# Anatomy and Physiology of Animals/Endocrine System

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**PREPARED BY ARNOLD WAMUKOTA, BUSIA**

## Objectives

After completing this section, you should know:

-   The characteristics of endocrine glands and hormones
-   The position of the main endocrine glands in the body
-   The relationship between the pituitary gland and the hypothalamus
-   The main hormones produced by the two parts of the pituitary gland
    and their effects on the body
-   The main hormones produced by the pineal, thyroid, parathyroid and
    adrenal glands, the pancreas, ovary and testicle in regard to their
    effects on the body
-   What is meant by homeostasis and feedback control
-   The homeostatic mechanisms that allow an animal to control its body
    temperature, water balance, blood volume and acid/base balance

## The Endocrine System

In order to survive, animals must constantly adapt to changes in the
environment. The **nervous** and **endocrine systems** both work
together to bring about this adaptation. In general the nervous system
responds rapidly to short-term changes by sending electrical impulses
along nerves and the endocrine system brings about longer-term
adaptations by sending out chemical messengers called hormones into the
blood stream. In general Endocrine system is represented by a set of
heterogeneous structure and origin of formations capable of internal
secretion, ie the release of biologically active substances (hormones)
that flow directly into the bloodstream.

For example, think about what happens when a male and female cat meet
under your bedroom window at night. The initial response of both cats
may include spitting, fighting and spine tingling yowling - all brought
about by the nervous system. Fear and stress then activates the adrenal
glands to secrete the hormone **adrenaline** which increases the heart
and respiratory rates. If mating occurs, other hormones stimulate the
release of ova from the ovary of the female and a range of different
hormones maintains pregnancy, delivery of the kittens and lactation.

`                                                PREPARED  By ARNOLD WERANGAI`

**Evolution of endocrine systems**

The most primitive endocrine systems seem to be those of the
neurosecretory type, in which the nervous system either secretes
neurohormones (hormones that act on, or are secreted by, nervous tissue)
directly into the circulation or stores them in neurohemal organs
(neurons whose endings directly contact blood vessels, allowing
neurohormones to be secreted into the circulation), from which they are
released in large amounts as needed. True endocrine glands probably
evolved later in the evolutionary history of the animal kingdom as
separate, hormone-secreting structures. Some of the cells of these
endocrine glands are derived from nerve cells that migrated during the
process of evolution from the nervous system to various locations in the
body. These independent endocrine glands have been described only in
arthropods (where neurohormones are still the dominant type of endocrine
messenger) and in vertebrates (where they are best developed).

It has become obvious that many of the hormones previously ascribed only
to vertebrates are secreted by invertebrates as well (for example, the
pancreatic hormone insulin). Likewise, many invertebrate hormones have
been discovered in the tissues of vertebrates, including those of
humans. Some of these molecules are even synthesized and employed as
chemical regulators, similar to hormones in higher animals, by
unicellular animals and plants. Thus, the history of endocrinologic
regulators has ancient beginnings, and the major changes that took place
during evolution would seem to centre around the uses to which these
molecules were put.

**Vertebrate endocrine systems**

Vertebrates (phylum Vertebrata) are separable into at least seven
discrete classes that represent evolutionary groupings of related
animals with common features. The class Agnatha, or the jawless fishes,
is the most primitive group. Class Chondrichthyes and class Osteichthyes
are jawed fishes that had their origins, millions of years ago, with the
Agnatha. The Chondrichthyes are the cartilaginous fishes, such as sharks
and rays, while the Osteichthyes are the bony fishes. Familiar bony
fishes such as goldfish, trout, and bass are members of the most
advanced subgroup of bony fishes, the teleosts, which developed lungs
and first invaded land. From the teleosts evolved the class Amphibia,
which includes frogs and toads. The amphibians gave rise to the class
Reptilia, which became more adapted to land and diverged along several
evolutionary lines. Among the groups descending from the primitive
reptiles were turtles, dinosaurs, crocodilians (alligators, crocodiles),
snakes, and lizards. Birds (class Aves) and mammals (class Mammalia)
later evolved from separate groups of reptiles. Amphibians, reptiles,
birds, and mammals, collectively, are referred to as the tetrapod
(four-footed) vertebrates.

