Page 59 - 86 human physiology part-2

Chapter 14

norepinephrine, whose functions are very similar but not identical. The adrenal medulla is derived
embriogically from neural tissue. It has been likened to an overgrown sympathetic ganglion whose cell
bodies do not send out nerve fibers, but release their active substances directly into the blood, thereby
fulfilling the criteria for an endocrine gland. In controlling epinephrine secretion, the adrenal medulla
behaves just like any sympathetic ganglion, and is dependent upon stimulation by sympathetic
preganglionic fibers.

Epinephrine promotes several responses, all of which are helpful in coping with emergencies: the
blood pressure rises, the heart rate increases, the glucose content of the blood rises because of glycogen
breakdown, the spleen contracts and squeezes out a reserve supply of blood, the clotting time
decreases, the pupils dilate, the blood flow to skeletal muscles increase, the blood supply to intestinal
smooth muscle decreases and hairs become erect. These adrenal functions, which mobilize the
resources of the body in emergencies, have been called the fight-or-flight response. Norepinephrine
stimulates reactions similar to those produced by epinephrine, but is less effective in conversion of
glycogen to glucose.

The significance of the adrenal medulla may seem questionable since the complete removal of the
gland causes few noticeable changes; humans can still exhibit the flight-or-fight response. This occurs
because the sympathetic nervous system complements the adrenal medulla in stimulating the fight-or-
flight response, and the absence of the hormonal control will be compensated for by the nervous
system.

Pancreas

The pancreas is very important organ in the digestion system and the circulatory system because it
helps to maintain our blood sugar levels. The pancreas is considered to be part of the gastrointestinal
system. It produces digestive enzymes to be released into the small intestine to aid in reducing food
particles to basic elements that can be absorbed by the intestine and used by the body. It has another
very different function in that it forms insulin, glucagon and other hormones to be sent into the
bloodstream to regulate blood sugar levels and other activities throughout the body.

It has a pear-shape to it and is approximately 6 inches long. It is located in the middle and back
portion of the abdomen. The pancreas is connected to the first part of the small intestine, the
duodenum, and lies behind the stomach. The pancreas is made up of glandular tissue: any substance
secreted by the cells of the pancreas will be secreted outside of the organ.

The digestive juices produced by the pancreas are secreted into the duodenum via a Y-shaped duct,
at the point where the common bile duct from the liver and the pancreatic duct join just before entering
the duodenum. The digestive enzymes carried into the duodenum are representative of the exocrine
function of the pancreas, in which specific substances are made to be passed directly into another
organ.

The pancreas is unusual among the body's glands in that it Note:
also has a very important endocrine function. Small groups of The pancreas is both an exocrine and
special cells called islet cells throughout the organ make the an endocrine organ.
hormones of insulin and glucagon. These, of course, are hormones that are critical in regulating blood
sugar levels. These hormones are secreted directly into the bloodstream to affect organs all over the
body.

274 | Human Physiology