closes depending upon the action of eating and swallowing or rumination and mastication.
Whew, all this science stuff. Why don't we just say that there are two tubes, one carrying air into and out of the cow and another one that carries food from the mouth to the rumen and sometimes from the rumen back to the mouth? This is basically true, after we include the little epiglottis, the traffic light directing food or air, and the smooth muscled sphincter, which also controls the flow of food and avoids plugging. Of course the nerves control all this, as directed by the brain.
What controls the brain? We can answer that in many ways, but for the purpose of this study on digestion, it is a drop in blood glucose that tells bossy, hey, let's get up and eat so our blood glucose level is maintained (remember the word homeostasis?). An added impetus is the wonderful smell of alfalfa hay and corn silage and what is this.....broken Oreo cookies called bakery by-product?! Oh my!
You get the picture. Now the last bit of lesson about this part of the digestive system is this: moisture. That's right, water. Why? Water is the most important nutrient for the cow, for many reasons. We would do well to remember that there are difficulties eating a diet too dry. What is too dry? Perhaps the nutrition people can answer this one more accurately, but certainly a diet based mainly of hay and dry cereal grains is a dry one. All is not lost, of course, because the salivary glands help ease this problem by secreting greater fluid volumes in the mouth, so the bolus formation is easier, and then swallowing is easier.
So water is important as an aid in the physical movement of foodstuffs from the mouth to the rumen. If not a part of the feed mix, then a part of the ration found as drinking water in clean, well kept water troughs. Believe me, this fact cannot be overstated! Remember the word homeostasis.....water is the most essential ingredient in the ration because it is singularly the most important component of staying alive.
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