The human endocrine system is the product of millions of years of
evolution. and it should not be surprising that the endocrine glands and
associated hormones of the human endocrine system have their
counterparts in the endocrine systems of more primitive vertebrates. By
examining these animals it is possible to document the emergence of the
hypothalamic-pituitary-target organ axis, as well as many other
endocrine glands, during the evolution of fishes that preceded the
origin of terrestrial vertebrates.

**The hypothalamic-pituitary-target organ axis**

The hypothalamic-pituitary-target organ axes of all vertebrates are
similar. The hypothalamic neurosecretory system is poorly developed in
the most primitive of the living Agnatha vertebrates, the hagfishes, but
all of the basic rudiments are present in the closely related lampreys.
In most of the more advanced jawed fishes there are several
well-developed neurosecretory centres (nuclei) in the hypothalamus that
produce neurohormones. These centers become more clearly defined and
increase in the number of distinct nuclei as amphibians and reptiles are
examined, and they are as extensive in birds as they are in mammals.
Some of the same neurohormones that are found in humans have been
identified in nonmammals, and these neurohormones produce similar
effects on cells of the pituitary as described above for mammals.

Two or more neurohormonal peptides with chemical and biologic properties
similar to those of mammalian oxytocin and vasopressin are secreted by
the vertebrate hypothalamus (except in Agnatha fishes, which produce
only one). The oxytocin-like peptide is usually isotocin (most fishes)
or mesotocin (amphibians, reptiles, and birds). The second peptide is
arginine vasotocin, which is found in all nonmammalian vertebrates as
well as in fetal mammals. Chemically, vasotocin is a hybrid of oxytocin
and vasopressin, and it appears to have the biologic properties of both
oxytocin (which stimulates contraction of muscles of the reproductive
tract, thus playing a role in egg-laying or birth) and vasopressin (with
either diuretic or antidiuretic properties). The functions of the
oxytocin-like substances in non-mammals are unknown.

The pituitary glands of all vertebrates produce essentially the same
tropic hormones: thyrotropin (TSH), corticotropin (ACTH), melanotropin
(MSH), prolactin (PRL), growth hormone (GH), and one or two
gonadotropins (usually FSH-like and LH-like hormones). The production
and release of these tropic hormones are controlled by neurohormones
from the hypothalamus. The cells of teleost fishes, however, are
innervated directly. Thus, these fishes may rely on neurohormones as
well as neurotransmitters for stimulating or inhibiting the release of
tropic hormones.

Among the target organs that constitute the
hypothalamic-pituitary-target organ axis are the thyroid, the adrenal
glands, and the gonads. Their individual roles are discussed below.

**The thyroid axis**

Thyrotropin secreted by the pituitary stimulates the thyroid gland to
release thyroid hormones, which help to regulate development, growth,
metabolism, and reproduction. In humans, these thyroid hormones are
known as triiodothyronine (T3) and thyroxine (T4). The evolution of the
thyroid gland is traceable in the evolutionary development of
invertebrates to vertebrates. The thyroid gland evolved from an
iodide-trapping, glycoprotein-secreting gland of the protochordates (all
nonvertebrate members of the phylum Chordata). The ability of many
invertebrates to concentrate iodide, an important ingredient in thyroid
hormones, occurs generally over the surface of the body. In
protochordates, this capacity to bind iodide to a glycoprotein and
produce thyroid hormones became specialized in the endostyle, a gland
located in the pharyngeal region of the head. When these iodinated
proteins are swallowed and broken down by enzymes, the iodinated amino
acids known as thyroid hormones are released. Larvae of primitive
vertebrate lampreys also have an endostyle like that of the
protochordates. When a lamprey larva undergoes metamorphosis into an
adult lamprey, the endostyle breaks into fragments. The resulting clumps
of endostyle cells differentiate into the separate follicles of the
thyroid gland. Thyroid hormones actually direct metamorphosis in the
larvae of lampreys, bony fishes, and amphibians. Thyroids of fishes
consist of scattered follicles in the pharyngeal region. In tetrapods
and a few fishes, the thyroid becomes encapsulated by a layer of
connective tissue.

