1. Pancreatic Secretions

Exocrine pancreas secretion occurs after the ingestion of a meal, and is associated with the appearance of the acidified chyme from the stomach in the duodenum. There is a need to neutralize the pH of the incoming acid. As a consequence there is a coupled secretion by the exocrine pancreas of the pancreatic juice, which is a rich source of HCO3− (bicarbonate, pH ~7) and also of many enzymes. The composition of the human pancreatic juice is given in Table 1.

Table1. Composition of Human Pancreatic Juice ^a

Many of the potent enzymes are secreted in inactive pro forms and must be converted separately to active enzymes in the lumen of the intestine. The pancreatic secretion is mainly dependent upon the actions of CCK; in addition, the stimulation of appropriate cholinergic or peptidergic nerves leading to the pancreas can stimulate exocrine pancreatic secretions.

Pancreatic secretions of electrolytes and H2O from the ductular and centroacinar cells are primarily under the influence of secretin. Secretin is known to stimulate intra cellular cAMP in these cells, which activates unknown factors necessary to alter the secretory cell membrane permeability to sodium and hydrogen ions. Thus, there is an increased exchange of external Na+ ions for internal H+ ions. The increased concentration of extracellular H+ lowers the local pH, which then increases the pro duction of CO2 from circulating bicarbonate. This CO2 then diffuses into the cells, where it combines with water to form carbonic acid (mediated by carbonic anhydrase) and results in the production of HCO3−, which can be utilized for secretion.

The pancreatic secretion of digestive enzymes that occurs in the acinar cells of the exocrine pancreas is under the influence of CCK. In the pancreatic acinar cells, CCK interacts with a CCK-A receptor present on the outer cell membrane. This activates a G protein-coupled signal transduction pathway to ultimately stimulate the secretion of amylase, trypsinogen, and chymotrypsinogen stored in secretory granules. In contrast when CCK binds to the CCK-B receptor it has stimulatory effects on the vagus nerve and has functions relating to anxiety and the perception of pain.

2. Biliary Secretion

The liver produces 500–1500 mL of bile per day. Bile canaliculi also known as bile capillaries are thin tubes that receive bile secreted by hepatocytes. The bile canaliculi eventually merge and form bile ductules. The bile passes through canaliculi to the hepatic bile ducts and then into the common hepatic duct which drains directly into the duodenum. Table 2 summarizes the major organic com ponents present in human bile.

Table2. Major Organic Components of Human Bile ^a

As bile flows through the bile ducts it is diluted by addition of a watery, bicarbonate-rich secretion from ductal epithelial cells. Then the bile passes through small ducts that empty directly into the upper portion of the duodenum. This results in the release of secretin from S cells in the small intestine. Secretin stimulates the pancreatic secretion of bicarbonate and water.

Similarly, the exocrine pancreas has ducts that are arranged in clusters called acini that merge into the main pancreatic duct which also drains directly into the duodenum. This results in the secretion of CCK by the I cells in the duodenum and jejunum, which results also in the stimulation of secretion of ancinar pancreatic amylase, trypsin, and chymotrypsin secretions directly into the duodenum and separately stimulation of the contraction of the gallbladder. As yet there is no detailed mechanism describing how CCK mediates gallbladder emptying.

Bile is a watery mixture of inorganic and organic compounds that are synthesized by the liver and then stored in the gallbladder. Table 2 presents the major organic components of human bile. These constituents are secreted into the intestinal lumen in response to the presence of dietary fat and they act as detergents to disrupt and disperse the oil droplets.

3. Intestinal Secretion

Several gut hormones, including VIP, GIP, secretin, CCK, and glucagon, and prostaglandins E1, E2, and F2α have been shown to affect the small intestine and colon by either inhibiting the active absorption of electrolytes and H2O or stimulating the secretion of H2O and electrolytes. Both VIP and the prostaglandins are potent stimulators of the adenylate cyclase-cAMP system, and it is probable that this is the basis for their actions on the intestinal secretion of H2O and electrolytes. The cellular mechanisms of the other gut hormones in this system are not yet known.

اخر الاخبار

اخبار العتبة العباسية المقدسة

قسم الشؤون الدينية يطلق الدورة التطويرية والفقهية الخامسة لملاكات العتبة العباسية المقدسة

قسم الشؤون الفكرية ينظّم ختمة قرآنية رمضانية لطلبة العلوم الدينية الأفارقة في النجف

شعبة الخطابة تقيم مجلسا للوعظ والإرشاد في الصحن الشريف تزامناً مع حلول شهر رمضان المبارك

قسم التطوير يختتم دورة إعداد المدربين لملاكات مديرية تربية كربلاء

المزيد

الآخبار الصحية

مفاجأة.. تحسين القدرة على التحمل لا يعتمد العضلات فقط

المرتفعات والأكسجين.. دراسة "مبشرة" لمرضى السكري

علماء يبتكرون "آلية" تتوقع موعد ظهور أعراض الزهايمر

ماذا يحدث لجسمك عند التوقف عن تناول الخضراوات والفواكه؟

المزيد

الآخبار التكنلوجية

رصد "القرش النائم" في أحد أقسى الأماكن على وجه الأرض

أسماك أعماق البحر تكشف نظاما بصريا يتجاوز "المألوف"

زوكربيرغ أمام القضاء في قضية إدمان المراهقين

"علي بابا" تكشف عن نموذج جديد للذكاء الاصطناعي

المزيد

مواضيع ذات صلة

Hormone-Secreting Cells: Their Distribution in the Gastroenteropancreatic Complex

Leptin Functions as a Satiety Factor

Glucagon and Glucagon-like Peptides— Biosynthesis and Secretion

Adult Growth Hormone Deficiency

Hypopituitary crisis

Thyroid Replacement in TSH Deficiency

Glucagon’s Structure

Insulin and Its Glucose Transporter

Insulin Receptor

Insulin Structure

Biological Actions of T3

Adrenal Replacement in ACTH Deficiency