Physiology Of Secretion

l

SECRETION  The

process of elaborating a specific product as a result of the activity of a gland which may range from separating a specific substance of the blood to the elaboration of a new chemical substance.

 Glands

are those organs or tissues in which there are cells encaged in the synthesis , storage & discharge of secretory products . There are two major categories of gland in the body .

ENDOCRINE GLANDS  Synthesize

hormones & secrete them into extracellular spaces from they enters the blood stream or lymphatic system .  They do not possess a duct system & the secretion enters the blood stream rapidly & it does not leave the body until it is ultimately metabolized or excreted .

EXOCRINE GLANDS  Their

secretions are discharged from the secretory cells onto a surface & not into the extra cellular spaces .  The secretion passes through a system of ducts ( which may be a passive transport system or may modify the secretion ) which may transport it to the surface or the site of discharge . The secretion may be on body surface or on to an internal surface like the intestine .

MODE OF SECRETION Holocrine secretion  HOLOCRINE

= Wholly secreted ; denoting that type of glandular secretion in which the entire secreting cell, along with its accumulated secretion , forms the secreted matter of the gland .  The secretion from a gland consists of complete , usually degenerate cells like the sebaceous glands of the skin .

Apocrine secretion  APOCRINE = Denoting that type of

glandular secretion in which the free end or apical portion of the secreting cell is cast off along with the secretory products that have accumulated therein .  In apocrine glands , a variable but substantial amount of cytoplasm is supposed to be lost during secretion .  It is now known that , in mammary gland & axillary sweat glands , very little or even no cytoplasmic ground substance is lost when the secretion leaves the cell .

Merocrine secretion  MEROCRINE

= Partly secreted ; denoting the type of glandular secretion in which the secreting cell remains intact throughout the process of formation & discharge of the secretory products as in the salivary & pancreatic glands .  The secretory product diffused from the cell so that no cellular contents are lost .

TYPES OF SECRETION  Serous

: is clear , low viscosity fluid containing proteins .  Mucous : is opaque high viscosity fluid containing high proportion of glycoprotein.  Mixed

THE SECRETORY PROCESS Transport of water & raw materials into the cell from plasma . 2. Their movement through the cytoplasm together with synthesis & storage . 3. The extrusion of the product into: A. a lumen in case of exocrine, or B. into plasma of blood in case of endocrine gland . 1.

MOVEMENT OF WATER & SOLUTE THROUGH MEMBRANES

 Pressure

gradients across the membrane created by hydrostatic or osmotic forces moves the water across the membrane .

 Some

solute passively transferred by hydrostatic & osmotic forces , but this is limited by water & lipid solubility & the charge , size & shape of the molecules .  In some glands , the secreted fluid contains one or more constituents in higher concentration than in plasma , exerts an osmotic pull on fluid across the cell into the secretion .

Regulation of secretion Higher centres

Hypothalamus Short feedback Loop eg. LH, ACTH, GH

Releasing

hormone

Pituitary Pituitary

hormone

Target gland

Hormone

Long feedback Loop eg. Thyroxine, Cortisol

Hypothalamus regulates secretion of hormones from anterior and posterior pituitary ,ANS regulates secretion of hormones from adrenal medulla and pancreas Sympathetic nerves stimulate the secretion of adrenal medulla Parasympathetic nerves stimulate the secretion of pancreatic islets sex hormones from gonads regulate the hypothalamus