**The adrenal axis**

The adrenal axes in mammals and in nonmammals are not constructed along
the same lines. In mammals the adrenal cortex is a separate structure
that surrounds the internal adrenal medulla; the adrenal gland is
located atop the kidneys. Because the cells of the adrenal cortex and
adrenal medulla do not form separate structures in nonmammals as they do
in mammals, they are often referred to in different terms; the cells
that correspond to the adrenal cortex in mammals are called inter renal
cells, and the cells that correspond to the adrenal medulla are called
chromaffin cells. In primitive non mammals the adrenal glands are
sometimes called inter renal glands.

In fishes the interrenal and chromaffin cells often are embedded in the
kidneys, whereas in amphibians they are distributed diffusely along the
surface of the kidneys. Reptiles and birds have discrete adrenal glands,
but the anatomical relationship is such that often the "cortex" and the
"medulla" are not distinct units. Under the influence of pituitary
adrenocorticotropin hormone, the interrenal cells produce steroids
(usually corticosterone in tetrapods and cortisol in fishes) that
influence sodium balance, water balance, and metabolism.

**The gonadal axis**

Gonadotropins secreted by the pituitary are basically LH-like and/or
FSH-like in their actions on vertebrate gonads. In general, the FSH-like
hormones promote development of eggs and sperm and the LH-like hormones
cause ovulation and sperm release; both types of gonadotropins stimulate
the secretion of the steroid hormones (androgens, estrogens, and, in
some cases, progesterone) from the gonads. These steroids produce
effects similar to those described for humans. For example, progesterone
is essential for normal gestation in many fishes, amphibians, and
reptiles in which the young develop in the reproductive tract of the
mother and are delivered live. Androgens (sometimes testosterone, but
often other steroids are more important) and estrogens (usually
estradiol) influence male and female characteristics and behaviour.

**Control of pigmentation**

Melanotropin (melanocyte-stimulating hormone, or MSH) secreted by the
pituitary regulates the star-shaped cells that contain large amounts of
the dark pigment melanin (melanophores), especially in the skin of
amphibians as well as in some fishes and reptiles. Apparently, light
reflected from the surface stimulates photoreceptors, which send
information to the brain and in turn to the hypothalamus. Pituitary
melanotropin then causes the pigment in the melanophores to disperse and
the skin to darken, sometimes quite dramatically. By releasing more or
less melanotropin, an animal is able to adapt its colouring to its
background.

**Growth hormone and prolactin**

The functions of growth hormone and prolactin secreted by the pituitary
overlap considerably, although prolactin usually regulates water and
salt balance, whereas growth hormone primarily influences protein
metabolism and hence growth. Prolactin allows migratory fishes such as
salmon to adapt from salt water to fresh water. In amphibians, prolactin
has been described as a larval growth hormone, and it can also prevent
metamorphosis of the larva into the adult. The water-seeking behaviour
(so-called water drive) of adult amphibians often observed prior to
breeding in ponds is also controlled by prolactin. The production of a
protein-rich secretion by the skin of the discus fish (called "discus
milk") that is used to nourish young offspring is caused by a
prolactin-like hormone. Similarly, prolactin stimulates secretions from
the crop sac of pigeons ("pigeon" or "crop" milk), which are fed to
newly hatched young. This action is reminiscent of prolactin's actions
on the mammary gland of nursing mammals. Prolactin also appears to be
involved in the differentiation and function of many sex accessory
structures in nonmammals, and in the stimulation of the mammalian
prostate gland. For example, prolactin stimulates cloacal glands
responsible for special reproductive secretions. Prolactin also
influences external sexual characteristics such as nuptial pads (for
clasping the female) and the height of the tail in male salamanders.

**Other vertebrate endocrine glands**

**The pancreas**

The pancreas in nonmammals is an endocrine gland that secretes insulin,
glucagon, and somatostatin. Pancreatic polypeptide has been identified
in birds and may occur in other groups as well. Insulin lowers blood
sugar (hypoglycemia) in most vertebrates, although mammalian insulin is
rather ineffective in reptiles and birds. Glucagon is a hyperglycemic
hormone (it increases the level of sugar in the blood).

In primitive fishes the cells responsible for secreting the pancreatic
hormones are scattered within the wall of the intestine. There is a
trend toward progressive clumping of cells in more evolutionarily
advanced fishes, and in a few species the endocrine tissue forms only
one or a few large islets. As a rule, most fishes lack a discrete
pancreas, but all tetrapods have a fully formed exocrine and endocrine
pancreas. The endocrine cells of all tetrapods are organized into
distinct islets as described for humans, although the abundance of the
different cell types often varies. For example, in reptiles and birds
there is a predominance of glucagon-secreting cells and relatively few
insulin-secreting cells.

