Here, they are absorbed into the blood through the epithelial cells and into the blood stream. From the bloodstream, they may enter the tissue fluid and then into the cell where they are metabolized and converted into active form. During this process, the drugs encounter several obstacles in their path that they have to overcome. They occur at almost every stage of the process from administering to action at the target sites. The main barriers are the cell plasma membrane, epithelial cells, endothelial cells, blood capillaries, glomerular membrane, capillaries and renal tubules among others. The plasma membrane is a lipid barrier. It only allows passage of lipophilic molecules and hydrophilic molecules, like water, into the cell. It is partially permeable to larger molecules such as larger lipophilic ones. The intestine is the main site for absorption be it drugs or food. The gastrointestinal epithelium also acts as great barrier to the movement of drugs. This layer is a single cell thick. These cells are closely packed together making it difficult to for drugs to move from the intestines into the blood stream. The layer is covered with a mucosal membrane which bars the movement and absorption of the drug. The drug has to go through two plasma membranes to get to the bloodstream. In the bloodstream, the drug is transported to various tissues where the vascular endothelium must be overcome for the drug to enter the tissue. The cells may so closely packed together that they impede the passage of large drug molecules into the tissue fluid. A good example of vessels with tightly packed epithelial cells is the capillaries at the central nervous system. The endothelial cells may also be loosely packed making it easy for the body to eliminate the drug. The spleen has ‘leaky’ epithelial lining. There is also the capillary membrane that is very selective of the size of molecules that go through. They allow movement of both polar and non-polar molecules. This is useful in the kidney during elimination of drug and waste products that are polar. The kidney is a site for drug elimination as well as re-absorption. Due to the nature of its membrane, only non-ionized and lipid compounds get reabsorbed (columbia.edu). The method of transportation also affects drug activity. Methods such as active transport require a lot of energy to perform, therefore, delaying the time of action of the drug. There could be delays in the time taken for the stomach to empty its contents into the ileum, thus, postponing absorption. Intestinal motility is also crucial when bringing the drug into contact with the cells of the epithelium for absorption. The duration of contact between the cells the drug should be adequate to promote absorption. For absorption to be successful, the flow of blood should be high for maximum absorption. Blood determines the diffusion gradient. Other factors that affect the rate of drug absorption include: the food we eat, the pH of the gastrointestinal environment, age and state of health (Washington et.al, 2000). Question 2 The half life of a drug is the time it takes to for a drug to move from full pharmacological, physiological and radioactive potency to half. This information is useful to the nurse in that is informs him or her that the drug might not be as effective. The nurse should therefore request fro the replacement of the drug with a potent one. This will ensure a patient receives proper treatment using active drugs.