Gases or liquids can be unevenly distributed between two areas. If one area has a higher concentration than the other then the differance between these two areas is termed the concentration gradient. The equality is then corrected by the movement of the molecules down this so called gradient from the region of high concentration to that of low. This process is passive as the molecules do not have to be forced to do this and it is reffered to as diffusion.

The duodenum is the proximal part of the small intestine and extends from the pylorus of the stomach to the jejunum. It has descending and ascending portions and both portions have digestive and absorptive functions.

Elephant anatomy is very much comparable to the horse and rabbit. Microbes are present in the hindgut that produce Volatile Fatty Acids (VFAs). VFAs make a substantial contribution to the elephant's total energy requirements. Food has a relatively fast transit time and as a result, elephants have a low digestive efficiency (44% as opposed to 60% in horses). A fast transit time is achieved by a short GIT, reduced caecum and increased GIT diameter. Their digestive strategy is to pass as large a quantity of low quality food through their digestive tract within a short period of time.

There are various hormones that influence the structure of the skin. These influences may be made apparent by the repeated long-term administration of various glucocorticoids or their analogues. Endogenous imbalances are generally seen in adult mature animals although congenital forms have been seen, especially with hypothyroidism. The hormones implicated as important for maintaining skin structure are thyroxine, cortisol and estradiol. Deficiencies or excessive production may result from abberations in the function of the hypothalamic-adrenal axis, the adrenal gland, thyroid gland or the gonads.

Comprised of a group of duct-less glands with limited or no anatomical contact with each other, the endocrine system integrates and controls metabolic activity through the secretion of hormones into the vascular system. These hormones may have their effects on tissues and organs far from where they were produced.

Eosinophils are a similar size to neutrophils, have a bilobed nucleus and are characterised by the large eosinophilic granules present in their cytoplasm. Produced in the bone marrow they migrate into circulation briefly before moving into tissue where they survive for around six hours. The proportion of eosinophils circulating depends on the state of the animal. Normally numbers are very low but will rise considerably during a parasitic infection or allergic reaction.

As in any animal, anaesthesia in a horse carries a risk, although it is much higher than that of other domestic species. It is therefore important to try and minimise these risks as much as possible when performing any procedure. Problems can be encountered at any stage of the anaesthetic and so each stage shall be considered separately here.

Also known as red blood cells (RBCs). Erythrocytes deliver oxygen to, and remove carbon dioxide from tissues. Erythrocytes are derived from the stem cell (CFU-GEMM) and formed in a process known as erythropoiesis.

Erythrocytes contain no nucleus and are thus only produced from stem cells. During the fetal stage production is in both the liver and spleen however production is transferred to the bone marrow (red marrow) in the final stages of gestation. Initially erythropoiesis occurs in all bones, however after puberty production is limited to membranous bones (ribs, vertebrae, pelvic bones etc.) as the long bones contain adipose tissue in place of red marrow.

A diagnostic test is an objective method of deciding whether an animal has a disease, or not. Decisions made following diagnostic testing are usually dichotomous e.g. treat or do not treat the animal, therefore diagnostic tests are usually interpreted as dichotomous outcomes (diseased or non-diseased). In this case, if a diagnostic test is measuring a continuous outcome e.g. antibody titre then a cut-off for classifying animal’s as positive or negative must be selected. The figure below shows that whereever the cut-off is selected there is usually some overlap between results i.e. some diseased animals will have the same value as non-diseased animals and resulting in some false-positive and false-negative results.

Transfer of passive immunity in the bovine neonate occurs solely through maternal colostrum. This is in contrast to humans where placental transfer of immunity via specific Fc receptors is the predominant mechanism. However, inadequate transfer of immunity is a relatively commonly diagnosed problem affecting young stock. This is particularly the case in modern Holstein dairy herds, where large milk yields dilute the antibody concentration in the colostrum meaning relatively more must be consumed to have the same immunity transferred. Due to the lower yields and possibly other genetic reasons, this tends to be less of a problem in suckler herds.

Prior to birth the foetus is not capable of respiratory function and thus relies on the maternal circulation to carry out gas, nutrient and waste exchange. The foetal and maternal blood never mix, instead they interface at the placenta. Consequently the liver and the lungs are non-functional, and a series of shunts exist in the foetal circulation so that these organs are almost completely by-passed.

The forebrain (proencephalon) is the largest part of the brain, most of which is cerebrum. Other important structures found in the forebrain include the thalamus , the hypothalamus and the limbic system. The cerebrum is divided into two cerebral hemispheres connected by a mass of white matter known as the corpus callosum.

The gall bladder stores bile produced in the liver. Bile is important in the digestion of lipids. The gall bladder forms as an outgrowth of the bile duct, as a secondary hollow at the posterior edge of the original hepatic rudiment. The cystic duct joins the common bile duct which enters the duodenum at the major duodenal papillae (with the pancreatic duct) on the dorsal surface of the duodenum.

The air in the alveoli is renewed regularly, thanks to the ventilation process. Gas exchange in the lungs takes place between the blood in the capillary network surrounding the alveoli, and the air in the alveoli itself.

Gastrulation is the process of forming the three germ layers; ectoderm, mesoderm and endoderm. It is achieved through a series of highly coordinated cell movements. Cells that will form the endodermal and mesodermal organs are brought inside the embryo, whilst cells that will form ectoderm move to spread out over the outside of the embryo.

The glomerular filtration rate or GFR is the amount of fluid filtered from the capillaries into the Bowmans capsule of the kidneys per unit time.

The endoderm will form the lining of the gut and the organs that develop from it. Splanchnic mesoderm surrounds the endoderm and orginates from the lateral plate mesoderm. It will form the smooth muscle of the gut that are used in peristalsis.

The Guttural Pouch is present only in members of the order Perissodactyla (nonruminant ungulates: horses, tapirs, rhinoceros) and another small band of small mammals including Hyraxes, certain bats and a South American mouse.