**Calcium-regulating hormones**

Fishes have no parathyroid glands: these glands first appear in
amphibians. Although the embryological origin of parathyroid glands of
tetrapods is well known, their evolutionary origin is not. Parathyroid
hormone raises blood calcium levels (hypercalcemia) in tetrapods. The
absence in most fishes of cellular bone, which is the principal target
for parathyroid hormone in tetrapods, is reflected by the absence of
parathyroid glands.

Fishes, amphibians, reptiles, and birds have paired pharyngeal
ultimobranchial glands that secrete the hypocalcemic hormone calcitonin.
The corpuscles of Stannius, unique glandular islets found only in the
kidneys of bony fishes, secrete a peptide called hypocalcin. Fish
calcitonins differ somewhat from the mammalian peptide hormone of the
same name, and fish calcitonins have proved to be more potent and have a
longer-lasting action in humans than human calcitonin itself.
Consequently, synthetic fish calcitonin has been used to treat humans
suffering from various disorders of bone, including Paget's disease. The
secretory cells of the ultimobranchial glands are derived from cells
that migrated from the embryonic nervous system. During the development
of a mammalian fetus, the ultimobranchial gland becomes incorporated
into the developing thyroid gland as the "C cells" or "parafollicular
cells."

**Gastrointestinal hormones**

Little research has been done on gastrointestinal hormones in
nonmammals, but there is good evidence for a gastrinlike mechanism that
controls the secretion of stomach acids. Peptides similar to
cholecystokinin are also present and can stimulate contractions of the
gall bladder. The gall bladders of primitive fishes contract when
treated with mammalian cholecystokinin.

**Other mammalian-like endocrine systems**

**The renin-angiotensin system**

The renin-angiotensin system in mammals is represented in nonmammals by
the juxtaglomerular cells that secrete renin associated with the kidney.
The macula densa that functions as a detector of sodium levels within
the kidney tubules of tetrapods, however, has not been found in fishes.

**The pineal complex**

In fishes, amphibians, and reptiles, the pineal complex is better
developed than in mammals. The nonmammalian pineal functions as both a
photoreceptor organ and an endocrine source for melatonin. Effects of
light on reproduction in fishes and tetrapods are mediated at least in
part through the pineal, and it has been implicated in a number of daily
and seasonal biorhythmic phenomena.

**Prostaglandins**

Many tissues of nonmammals produce prostaglandins that play important
roles in reproduction similar to those discussed for humans and other
mammals.

**The liver**

As in mammals, the liver of several nonmammalian species has been shown
to produce somatomedin-like growth factors in response to stimulation by
growth hormone. Similarly, there is evidence that prolactin stimulates
the production of a related growth factor, which synergizes (cooperates)
with prolactin on targets such as the pigeon crop sac.

**Unique endocrine glands in fishes**

In addition to the corpuscles of Stannius and the ultimobranchial
glands, most fishes have a unique neurosecretory neurohemal organ, the
urophysis, which is associated with the spinal cord at the base of the
tail. Although the functions of this caudal (rear) neurosecretory system
are not now understood, it is known to produce two peptides, urotensin I
and urotensin II. Urotensin I is chemically related to a family of
peptides that includes somatostatin; urotensin II is a member of the
family of peptides that includes mammalian corticotropin-releasing
hormone (CRH). There are no homologous structures to either the
corpuscles of Stannius or the urophysis in amphibians, reptiles, or
birds.

**Invertebrate endocrine systems**

Advances in the study of invertebrate endocrine systems have lagged
behind those in vertebrate endocrinology, largely due to the problems
associated with adapting investigative techniques that are appropriate
for large vertebrate animals to small invertebrates. It also is
difficult to maintain and study appropriately some invertebrates under
laboratory conditions. Nevertheless, knowledge about these systems is
accumulating rapidly.

