Body substances, endogenous exogenous

The chemical transmitters may be small molecules— notably acetylcholine, norepinephrine, epinephrine, serotonin, dopamine, or histamine. Acetylcholine and norpeinephrine are the dominant neurotransmitters in the parasympathetic and sympathetic nervous systems, respectively. Dopamine and serotonin are employed primarily in the central nervous system. Neurotransmitters may also be more complex peptides (small proteins) such as substance P, vasopressin, endorphins, and enkephalins. The latter agents are of particular importance to our considerations of opium since they represent the endogenous opiates—agents that exist within the body whose actions are mimicked by exogenous, or outside, agents such as morphine, heroin, codeine, and so on. These neurotransmitters serve to convey information between neurons across the synaptic cleft (the junction where two neurons meet) or at the neuroeffector junction (the site between neuron and an innervated organ such as muscle or secretory gland). 

Capillaries are the exchange vessels of the body. They have structural variations to allow different levels of metabolic exchange (of exogenous and endogenous substances) between blood and the surrounding tissues. The structure of the walls varies depending on their resident tissue. There are three major types of blood capillaries continuous fenestrated and sinusoidal (discontinuous) [1] ... 

After traversing the proximal and distal regions of the nephron, the filtrate eventually exits into the ureter, which leads to the urinary bladder. Molecules and ions that the body needs are reabsorbed back across the epithelial layer of the tubules into the blood. The human kidneys produce approximately 180 liters of glomerular filtrate per day, with the tubules reabsorbing all but 1.5 liters of water and many dissolved endogenous and exogenous substances. 

Exogenous and endogenous substances which may cause damage to the body must be broken down and/ or detoxified and made water-soluble, so that they are capable of being excreted in the stool or urine. This especially applies to lipophilic or corpuscular substances which cannot be excreted and may therefore accumulate in the body at potentially toxic levels. The liver is the central organ for the degradation andlor detoxification of superfluous and harmful substances as well as their excretion from the body. 

The natural function of the skin is the protection of the body against the loss of endogenous substances such as water and undesired influences from the environment caused by exogenous substances. This implies that the skin acts as a barrier for diffusion of substances through the underlying tissue. Overcoming this natural barrier is the main problem in dermal/transdermal administration of drugs. Methods to decrease the diffusional barrier include the use of penetration enhancers. 

The design of natural polymer-based artificial ECMs for liver tissue engineering is very important because the liver plays important roles in our body, such as metabolism, storage, and synthesis and release of carbohydrates, vitamins, lipids, proteins, and detoxification, inactivation of endogenous and exogenous substances and activation of precursor molecules (Zakim and Boyer, 1990). Also, hepatocytes as the parenchymal cells among liver cells are anchorage-dependent cells and the hepatocytes lose their fiver-specific functions and viability without an optimal artificial ECM (Chan et al., 2004). 

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