A knowledge of normal anatomy is helpful in understanding how to treat a disease when things have gone wrong.
The Swallowing Mechanism
When learning about the normal workings of the gullet, stomach and diaphragm, the first element to understand is the action of normal swallowing. The only part of normal swallowing of which we are conscious takes place at the back of the tongue and in the throat area. From then on, once the food or the liquid reaches the top of the oesophagus, the process is an active but unconscious one that takes place in the oesophagus. This process is completed when the food or drink enters the stomach.
It is easy to imagine that the oesophagus is simply an inactive tube through which the food that we have swallowed passes by gravity to the stomach. However, what really takes place in swallowing is very different. We transfer food towards the back of the throat (the pharynx) using the tongue. Once the food reaches the pharynx, swallowing becomes automatic. In other words we can feel the food in our throat but we are unable to prevent the action of swallowing. The movement is now under the control of the autonomic nervous system, which also controls the movements of food through the rest of the digestive system without us being aware of it.
We can divide the autonomic (or subconscious) phase of swallowing into two sub phases: what takes place in the pharynx and what happens in the oesophagus.
In the Pharynx
When food or drink hits the pharynx (back of the throat), it stimulates two muscle reflexes. The first one shuts off the passages back into the mouth, the back of the nose, and the lungs. The second one squeezes the food down into the top of the oesophagus. The aim of this is to ensure that we do not inhale and swallow at the same time. If food or drink enters the lungs, it is a disaster that can lead to very sudden death or aspiration pneumonia.
In the Oesophagus
The oesophagus is a very muscular tube. The action of the autonomic nervous system causes it to contract and relax in a very coordinated fashion, so that it actively pushes liquids and solids onwards down into the stomach. These rippling muscle contractions are known as peristalsis. They take place throughout the whole length of the gut from the oesophagus, through the small intestines to the large intestine and to the anus. They are the means by which food and then faeces are passed onwards. If this process fails then food just sticks. Any liquid may trickle downwards, but solids will remain in a lump, stretching the walls of the tube and causing considerable pain and discomfort.
There are three recognisable types of muscular contraction in the oesophagus.
- Once we start to swallow, primary peristaltic waves ripple down the oesophagus, pushing food in front of them at a speed of approximately 5 cm per second.
- If the primary peristaltic waves are unable to manage to empty the contents of the oesophagus down into the stomach, then a second peristalsis wave commences, about halfway down the oesophagus. This reinforces the primary wave, and sometimes it may be felt as an uncomfortable almost indescribable feeling deep within the central chest.
- Tertiary muscle contractions in the oesophagus have been identified by radiologists who have vast experience of watching barium swallow x-rays. These tertiary muscle contractions take place in one segment of the oesophagus at a time. They do not appear to be involved in swallowing and they do not propel food onwards. Their purpose is not known, but it is possible that they are simply a way of maintaining the muscular tone of the oesophagus between meals, whilst waiting for the next piece of food to come down.
Peristalsis, both primary and secondary, is vital for transferring solid and semisolid food from the back of the pharynx into the stomach. As long as we are upright when we drink, however, peristalsis is not absolutely essential for liquids. Once we have swallowed some liquid, provided there is no obstruction to its flow, it will reach the stomach by gravity alone. However, peristalsis is required to ensure that food does not return upwards to the throat. Without peristalsis, if you were to swallow whilst lying flat or standing on your head, the food or drink would simply flow back into your mouth.
At its lower end, the oesophagus passes through a hole – the hiatus – in the diaphragm, which is a sheet of muscle that separates the contents of the chest from those of the abdomen. Below the diaphragmatic hiatus, the oesophagus becomes the upper part of the stomach. How it does so, and how it relates to the diaphragm, are crucial in determining whether or not a patient has a hiatus hernia.
The Junction of the Oesophagus and Stomach.
It is important that food flows in a one-way direction within the oesophagus, and it is even more vital that, once the food has reached the stomach, it remains there, and can only travel forwards into the duodenum. The digestive juices within the stomach are extremely acidic and are designed to digest proteins. The stomach wall is extremely well protected against its own acidic juices with a thick layer of mucus, to protect it against the protein digestive enzymes, and bicarbonate, to protect against acid. However, the oesophagus is not protected at all in this way. Therefore stomach juices that reflux up into the oesophagus are a powerful irritant. The acidic gastric juices can cause ulcers, inflammation, friability, erosions and eventually scarring and constriction at the lower end of the oesophagus.
Therefore, several mutually cooperative mechanisms exist in order to ensure that once food has passed into the stomach, it is unable to flow backwards up into the oesophagus.
The lower 5 cm or so of the oesophagus lie under the diaphragm within the abdominal cavity. The oesophagus meets the stomach at its upper right-hand surface, not quite at the top. If the stomach were the face of a clock, and you were looking at it from in front, then the junction between the oesophagus and stomach would lie at approximately 11 o'clock.