All phyla in the animal kingdom that have a nervous system also possess
neurosecretory neurons. The results of studies on the distribution of
neurosecretory neurons and ordinary epithelial endocrine cells imply
that the neurohormones were the first hormonal regulators in animals.
Neurohemal organs appear first in the more advanced invertebrates (such
as mollusks and annelid worms), and endocrine epithelial glands occur
only in the most advanced phyla (primarily Arthropoda and Chordata).
Similarly, the peptide and steroid hormones found in vertebrates are
also present in the nervous and endocrine systems of many invertebrate
phyla. These hormones may perform similar functions in diverse animal
groups. With more emphasis being placed on research in invertebrate
systems, new neuropeptides are being discovered initially in these
animals, and subsequently in vertebrates.

The endocrine systems of some animal phyla have been studied in detail,
but the endocrine systems of only a few species are well known. The
following discussion summarizes the endocrine systems of five
invertebrate phyla and the two invertebrate subphyla of the phylum
Chordata, a phylum that also includes Vertebrata, a subphylum to which
the backboned animals belong

## Endocrine Glands And Hormones

Hormones are chemicals that are secreted by **endocrine glands**. Unlike
exocrine glands (see chapter 5), endocrine glands have no ducts, but
release their secretions directly into the blood system, which carries
them throughout the body. However, hormones only affect the specific
**target organs** that recognize them. For example, although it is
carried to virtually every cell in the body, **follicle stimulating
hormone** (FSH), released from the **anterior pituitary gland**, only
acts on the follicle cells of the ovaries causing them to develop.

A nerve impulse travels rapidly and produces an almost instantaneous
response but one that lasts only briefly. In contrast, hormones act more
slowly and their effects may be long lasting. Target cells respond to
minute quantities of hormones and the concentration in the blood is
always extremely low. However, target cells are sensitive to subtle
changes in hormone concentration and the endocrine system regulates
processes by changing the rate of hormone secretion.

The main endocrine glands in the body are the **pituitary, pineal,
thyroid, parathyroid**, and **adrenal glands**, the **pancreas,
ovaries** and **testes**. Their positions in the body are shown in
diagram 16.1.

![](Anatomy_and_physiology_of_animals_Main_endocrine_organs_of_the_body.jpg "Anatomy_and_physiology_of_animals_Main_endocrine_organs_of_the_body.jpg")

Diagram 16.1 - The main endocrine organs of the body

## The Pituitary Gland And Hypothalamus

The **pituitary gland** is a pea-sized structure that is attached by a
stalk to the underside of the cerebrum of the brain (see diagram 16.2).
It is often called the "master" endocrine gland because it controls many
of the other endocrine glands in the body. However, we now know that the
pituitary gland is itself controlled by the **hypothalamus**. This small
but vital region of the brain lies just above the pituitary and provides
the link between the nervous and endocrine systems. It controls the
**autonomic nervous system**, produces a range of hormones and regulates
the secretion of many others from the pituitary gland (see Chapter 7 for
more information on the hypothalamus).

The pituitary gland is divided into two parts with different functions -
the **anterior** and **posterior pituitary** (see diagram 16.3).

![](Anatomy_and_physiology_of_animals_Position_of_the_pituitary_gland_and_hypothalamus.jpg "Anatomy_and_physiology_of_animals_Position_of_the_pituitary_gland_and_hypothalamus.jpg")

Diagram 16.2 - The position of the pituitary gland and hypothalamus

![](Anterior_and_posterior_pituitary.jpg "Anterior_and_posterior_pituitary.jpg")

Diagram 16.3 - The anterior and posterior pituitary

The **anterior pituitary gland** secretes hormones that regulate a wide
range of activities in the body. These include:

:   1\. **Growth hormone** that stimulates body growth.
:   2\. **Prolactin** that initiates milk production.
:   3\. **Follicle stimulating hormone (FSH**) that stimulates the
    development of the **follicles** of the ovaries. These then secrete
    **oestrogen** (see chapter 6).
:   4\. **melanocyte stimulating hormone (MSH**) that causes darkening
    of skin by producing melanin
:   5\. **lutenizing hormone (LH**) that stimulates ovulation and
    production of progesterone and testosterone

The **posterior pituitary gland**

:   1\. Antidiuretic Hormone (ADH), regulates water loss and increases
    blood pressure
:   2\. Oxytocin, milk \"let down\"

## The Pineal Gland

The **pineal gland** is found deep within the brain (see diagram 16.4).
It is sometimes known as the 'third eye" as it responds to light and day
length. It produces the hormone **melatonin**, which influences the
development of sexual maturity and the seasonality of breeding and
hibernation. **Bright light inhibits melatonin secretion** Low level of
melatonin in bright light makes one feel good and this increases
fertility. High level of melatonin in dim light makes an animal tired
and depressed and therefore causes low fertility in animals.

