Human Anatomy and Physiology - I. V. Gayvoronsky 2011

Anatomy and Physiology of the Digestive System
Pancreas

The Pancreas is the second largest digestive gland in The Human Body. It is located in the retroperitoneal space of the Abdominal cavity AND lies against THE Vertebral Column at the level of the I — II lumbar vertebrae. In an adult, it weighs 70 — 80 g and measures 16 — 22 cm in length. Anatomically, the pancreas is divided into three parts: the HEAD, body, and tail. The head of the pancreas is cradled within the C-shaped loop of the duodenum.

Only the anterior surface of the pancreas is covered by the serous membrane. Most of the organ is in contact with the adipose tissue of the lumbar region.

The gland consists of glandular tissue proper (the parenchyma) and excretory ducts. The latter channel pancreatic juice from the lobules into the main pancreatic duct, which empties into the duodenum alongside the common Bile duct. Frequently, both ducts merge to form the hepatopancreatic ampulla. This ampulla is encircled by a layer of smooth Muscle known as the sphincter of Oddi, which regulates the flow of BILE AND PANCREATIC juice into the duodenum.

The pancreas Functions as both an exocrine and an endocrine gland. The Structural and functional unit of the pancreas as an exocrine organ is the acinus. Its Cells produce pancreatic juice, which contains numerous digestive Enzymes. Interspersed between the lobules in the tail region are specialized clusters of cells called the islets of Langerhans, which serve as the structural and functional units of the endocrine pancreas. Their cells secrete Hormones—specifically Insulin and Glucagon—directly into the bloodstream. The endocrine function of the pancreas is discussed in greater detail in Chapter 18, "The Endocrine System".

Over a 24-hour period, 1.5 — 2.0 L of pancreatic juice is produced, with a pH ranging from 7.8 to 8.4. Consequently, it has a weakly alkaline reaction and helps neutralize the Hydrochloric acid entering the duodenum with chyme from The Stomach. Water makes up the majority of pancreatic juice. The dry residue consists of organic substances and inorganic ions (Na⁺, К⁺, НСО₃^-, Cl^-, etc.). Organic compounds are predominantly enzymes, the most important of which are Trypsin, Chymotrypsin, carboxypeptidase, amylase, lipase, Ribonuclease, and deoxyribonuclease.

Trypsin, chymotrypsin, and carboxypeptidase are protein-digesting enzymes (proteases). The first two break down large peptide molecules into smaller fragments. Unlike Pepsin, they are active in an alkaline environment. Carboxypeptidase cleaves terminal Amino Acids from Polypeptides, preparing them for intestinal absorption. Trypsin is formed from the zymogen trypsinogen through the action of a specialized enzyme, enterokinase (found in intestinal juice), which cleaves off a six-amino-acid residue. Chymotrypsin is derived from chymotrypsinogen through activation by active trypsin. Pancreatic amylase breaks down CARBOHYDRATES. Lipase acts on fats that have been previously emulsified by bile, breaking lipid molecules down into glycerol and Fatty acids. Ribonuclease and deoxyribonuclease are nucleolytic enzymes that break down RNA and DNA, respectively.

Pancreatic enzymes are quite potent; to prevent auto-Digestion, the very cells that secrete Proteolytic Enzymes also produce a specialized substance known as trypsin inhibitor. This substance prevents the premature activation of trypsin within the pancreas itself.

The secretion of pancreatic juice is regulated by neural and humoral mechanisms. The entry of chyme into the duodenum reflexively increases juice production. Secretion is also stimulated by substances such as secretin, cholecystokinin, and acetylcholine. Conversely, inhibitory effects are exerted by glucagon, Somatostatin, and adrenaline. The parasympathetic Nervous system stimulates pancreatic secretion, whereas the sympathetic nervous system inhibits it.

Thus, pancreatic juice plays a vital role in digestion by facilitating the Breakdown of Proteins, fats, and carbohydrates.