| Principles in structure function and disease
As Biological Unit
The Colon
Links and Connections
The (name organ) is connected to
Units to Unity
The (name organ) is part of
Dependence and Independence
The (name organ) cannot function alone and requires centrally based innervations and signaling in addition to the
Time Growth and Aging
Embryo highlights growth and aging highlights.
Space
The (name organ) space lies within the
Forces
Forces that relate to (name organ) function include….
Interactions
While it …
States of Being Health and Disease
During health …..
As a structure Every structure can be described with having a characteristic size, shape, position, and character or nature. Whether we are discussing the structure at a subcellular, cellular, histological, or macroscopic level, these descriptive principles will pertain. Similarly when these features change in disease, we will recognise this disease when we identify a change in any or all of these parameters. It is therefore of extreme importance that we know or have access to the normal features of the cells and organs of the body if we want to be able to detect disease. The enlargement of an organ is a classical example of how we identify disease. A colon that is enlarged or dilated in its upstream portion and decompressed in its downstream portion will imply that there is an obstruction. The recognition of these structural changes are the key elements of diagnosis in clinical medicine and in radiology. Similarly a mucosal cell that is too large will also reflect aberrance in the cell and together with other features of structural change such as an alteration in the way the cell appears with different staining techniques, will be aids to the pathologist of a more specific diagnosis. While size, shape, and position in space are easy descriptors to understand, characater needs a tad more discussion. Newton described that for every action there is an equal and opposite reaction. In other words if you do something to a cell, tissue or an organ or even a person, you will get a reaction. If for example you press your finger on the liver you will be able to describe the liver as being rubbery, soft or hard. Now the liver normally is a fairly soft organ, and if you notice that it is hard then you will have noticed a character change and this will be a clue that the liver is abnormal and a hunt for a relevant diagnosis and therefore a relevant therapy will be the function of the physician. The stains that are used by pathologists are a way of interrogating the character of a cell or a tissue. By applying a staining technique the pathologist challenges the cell to see how it will react to the stain and in so doing is able to characterise the cell. Radiologists will subjet the organs to X-ray interrogation, ultrasound waves or electromagnetic fields and magnets in an attempt to explore how the tissues react to the challenge. Thus if a patient undergoes a chest X-ray, the X-rays are accelerated through the chest, and the image reflects how the tissue has reacted to the waves. Thus normal lung tissue will offer little resistance to the X-rays as they pass through , while bone on the other hand will offer significant resistance. The lungs appear black on the image and the bones appear white reflecting the reaction of these tissues to the X-ray path and thus their distinguishing characteristics. The liver and other “soft tissues” of the body will offer an intermediate resistance to the X-rays and therefore will appear gray. A mass in the lung will offer a “soft tissue” resistance to the path and will present as a gray structure where black air should have been. This reflects a change in the character of lung and is the first clue that all is not well with the patient. The characater of a person is also gauged as to how they react to a given situation or provocation. If you for example press your finger on persons nose (the way you pressed on the liver) the reaction will represent in some measure their nature. Of course humans do not have the same black and white action reaction cycle, but the principles apply.
As a tube
In general the body is made up of tubes and factories. The factories process and produce and the tubes transport. The colon is a muscular and tubular organ and is specifically designed for water and electrolyte reabsorbtion and the storage, transport and evacuation of feces. As in any functioning system, whether it is biological or otherwise, there are controlling influences that govern supply and demand, and in the case of the body these controlling systems are executed through the nerves and hormones of the body.
As a tube the colon is subject to the laws, principles, and physics of tubes. Relevant factors relating to rate and velocity of tubular flow include radius, length, pressure differences, type of content being moved, and resistance within the tubular system. The function of the colon is to mix the chyme and absorb the remaining water in the right colon, while slowly moving the product onward to the left colon where storage is the major function. Finally evacuation requires rapid movement and expulsion of a relatively bulky product.The colon needs to have a large surface area to increase its absorptive powers, which it accomplishes by folding its mucosa inward to form crypts, the so called crypts of Lieberkuhn. The small bowel also uses a similar structural trick of increasing its functional surface area by its villous microstructure, while the lung bulges its surface area by forming grape like sacculations called alveoli around its ductal system.