![](Anatomy_and_physiology_of_animals_Pineal_gland.jpg "Anatomy_and_physiology_of_animals_Pineal_gland.jpg")

Diagram 16.4 - The pineal gland

## The Thyroid Gland

The **thyroid gland** is situated in the neck, just in front of the
windpipe or trachea (see diagram 16.5). It produces the hormone
**thyroxine**, which influences the rate of growth and development of
young animals. In mature animals it increases the rate of chemical
reactions in the body.

Thyroxine consists of 60% **iodine** and too little in the diet can
cause **goitre**, an enlargement of the thyroid gland. Many inland soils
in New Zealand contain almost no iodine so goitre can be common in stock
when iodine supplements are not given. To add to the problem, chemicals
called **goitrogens** that occur naturally in plants like kale that
belong to the **cabbage family**, can also cause goitre even when there
is adequate iodine available.

![](Anatomy_and_physiology_of_animals_Thyroid_&_parathyroid_glands.jpg "Anatomy_and_physiology_of_animals_Thyroid_&_parathyroid_glands.jpg")

Diagram 16.5 - The thyroid and parathyroid glands

## The Parathyroid Glands

The **parathyroid glands** are also found in the neck just behind the
thyroid glands (see diagram 16.5). They produce the hormone
**parathormone** that regulates the amount of **calcium** in the blood
and influences the excretion of **phosphates** in the urine.

## The Adrenal Gland

The **adrenal glands** are situated on the cranial surface of the
kidneys (see diagram 16.6). There are two parts to this endocrine gland,
an outer **cortex** and an inner **medulla**.

![](Anatomy_and_physiology_of_animals_Adrenal_glands.jpg "Anatomy_and_physiology_of_animals_Adrenal_glands.jpg")

Diagram 16.6 - The adrenal glands

The **adrenal cortex** produces several hormones. These include:

:   1\. **Aldosterone** that regulates the concentration of **sodium and
    potassium** in the blood by controlling the amounts that are
    secreted or reabsorbed in the kidney tubules.
:   2\. **Cortisone** and **hydrocortisone** (cortisol) that have
    complex effects on glucose, protein and fat metabolism. In general
    they increase metabolism. They are also often administered to
    animals to counteract allergies and for treating arthritic and
    rheumatic conditions. However, prolonged use should be avoided if
    possible as they can increase weight and reduce the ability to heal.
:   3\. **Male and female sex hormones** similar to those secreted by
    the ovaries and testes.

The hormones secreted by the adrenal cortex also play a part in
"**general adaptation syndrome**" which occurs in situations of
prolonged stress.

The **adrenal medulla** secretes **adrenalin** (also called
**epinephrine**). Adrenalin is responsible for the so-called flight
fight, fright response that prepares the animal for emergencies. Faced
with a perilous situation the animal needs to either fight or make a
rapid escape. To do either requires instant energy, particularly in the
skeletal muscles. Adrenaline increases the amount of blood reaching them
by causing their blood vessels to dilate and the heart to beat faster.
An increased rate of breathing increases the amount of oxygen in the
blood and glucose is released from the liver to provide the fuel for
energy production. Sweating increases to keep the muscles cool and the
pupils of the eye dilate so the animal has a wide field of view.
Functions like digestion and urine production that are not critical to
immediate survival slow down as blood vessels to these parts constrict.

Note that the effects of adrenalin are similar to those of the
sympathetic nervous system.

`                                                              PREPARED BY ARNOLD WAMUKOTA`

## The Pancreas

In most animals the **pancreas** is an oblong, pinkish organ that lies
in the first bend of the small intestine (see diagram 16.7). In rodents
and rabbits, however, it is spread thinly through the mesentery and is
sometimes difficult to see.

