R–S

relaxin A peptide HORMONEwith poorly under-
stood roles in reproduction and CHILDBIRTH. Relaxin
receptors are widespread throughout the body in
both men and women. In women the OVARIES, cor-
pus luteum, and breasts—and the PLACENTAduring
PREGNANCY—produce relaxin. In men, the PROSTATE
GLANDand the seminal vesicles produce relaxin.
Relaxin is biochemically similar to INSULINand has
numerous effects on smooth MUSCLEtissue and col-
lagen (connective tissue) in the UTERUS, reproduc-
tive tract, cardiovascular system, urinary system,
and gastrointestinal system. During pregnancy
relaxin facilitates collagen remodeling, the alter-
ations that take place in the collagen structures of
the ligaments and tendons that support the enlarg-
ing uterus. During CHILDBIRTHrelaxin appears to, as
the name implies, relax the smooth muscles of the
CERVIXand uterus after contractions. In men relaxin
may facilitate the transportation of SPERMthrough
the seminal vesicles.
For further discussion of relaxin within the
context of the endocrine system’s structure and
function please see the overview section “The
Endocrine System.”
See also CHORIONIC GONADOTROPIN; ESTROGENS;
OXYTOCIN; PROLACTIN; PROGESTERONE.

renin A peptide HORMONE the KIDNEYS produce
that sets in motion the sequence of events to con-
vert angiotensinogen, an inert enzyme the LIVER
stores, to angiotensin II, a potent vasoconstrictor
(chemical that causes the blood vessels to narrow
and stiffen, which raises BLOOD PRESSURE).
Angiotensin II also stimulates the adrenal cortex
of the ADRENAL GLANDSto release ALDOSTERONE. The
kidneys secrete renin whenever they sense a
reduction in the fluid volume in the BLOODthat
passes through them. Aldosterone causes the kid-

neys to increase the amount of sodium they with-

hold from the blood, which in turn draws more

water back into the blood circulation. In combina-

tion, these actions increase blood volume and

blood pressure. Aldosterone’s presence in the

blood circulation and its actions on the kidneys

also suppress renin secretion. The sequence of

events is the renin–angiontensin–aldosterone

(RAA) system, the body’s key mechanism for reg-

ulating blood pressure and blood volume.

The RAA balance adjusts nearly continuously,

as the blood’s pressure and volume fluctuate with

body activities down to even the most minute

metabolic alterations. The release of renin may

surge when a person stands up, for example, and

slow when a person lies down, and even when a

person transitions from sleep to wake. The RAA

system relies on the proper functioning of all three

component elements; dysfunction of any results in

HYPERTENSION(high blood pressure). Kidney dis-

ease, particularly RENAL FAILURE, and HYPERALDOS-

TERONISMare the most important health conditions

that influence the RAA system because both inter-

fere with the release of renin. The excessive

aldosterone in the blood circulation with hyperal-

dosteronism suppresses renin release, and kidney

disease may interfere with the ability of the kid-

neys to sense fluid volume or may damage the

cells that synthesize renin.

For further discussion of renin within the con-

text of the endocrine system’s structure and func-

tion, please see the overview section “The

Endocrine System.”

See also ANTIDIURETIC HORMONE(ADH).

somatostatin A peptide HORMONEthe delta cells

of the ISLETS OFLANGERHANSin the PANCREASprima-

rily synthesize (produce). The HYPOTHALAMUSand

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