Circulatory system structure

When cells lie adjacent to each other in animal tissues, they are often connected by gap junction structures, which permit the passive flow of small molecules from one cell to the other. Such junctions essentially connect the cells metabolically, providing a means of chemical transfer and communication. In certain tissues, such as heart muscle that is not innervated, gap junctions permit very large numbers of cells to act synchronously. Gap junctions also provide a means for transport of nutrients to cells disconnected from the circulatory system, such as the lens cells of the eye. 

The surface area in the luminal side of the small intestine per unit length of the serosal (blood) side is enormous in the proximal jejunum, and steadily decreases (to about 20% of the starting value [62]) in the distal portions of the small intestine. The surface area is increased threefold [69] by ridges oriented circumferentially around the lumen. Similar folds are found in all segments of the GIT, except the mouth and esophagus [66]. Further 4—10-fold expansion [62,69] of the surface is produced by the villi structures, shown schematically in Fig. 2.4. The layer of epithelial cells lining the villi structures separate the lumen from the circulatory system. Epithelial cells are made in the crypt folds of the villi, and take about... 

The adenohypophysis does not have a direct anatomical connection with the hypothalamus therefore, regulation of hormone secretion by way of neuronal signals is not possible. Instead, these two structures are associated by a specialized circulatory system and the secretion of hormones from the adenohypophysis is regulated by hormonal signals from the hypothalamus (see Figure 10.2). Systemic arterial blood is directed first to the hypothalamus. The exchange of materials between the blood and the interstitial fluid of the hypothalamus takes place at the primary capillary plexus. The blood then flows to the adenohypophysis through the hypothalamic-hypophyseal portal veins. Portal veins are blood vessels that connect two capillary beds. The second capillary bed in this system is the secondary capillary plexus located in the adenohypophysis. 

Internal structure of the tube worm Riftia pachyptila. (a) Oxygen, sulfide, and carbon dioxide are absorbed through the plume filaments and transported In the blood to the cells of the trophosome. (b) The chemicals are absorbed into these cells, which contain dense colonies of sulfur bacteria, where they are converted to organic compounds and (c) passed back into the circulatory system to act as an energy source for the worms. Source-. From Childress, J. J., et al. (1987). Scientific American, 256, 114-121. 

After highlighting the present top ten pharmaceuticals, let us now cover a number of other important drugs, both prescription and over-the-counter. We will attempt to categorize them by physiological action but will emphasize chemical structure and synthesis where appropriate. Our first type will be drugs affecting the heart. Cardiovascular agents are used for their action on the heart or on other parts of the vascular system. They modify the total output of the heart or the distribution of blood to certain parts of the circulatory system. 

Structurally, the skin consists of the outermost epidermis which is essentially composed of dead squamous cells sloughed off from the underlying dermis (Figure 7.1). The dermis lies on top of subdermal layers, which further down gives access to nerve endings and capillaries of the circulatory system. 

Traditionally the technique of the medical physicist, magnetic resonance imaging (MRI) has long been used to investigate the internal structure of the human body and the transport processes occurring within it for example, MRI has been used to characterize drug transport within damaged tissue and blood flow within the circulatory system. It is therefore a natural extension of medical MRI to implement these techniques to study flow phenomena and chemical transformations within catalysts and catalytic reactors. 

The circulatory system of fish is also unique structurally and functionally. Structurally, the membranous nature of the vasculature makes for a friable high-capacitance system under low pressure. Low blood flows result in somewhat longer distributional phases for many drugs. Processes such as heart rate and stroke volume that influence drug distribution are themselves influenced by external factors such as temperature and stress. In addition, total plasma protein content differs in fish as compared to mammals. Total plasma protein in the trout and flounder is approximately one-half that of mammals such as dogs and cats. For many compounds protein binding is considerably lower in fish than their mammalian counterparts (19, 20). 

The structure of an enzyme can also vary within a person, since different genes may encode enzymes that catalyse the same reaction. These enzymes are known as isozymes. Isozymes are often specific for different types of tissue. For example, lactate dehydrogenase (LDH) is produced in two forms, the M-type (muscle) and the H-type (heart). The M-type is predominates in tissue subject to anaerobic conditions, such as skeletal muscle and liver tissue, whereas the H-type predominates in tissue under aerobic conditions, such as the heart. Isozymes may be used as a diagnostic aid. For example, the presence of H-type LDH in the blood indicates a heart attack, since heart attacks cause the death of heart muscle with the subsequent release of H-type LDH into the circulatory system. 

