Ecology

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Monday, 04 September 2017 17:01
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Ecology (from Greek: οἶκος, "house", or "environment"; -λογία, "study of") is the scientific analysis and study of interactions among organisms and their environment. It is an interdisciplinary field that includes biology, geography, and Earth science. Ecology includes the study of interactions that organisms have with each other, other organisms, and with abiotic components of their environment. Topics of interest to ecologists include the diversity, distribution, amount (biomass), and number (population) of particular organisms, as well as cooperation and competition between organisms, both within and among ecosystems. Ecosystems are composed of dynamically interacting parts including organisms, the communities they make up, and the non-living components of their environment. Ecosystem processes, such as primary production, pedogenesis, nutrient cycling, and various niche construction activities, regulate the flux of energy and matter through an environment. These processes are sustained by organisms with specific life history traits, and the variety of organisms is called biodiversity. Biodiversity, which refers to the varieties of species, genes, and ecosystems, enhances certain ecosystem services.
 
Ecology is not synonymous with environment, environmentalism, natural history, or environmental science. It is closely related to evolutionary biology, genetics, and ethology. An important focus for ecologists is to improve the understanding of how biodiversity affects ecological function. Ecologists seek to explain:
 
  • Life processes, interactions, and adaptations
  • The movement of materials and energy through living communities
  • The successional development of ecosystems
  • The abundance and distribution of organisms and biodiversity in the context of the environment.
 
There are many practical applications of ecology in conservation biology, wetland management, natural resource management(agroecology, agriculture, forestry, agroforestry, fisheries), city planning (urban ecology), community health, economics, basic and applied science, and human social interaction (human ecology). For example, the Circles of Sustainability approach treats ecology as more than the environment 'out there'. It is not treated as separate from humans. Organisms (including humans) and resources compose ecosystems which, in turn, maintain biophysical feedback mechanisms that moderate processes acting on living (biotic) and non-living (abiotic) components of the planet. Ecosystems sustain life-supporting functions and produce natural capital like biomass production (food, fuel, fiber, and medicine), the regulation of climate, global biogeochemical cycles, water filtration, soil formation, erosion control, flood protection, and many other natural features of scientific, historical, economic, or intrinsic value.
 
The word "ecology" ("Ökologie") was coined in 1866 by the German scientist Ernst Haeckel (1834–1919). Ecological thought is derivative of established currents in philosophy, particularly from ethics and politics. Ancient Greek philosophers such as Hippocrates and Aristotle laid the foundations of ecology in their studies on natural history. Modern ecology became a much more rigorous science in the late 19th century. Evolutionary concepts relating to adaptation and natural selection became the cornerstones of modern ecological theory.

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Last modified on Monday, 04 September 2017 17:15
Dayyal Dg.

Medical Laboratory Technician at National Institute of Cardiovascular Diseases, Karachi. | Author/Writer/Blogger

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  • NORMAL GASTRIC ANATOMY AND PHYSIOLOGY
    Anatomically, stomach is divided into four parts: cardia, fundus, body, and pyloric part. Cardia is the upper part surrounding the entrance of the esophagus and is lined by the mucus-secreting epithelium. The epithelium of the fundus and the body of the stomach is composed of different cell types including: (i) mucus-secreting cells which protect gastric mucosa from self-digestion by forming an overlying thick layer of mucus, (ii) parietal cells which secrete hydrochloric acid and intrinsic factor, and (iii) peptic cells or chief cells which secrete the proteolytic enzyme pepsinogen. Pyloric part is divided into pyloric antrum and pyloric canal. It is lined by mucus-secreting cells and gastrin-secreting neuroendocrine cells (G cells) (Figure 859.1).
     
    Figure 859.1 Parts of stomach and their lining cells
    Figure 859.1 Parts of stomach and their lining cells 
     
    In the stomach, ingested food is mechanically and chemically broken down to form semi-digested liquid called chyme. Following relaxation of pyloric sphincter, chyme passes into the duodenum.
     
    There are three phases of gastric acid secretion: cephalic, gastric, and intestinal.
     
    • Cephalic or neurogenic phase: This phase is initiated by the sight, smell, taste, or thought of food that causes stimulation of vagal nuclei in the brain. Vagus nerve directly stimulates parietal cells to secrete acid; in addition, it also stimulates antral G cells to secrete gastrin in blood (which is also a potent stimulus for gastric acid secretion) (Figure 859.2). Cephalic phase is abolished by vagotomy.
    • Gastric phase: Entry of swallowed food into the stomach causes gastric distension and induces gastric phase. Distension of antrum and increase in pH due to neutralization of acid by food stimulate antral G cells to secrete gastrin into the circulation. Gastrin, in turn, causes release of hydrochloric acid from parietal cells.
    • Intestinal phase: Entry of digested proteins into the duodenum causes an increase in acid output from the stomach. It is thought that certain hormones and absorbed amino acids stimulate parietal cells to secrete acid.
     