As a biological tube of the gastrointestinal system it will have the basic structural makeup that tubes have, including an inner mucosa, a submucosal layer that acts as the liason between the mucosa and the body, a muscular layer that helps in the transport and the outer skin which protects and is called the serosa or adventitia. Detail of the makeup will be discussed in the histological section.
As a physiological structure
As a phyiological structure it has to obey the rules of supply and demand which it executes through both simple and complex neurohormonal reflexes. A reflex consistsof a component that is able to sense, a component that integrates and controls the reflex, and a compoonent that is able to execute change. The sensors are in the form of chemical and mechanical sensors. The controlling bodies are at a spinal level, autonomic level, and also at cortical level. The aspects that execute change are at the level of the muscle and glands. The body can basically perform two functions. It can contract and secrete. It is astounding that through these two processes we have been able to advance humanity to the extent that we have. The sensors of the colon, are within the submucosal and muscular layers. In general the chemical sensors are within the submucosa while the mechanical sensors are in the muscular layer. The major function of the colon is to produce and transport feces. Mixing, churning and propulsive movement of the feces is involuntary and at a relatively slow pace as far as physiological processes go, but the final evacuation process is rapid and requires a voluntary effort.
Disease in the Colon
The colon is subject to the basic disease processes that all structures endure being subject to genetic and environmental influences. The unique environment of the colon as the sewerage line with undigested products, its rapid turnover of mucosal lining, its tube like structure as a means of transport brings it some specific disease entities. As all organs, its general response to acute disease is by acute inflammationan. This results in the five cardianal changes of acute inflammation including calor, (heat) dolor, (pain) rubor, (redness) tumor,(swelling) and functio laesa (loss of function) The first 4 of these were described by Celsus in 25AD while the last was added by Virchow in 1858. The calor and and rubor are as a result of increased blood flow, the tumor is caused by increased permeability and resulting edema, and the stretching caused by swelling together with other chemical changes result in the pain. All these combine to cause the loss of function. Infections, and immune disorders will commonly result in inflammatory changes. The rapid turnover of the mucosa and the toxic envonment often result in neoplastic change, often starting out as a benign lesion and subsequently undergoing malignant transformation. Because the colon is a pliable and accomodating of large volumes, tumors only become clinically apparent in advanced disease. Mechanical disorders such as large bowel obstruction, volvulus and herniations are not uncommon disorders of the colon, while the complex makeup of its circulation brings in a host of circulatory disorders including ischemic disease, hemorrhage, and angiodysplasia. Functional disorders including irritable bowel syndrome and constipation are probably the most common of the disorders that affect the colon. The dreaded complication of many of these diseases is perforation or rupture, which is a surgical emergency since it is tantamount to bacterial warfare with spillage of feces into the peritoneal cavity and an associated high risk of mortality.
Principles of Clinical Diagnosis
The colon is difficult examine. When it is empty it is soft and its borders cannot be distinguished. The distinction between a mass and feces cannot be diagnosed since they feel the same to the examining hand, though theoreticallly one should be able to deform the shape of the feces The silent abdomen, or the presence high pitched sounds in the approprate clinical setting may suggest obstruction. Onre of the most useful studies in the rectal examnation which will allow the prostae gland to be examined as well a providing a small amount of stool for the Guiac examination.
Principles of Imaging
From a clinial standpoint the most useful study is the colonoscopy which has the ability to both evaluate the mucosa as well as providing tissue for the pathologist by biopsy, and for treating certain conditions such as small polyps and bleeding ulcers. Colonoscopy is not always appropriate nor safe in certain situations. It requires concious sedation and since air has to be used to distend the walls, the extra luminal pressure may result in rupture when the wall is particulalrly friable. The barium related structure are a n alternate to evaluate mucosal disease while CT scanning provides a window to the wall and the home of the colon – the abdominal cavity and its contents. |