Vitamins and hormones are minor organic biomolecules, but both of them are required by animals for the maintenance of normal growth and health. They differ in that vitamins are not synthesized by animals and must be supplied in diets while hormones are secreted by specialized tissues and carried by the circulatory system to the target cells somewhere in the body to initiate/stimulate specific biochemical or physiological activities. Vitamins (Dyke, 1965) can be classified as water-soluble (B vitamins and vitamin C) or fat-soluble (vitamins A, D, E, and K) and act as cofactors for numerous enzyme catalyzed reactions or cellular processes. Hormones (Nornam and Litwack, 1997) can be classified structurally as follows ... 

Human serum albumin (HSA) is an important transporter of fatty acids, metabolites, drugs, and organic compounds in the circulatory system [93, 94], It is a single polypeptide chain consisting of 585 amino acids. Under physiological conditions (pH 7), HSA adopts a heart-shaped three-dimensional (3D) structure with three homologous domains I—III (Fig. 14) each domain contains two subdomains A and B, which consist of four and six a-helices, respectively [95, 96]. The X-ray structure shows that two halves of the albumin molecule... 

The reactions of phase 2 relate to the attachment of the bridge-carbohydrate residues to the polypeptide chain. There is evidence showing that this addition occurs while the polypeptide chain is still attached to, or perhaps still being synthesized on, the ribosomes.101-103 Thus, 14C-labeled 2-amino-2-deoxy-D-glucose, injected into the circulatory system of the rat, was incorporated into protein in the ribosomes of the rough endoplasmic-reticulum of the liver. Administration of puromycin caused release of the 14C-labeled glycoprotein, which could be isolated by acid-precipitation methods. Examination of the radioactivity data revealed that the subcellular structures most actively involved in glycoprotein synthesis were the ribosomes bound to the membrane, and not free polysomes. 

By week five, the buds of tissue which will become the limbs are in place. The structures which will become the skeleton, nervous system, and circulatory system... 

Figure 4 Balloon angioplasty with and without stent deployment, (a) In balloon angioplasty, a thin catheter is threaded through the circulatory system until the uninflated balloon at its tip penetrates the diseased artery at the point of blockage, as shown in the top diagram. The balloon is then inflated to expand the artery, as shown in the middle, before being deflated and withdrawn to allow blood flow to resume (bottom panel). (b) An increasingly common feature of angioplasty involves deployment of an expandable wire structure to help keep the artery from collapsing after the balloon is withdrawn. The procedure is the same as in (a), except that a wire stent is placed over the balloon before insertion (top). The stent expands when the balloon is inflated (middle) and retains its expanded form after the balloon and catheter are withdrawn (bottom), remaining in place after the procedure is complete to provide a permanent structural support for the arterial wall.

The squalestatins, e.g. 6.28, also known as the zaragozic adds, have attracted considerable interest as inhibitors of squalene synthase and hence of cholesterol biosynthesis and lipid deposition in the circulatory system. They are also inhibitors of farnesyl protein transferase and thus they may have other potentially useful biological applications. They are formed by Phoma spedes and also by Setosphaeria khartoumensis. The squalestatins are characterized by a dioxabicyclo-octane core bearing three carboxyl groups and two polyketide chains, one of which is attached as an ester. The biosynthetic incorporation of succinic acid into part of the bicyclo-octane, together with its oxygenation pattern, indicate that it may be derived via oxaloacetic acid. Both the polyketide chains have several pendant methyl groups attached to them, which arise from methionine, whilst benzoic add ads as a starter unit for one of the chains. These complex structures are thus the summation of several biosynthetic pathways. 

These compounds represent a possible example of an orally active soft drug. Even if the structure contains an ester moiety to allow enzymatic hydrolysis, it is possible to maintain activity for ester-containing drugs after oral administration. Indeed, many ester-containing drugs are orally administered. A study of the butyl ester prodrug of indomethacin in rats also showed that hydrolysis of the ester bond is mainly carried out in the circulatory system and the bond is barely hydrolyzed in the intestinal tract (204). 

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