The liver

The liver is the biochemical core of mammalian metabolism: factory, central depot, clearing house, defense unit, and waste disposal facility for the body, all rolled into one. Well over 1000 biochemical reactions take place in the liver, and its activities influence the farthest reaches of the body. In the face of this biochemical complexity, the challenge has been to identify the clinical signs and biochemical abnormalities that are relevant to the clinician. Clinicians have several tasks to hand when it has been established that liver damage is present. They must decide how significant is the damage, if it is the major cause of the clinical problem, or if it is a relatively insignificant consequence of disease elsewhere.

There is a world of difference between an animal with liver failure and an animal with some liver damage, in spite of the fact that several features might be the same in each case. The diagnosis of liver disease is now sufficiently advanced for the thorough clinician to be able to resolve these questions in most cases, in spite of the variability and nonspecificity of clinical signs. However, it is worth repeating the most important clinical concept: there is a great difference between liver damage and liver failure. This emphasis arises from the ready availability of biochemical tests which can indicate liver damage. However, positive results from such tests must be kept in perspective.

In the following paragraphs, diagnostic signposts are highlighted against the background of the normal structure–function relationships of the liver, and ultimately against underlying disease processes within liver tissue.

The aim of this book is to assist the fledgling clinician to acquire that ‘total view’ of disease so essential for the competent diagnostician. The typical veterinary medical student first encounters disease at the level of cells and tissues, amongst microscopes and cadavers and then proceeds rather abruptly to a very different world of lame horses, vomiting dogs, panting cats, scouring calves, stethoscopes, blood counts, electrocardiographs and anxious owners. In this switch from the fundamental to the business end of disease, the link between the two is often obscured. It is easy to forget that all clinical disease is the result of malfunction (hypofunction or hyperfunction) within one or several organ systems, and that such malfunction springs from some pathologic process within living tissues.

Although some disease processes are purely functional, in most instances the pathologic events involve structural alteration of the affected organ, which may or may not be reversible or repairable. At least one of the basic reactions of general pathology, such as necrosis, inflammation, neoplasia, atrophy or dysplasia, will be present.

The expert clinician, having recognized functional failure in a particular organ as the cause of a clinical problem, is easily able to conjure up a mental image of the likely underlying lesion and take effective steps to characterize it. This characterization of the underlying disease opens the way for establishing the etiology and appropriate prognosis and management. By contrast, the novice tends to stop short at the stage of identifying organ malfunction, neglecting the important step of characterizing and comprehending the nature of the tissue disease.

The skin, being the outward manifestation of the body, is often admired, but frequently misunderstood by the everyday observer. The ‘real’ organs, it would seem, are wrapped in the dark mysteries of the interior while the skin is somehow set apart – a sort of bloodless, hairy and inert covering, useful for turning into shoes, handbags or rugs.

The well-trained biologist knows better; the skin is a marvellous adaptation of living tissue directly confronting the stresses and hazards of the environment. It is one of the largest organs in the body and no other tissue has the same problems to overcome. Consider the requirements for its design. It must be strong but elastic, pliable and resilient, and must be able to repair itself quickly. It must be able to resist desiccation – a particularly difficult accomplishment for an exposed system of living cells. It must protect against onslaughts chemical, physical, thermal and microbiologic. It must provide for the reception of countless sensory inputs needed for adequate proprioception and it must provide visual, olfactory and tactile stimuli for sexual and special species-related behavior. In wild animals this includes camouflage.

Far from being inert, the skin is very much alive in spite of its large non-living keratin component. It is interlocked with general body metabolism and has especially strong links with the endocrine and nervous systems. Its construction ensures that it is able to carry out these special functions.

The mammalian urinary system is constructed to generate, store and finally expel the familiar liquid known as urine, a product extracted from the flowing bloodstream by the kidneys. Disorders of the urinary system relate largely to three activities – production, conduction and storage of urine. A failure of any of these activities threatens the health, and ultimately the life of the afflicted animal, which is another way of saying that the urinary system is a vital and indispensible organ system. The total failure of urine formation or expulsion will lead to death in a few days, while lesser degrees of malfunction can give rise to numerous clinical abnormalities.

The kidneys create urine as a vehicle for the elimination of toxic or useless metabolic end products while simultaneously regulating the homeostatic balance of a range of crucial substances. The kidneys are also the site of production of several important hormones and of the degradation of others. The activity of the kidneys is tightly integrated with, and greatly influenced by, the cardiovascular system, the neurohypophysis, the adrenal cortex and the parathyroid gland. Any primary disease in one may be reflected in disturbances of function in the other.

In the lower urinary tract, the storage and voiding of urine are heavily subject to, amongst other things, the state of the autonomic nervous system, spinal cord and pelvic nerves.

In the face of such a diversity of possible clinical ramifications and interactions, the clinician has no choice but to come to grips with the fascinating complexities of the urinary system in health and disease.

The nervous system is a communications network whose functions range from simple reflex activities to the complexities of awareness and conscious thought. Elements of the nervous system ramify to every part of the body, giving it a universal functional influence.

The functional integrity of the nervous system is directly related to the activity of neurons. When some noxious influence affects part of the nervous system, activity of the neurons in the area may cease, may be suppressed or may become excessive. In the central nervous system (CNS) these changes will inevitably affect other, interconnected, neurons and induce a change in their activity and coordination. The nervous system is arranged in a hierarchical manner so that dysfunction of one group of neurons is reflected as dysfunction of the subordinate neural pathways including those in the peripheral nervous system (PNS). The dysfunction may thus be projected widely throughout the system.

Fortunately for the clinician, the highly organized nature of the nervous system means that damage to a given area will always produce distinctively recognizable dysfunctions. With adequate clinical skills and an appreciation of the functional anatomy of the system, it is often possible to localize the seat of nervous dysfunction in an animal with neurologic disease.

The student is therefore advised to review the general anatomy of the nervous system along the lines indicated on p. 337, including the disposition of the meninges and the generation and flow-patterns of the cerebrospinal fluid.