    The secretion from the stomach is called as gastric juice. The chief constituents of the gastric juice are:
     
    • Hydrochloric acid (HCl): This is secreted by the parietal cells of the fundus and the body of the stomach. HCl provides the high acidic pH necessary for activation of pepsinogen to pepsin. Gastric acid secretion is stimulated by histamine, acetylcholine, and gastrin (Figure 859.2). HCl kills most microorganisms entering the stomach and also denatures proteins (breaks hydrogen bonds making polypeptide chains to unfold). Its secretion is inhibited by somatostatin (secreted by D cells in pancreas and by mucosa of intestine), gastric inhibitory peptide (secreted by K cells in duodenum and jejunum), prostaglandin, and secretin (secreted by S cells in duodenum).
    • Pepsin: Pepsin is secreted by chief cells in stomach. Pepsin causes partial digestion of proteins leading to the formation of large polypeptide molecules (optimal function at pH 1.0 to 3.0). Its secretion is enhanced by vagal stimulation.
    • Mucus
    • Intrinsic factor (IF): IF is necessary for absorption of vitamin B12 in the terminal ileum. It is secreted by parietal cells of stomach.
     
    Figure 859.2 Stimulation of gastric acid secretion
    Figure 859.2 Stimulation of gastric acid secretion. Three receptors on parietal cells stimulate acid secretion: histamine (H2) receptor, acetylcholine or cholinergic receptor, and gastrin/CCK-B receptor. Histamine is released by enterochromaffin-like cells in lamina propria. Acetylcholine is released from nerve endings. Gastrin is released from G cells in antrum (in response to amino acids in food, antral distention, and gastrin-releasing peptide). After binding to receptors, H+ is secreted in exchange for K+ by proton pump
  • CONTRAINDICATIONS TO GASTRIC ANALYSIS
    • Gastric intubation for gastric analysis is contraindicated in esophageal stricture or varices, active nasopharyngeal disease, diverticula, malignancy, recent history of severe gastric hemorrhage, hypertension, aortic aneurysm, cardiac arrhythmias, congestive cardiac failure, or non-cooperative patient.
    • Pyloric stenosis: Obstruction of gastric outlet can elevate gastric acid output due to raised gastrin (following antral distension).
    • Pentagastrin stimulation is contraindicated in cases with allergy to pentagastrin, and recent severe gastric hemorrhge due to peptic ulcer disease.
     
    Gastric analysis is not a commonly performed procedure because of following reasons:
     
    • It is an invasive and cumbersome technique that is traumatic and unpleasant for the patient.
    • Information obtained is not diagnostic in itself.
    • Availability of better tests for diagnosis such as endoscopy and radiology (for suspected peptic ulcer or malignancy); serum gastrin estimation (for ZE syndrome); vitamin assays, Schilling test, and antiparietal cell antibodies (for pernicious anemia); and tests for Helicobacter pylori infection (in duodenal or gastric ulcer).
    • Availability of better medical line of treatment that obviates need for surgery in many patients.
  • LABORATORY TESTS FOR GASTRIC ANALYSIS
    1. Hollander’s test (Insulin hypoglycemia test): In the past, this test was used for confirmation of completeness of vagotomy (done for duodenal ulcer).

      Hypoglycemia is a potent stimulus for gastric acid secretion and is mediated by vagus nerve. This response is abolished by vagotomy.

      In this test, after determining BAO, insulin is administered intravenously (0.15-0.2 units/kg) and acid output is estimated every 15 minutes for 2 hours (8 post-stimulation samples). Vagotomy is considered as complete if, after insulin-induced hypoglycemia (blood glucose < 45 mg/dl), no acid output is observed within 45 minutres.

      The test gives reliable results only if blood glucose level falls below 50 mg/dl at some time following insulin injection. It is best carried out after 3-6 months of vagotomy.

      The test is no longer recommended because of the risk associated with hypoglycemia. Myocardial infarction, shock, and death have also been reported.

    2. Fractional test meal: In the past, test meals (e.g. oat meal gruel, alcohol) were administered orally to stimulate gastric secretion and determine MAO or PAO. Currently, parenteral pentagastrin is the gastric stimulant of choice.

    3. Tubeless gastric analysis: This is an indirect and rapid method for determining output of free hydrochloric acid in gastric juice. In this test, a cationexchange resin tagged to a dye (azure A) is orally administered. In the stomach, the dye is displaced from the resin by the free hydrogen ions of the hydrochloric acid. The displaced azure A is absorbed in the small intestine, enters the bloodstream, and is excreted in urine. Urinary concentration of the dye is measured photometrically or by visual comparison with known color standards. The quantity of the dye excreted is proportional to the gastric acid output. However, if kidney or liver function is impaired, false results may be obtained. The test is no longer in use.

    4. Spot check of gastric pH: According to some investigators, spot determination of pH of fasting gastric juice (obtained by nasogastric intubation) can detect the presence of hypochlorhydria (if pH>5.0 in men or >7.0 in women).

    5. Congo red test during esophagogastroduodenoscopy: This test is done to determine the completeness of vagotomy. Congo red dye is sprayed into the stomach during esophagogastroduodenoscopy; if it turns red, it indicates presence of functional parietal cells in stomach with capacity of producing acid.
     
    REFERENCE RANGES
     
    • Volume of gastric juice: 20-100 ml
    • Appearance: Clear
    • pH: 1.5 to 3.5
    • Basal acid output: Up to 5 mEq/hour
    • Peak acid output: 1 to 20 mEq/hour
    • Ratio of basal acid output to peak acid output: <0.20 or < 20%

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