Ana and Physio

8/9/2019 Ana and Physio

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Size and Location of the Pancreas

The pancreas is a grayish-pink-colored gland about 12 to 15 cm (6-9 inches) long,

weighing about 60 g. It resembles a fish with its head and neck in the C-shaped curve of

the duodenum, its body extending horizontally behind the stomach, and its tail touching

the spleen. According to an old anatomical witticism, the romance of the abdomen is

the pancreas lying in the arms of the duodenum.

Structure of the pancreas

The pancreas is composed of two different types of glandular tissue, one exocrine

and one endocrine. Most of the tissue is exocrine, with a compound acinar arrangement.

The word acinarmeans that the cells are in a grapelike formation and that they release

their secretions into a microscopic duct within each unit. The word compoundindicates

that the ducts have branches. These tiny ducts unite to form larger ducts that eventually


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join the main pancreatic duct, which extends throughout the length of the gland from its

tail to its head. It empties into the duodenum at the same point as the common bile duct,

that is, at the major duodenal papilla. An accessory duct is frequently found extending

from the head of the pancreas into the duodenum, about 2 cm above the major papilla.

Imbedded between the exocrine units of the pancreas, like so many little islands,

lie clusters of endocrine cells called pancreatic islets. Although there are about million

of these tiny islands, they constitute only about 2% of the total mass of the pancreas.

Special staining techniques have revealed that several kinds of cells- mainly alpha cells

and beta cells- make up the islets. They are secreting cells, but their secretion passes into

blood capillaries rather than into ducts. Thus the pancreas is a dual gland- an exocrine, or

ductless, gland because of the pancreatic islets.

Functions of the Pancreas

The acinar units of the pancreas secrete the digestive enzymes found in pancreatic

juice. Hence, the pancreas plays an important part in digestion.

Beta cells of the pancreas secrete insulin, a hormone that exerts a major control

over carbohydrate metabolism.

Alpha cells secrete glucogan. It is interesting to note that glucogan, which isproduced so closely to insulin, has a directly opposite effect on carbohydrate metabolism.


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Neurohypophysis (posterior pituitary)

The neurohypophysis serves as a storage and release site for two hormones:

antidiuretic hormone (ADH) and oxytocin (OT). The cells of the neurohypophysis,

called pituicytes, do not themselves make these hormones. Instead, neurons whose bodies

are in either the supraoptic or theparaventricular nuclei of the hypothalamus synthesize

them. From the cell bodies of these neurons in the hypothalamus, the hormones pass

down along axons into the neurohypophysis. Instead of the chemical-releasing factors

that triggered secretion of hormones from the adenohypophysis, release of ADH and OT

into the blood is controlled by nervous stimulation.


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Antidiuretic hormone

The term antidiuresis literally means opposing the production of a large urine

volume. And this is exactly what antidiuretic hormone (ADH) does- it prevents the

formation of a large volume of urine. In preventing large losses of fluid through the

excretion of dilute urine, ADH helps the body conserve water. In other words, ADH

causes water retention in the body. ADH produces its effects by causing a portion of each

tubule in the kidneys to reabsorb water from the urine it has formed. When body

dehydrates, the increased osmotic pressure of the blood is detected by special

osmoreceptors near the supraoptic nucleus. This triggers the release of ADH from the

neurohypophysis. ADH causes water to be reabsorbed from the tubules of the kidneys

and returned to the blood. This increases the water content of the blood, restoring the

osmotic pressure to its normal lower level.


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The Hypothalamus

The hypothalamus is the inferior division of the diencephalon. It forms the

inferolateral walls of the third ventricle. On the underside of the brain, it lies between the

optic chiasma and the posterior boarder of the mamillary bodies. The mamillary bodies

are rounded bumps that bulge from the hypothalamic floor. The pituitary gland also

projects inferiorly from the hypothalamus. These gland secretes many hormones. The

hypothalamus, like the thalamus, contains about a dozen nuclei of gray matter.

Functionally, the hypothalamus is the main visceral control center of the body, regulating

many activities of the visceral organs. Its functions include the following:


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1. Control of the autonomic nervous system. The autonomic nervous system is the

system of peripheral motor neurons that regulates contraction of smooth and

cardiac muscle and the secretion of glands. The hypothalamus exerts control over

the autonomic neurons. In doing so, it regulates heart rate and blood pressure,

movement of the digestive tube, the secretion of sweat glands and salivary

glands, and many other visceral activities.

2. Center for emotional response. The hypothalamus lies at the center of the

emotional part of the brain, the limbic system. Regions involved in pleasure, rage,

sex drive, and fear have been located in the hypothalamus.

3. Regulation of body temperature. The bodys thermostat is in the hypothalamus.

Some hypothalamic neurons sends blood temperature that initiate the bodys

cooling or heating mechanisms as needed.

4. Hunger and thirst centers. By sensing the concentrations of nutrients and salts

in the blood, certain hypothalamic neurons mediate feelings hunger and thirst.

5. Sleep-wake cycles. Acting with other brain regions, the hypothalamus helps

regulate the complex phenomenon of sleep. The hypothalamus is responsible for

the timing of sleep cycle. The nuclei involved seem to be the suprachiasmatic

nucleus above the optic chiasma and the preoptic nucleus anterior to that. The

suprachiasmatic nucleus is the bodys biological clock, regulating many daily

rhythms. It receives information on daylight-darkness cycles from the eye through

the optic nerve, then sends signals to the preoptic nucleus. In response to such

signals, the preoptic nucleus induces sleep.

6. Control of the endocrine system. The hypothalamus controls the secretion of

hormones by the pituitary gland, which intern regulates many functions of the

visceral organs.

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