This junction is known as the cardia. It meets the stomach at an angle so that food slides easily into the bottom 90% of the stomach. This process is not unlike tipping a glass when you pour a drink into it, in order to to avoid froth and turbulence. The angle should also ensure that, if there is any reverse movement of food upwards inside the stomach, it passes by the entry from the oesophagus and ends up in the top part of the stomach, an area known as the fundus.
The fundus, which is the uppermost part of the stomach, acts as a safety valve and collects any gas that has been swallowed or produced during the process of digestion. The fundus of the stomach sits neatly underneath the diaphragm.
The Lower Esophageal Sphincter
At the cardia, just where the esophagus becomes the stomach, lies a ring of muscle, within the stomach wall. Try to imagine it like an elastic band around the tube, gripping it slightly and narrowing it. This is known as the gastroesophageal sphincter or lower oesophageal sphincter. There are sphincters at several important sites within the gut: apart from the one between the oesophagus and stomach, they also occur at the outlet of the stomach into the duodenum, between the small intestines and a large intestines and finally there is one at the anus which is the anal sphincter. These sphincters control the passage of food and food residues from one section of the digestive tract to the next. These sphincters prevent backflow of food and food residues.
So not only is the angle at which the oesophagus meets the stomach important, so also is the efficiency of the gastroesophageal sphincter. When it opens the ring of muscle relaxes to allow food and drink to pass from the oesophagus to the stomach. When it closes and the ring of muscle contracts it prevents the digested food in the stomach from flowing back up into the oesophagus. It is in effect a one-way valve.
In addition to the lower oesophageal sphincter muscles, another system of muscles keeps this above structure intact. These are the oblique muscle fibres within the wall of the oesophagus and stomach around the sphincter which keep the oesophagus and stomach at the appropriate angle to each other, in a sling-like support. Without the oblique muscles, the angle between the oesophagus and stomach would flatten out, and the bottom end of the oesophagus would be more open the backflow pressures.
Such backflow pressures are normal when the stomach, full of food, starts its job of digestion. Like the oesophagus, the stomach wall is subject to peristalsis, and although the movement is usually from the above going down, there are also chaotic churning waves, designed to mix the stomach contents thoroughly with the digestive juices of the stomach. If the sphincter and the oblique muscle fibres are not working properly, then this motion may force the stomach contents into the lower end of the oesophagus, which then leads to irritation of the oesophagus and the symptom of heartburn.
The abdominal pressure is an essential force which prevents backflow. There is a large difference between the pressures inside the thoracic and abdominal cavities. The pressure inside the thoracic cavity is kept relatively low, if not the lungs would be unable to expand. The pressure inside the abdominal cavity is much higher, because there are external pressures on the bowel helping to push its contents through and eventually out. The diaphragm is the muscular structure that maintains this large difference in pressure between the chest and abdominal cavities.
The diaphragm is a strong sheet of muscle that is attached in an umbrella-shaped circle around the lower margin of the ribs. Above the diaphragm are the lungs and heart. Below the diaphragm are the kidneys, the liver (on the right), the spleen (on the left), and the entire bowel from the stomach, through the small intestine and colon to the rectum and anus. The high pressure within the abdominal cavity means that if the muscles of the diaphragm are not working properly, some part of the contents of the abdominal cavity can be pushed through into the thoracic cavity. This is what is meant by a hernia.
There are holes in the diaphragm through which the oesophagus and the main blood vessels from and to the heart (the aorta and inferior vena cava) must pass. The blood vessels are at the back of the abdomen, just in front of the spine, but the oesophagus enters closer to the front of the diaphragm, through an opening of its own – the hiatus.
To ensure that nothing can slip upwards from the abdomen into the thoracic cavity through the hiatus there are strong muscles within the rim of the hiatus that hold it close to the oesophagus. These muscles are the diaphragmatic crura (from the Latin word for cross). These muscles crisscross around the oesophagus keeping it tightly in position.
This is an excellent setup, not only for preventing a hernia, but also for ensuring that the external pressure around the lower few centimetres of the oesophagus remains high. Therefore, even if the cardia is a bit inefficient and there is a risk that it may allow stomach contents to reflux up into the oesophagus, this is prevented simply by the high external pressure. This external pressure keeps the oesophagus collapsed until it is opened by the pressure of food and peristalsis. This external pressure applied to the lower part of the oesophagus is probably the most important mechanism for preventing the reflux of stomach acid into the lower oesophagus. When the cardia is pushed up into the chest cavity – as happens with a hiatus hernia – where the pressure around the oesophagus is much lower, then acid reflux from the stomach into the oesophagus becomes the rule, rather than the exception.
to be completed soon