![](Anatomy_and_physiology_of_animals_The_pancreas.jpg "Anatomy_and_physiology_of_animals_The_pancreas.jpg")

Diagram 16.7 - The pancreas

Most of the pancreas acts as an **exocrine gland** producing digestive
enzymes that are secreted into the small intestine. The endocrine part
of the organ consists of small clusters of cells (called **Islets of
Langerhans**) that secrete the hormone **insulin**. This hormone
regulates the amount of **glucose** in the blood by increasing the rate
at which glucose is converted to glycogen in the liver and the movement
of glucose from the blood into cells.

In **diabetes mellitus** the pancreas produces insufficient insulin and
glucose levels in the blood can increase to a dangerous level. A major
symptom of this condition is glucose in the urine.

## The Ovaries

A part of the reproductive system of all female vertebrates. Although
not vital to individual survival, the ovary is vital to perpetuation of
the species. The function of the ovary is to produce the female germ
cells or ova, and in some species to elaborate hormones that assist in
regulating the reproductive cycle.

The ovaries develop as bilateral structures in all vertebrates, but
adult asymmetry is found in certain species of all vertebrates from the
elasmobranchs to the mammals.

The ovary of all vertebrates functions in essentially the same manner.
However, ovarian histology of the various groups differs considerably.
Even such a fundamental element as the ovum exhibits differences in
various groups. See Ovum

The mammalian ovary is attached to the dorsal body wall. The free
surface of the ovary is covered by a modified peritoneum called the
germinal epithelium. Just beneath the germinal epithelium is a layer of
fibrous connective tissue. Most of the rest of the ovary is made up of a
more cellular and more loosely arranged connective tissue (stroma) in
which are embedded the germinal, endocrine, vascular, and nervous
elements.

The most obvious ovarian structures are the follicles and the corpora
lutea. The smallest, or primary, follicle consists of an oocyte
surrounded by a layer of follicle (nurse) cells. Follicular growth
results from an increase in oocyte size, multiplication of the follicle
cells, and differentiation of the perifollicular stroma to form a
fibrocellular envelope called the theca interna. Finally, a fluid-filled
antrum develops in the granulosa layer, resulting in a vesicular
follicle.

The cells of the theca intima hypertrophy during follicular growth and
many capillaries invade the layer, thus forming the endocrine element
that is thought to secrete estrogen. The other known endocrine structure
is the corpus luteum, which is primarily the product of hypertrophy of
the granulosa cells remaining after the follicular wall ruptures to
release the ovum. Ingrowths of connective tissue from the theca interna
deliver capillaries to vascularize the hypertrophied follicle cells of
this new corpus luteum; progesterone is secreted here.

`                                                           PREPARED BY ARNOLD WAMUKOTA`

## The Testes

Sperm need temperatures between 2 and 10 degrees Centigrade lower and
then the body temperature to develop. This is the reason why the testes
are located in a bag of skin called the scrotal sacs (or scrotum) that
hangs below the body and where the evaporation of secretions from
special glands can further reduce the temperature. In many animals
(including humans) the testes descend into the scrotal sacs at birth but
in some animals they do not descend until sexual maturity and in others
they only descend temporarily during the breeding season. A mature
animal in which one or both testes have not descended is called a
cryptorchid and is usually infertile.

The problem of keeping sperm at a low enough temperature is even greater
in birds that have a higher body temperature than mammals. For this
reason bird's sperm are usually produced at night when the body
temperature is lower and the sperm themselves are more resistant to
heat.

The testes consist of a mass of coiled tubes (the seminiferous or sperm
producing tubules) in which the sperm are formed by meiosis (see diagram
13.4). Cells lying between the seminiferous tubules produce the male sex
hormone testosterone.

When the sperm are mature they accumulate in the collecting ducts and
then pass to the epididymis before moving to the sperm duct or vas
deferens. The two sperm ducts join the urethra just below the bladder,
which passes through the penis and transports both sperm and urine.

Ejaculation discharges the semen from the erect penis. It is brought
about by the contraction of the epididymis, vas deferens, prostate gland
and urethra.

`                                             PREPARED BY ARNOLD WAMUKOTA`

## Summary

-   **Hormones** are chemicals that are released into the blood by
    **endocrine glands** i.e. Glands with no ducts. Hormones act on
    specific **target organs** that recognize them.
-   The main endocrine glands in the body are the **hypothalamus,
    pituitary, pineal, thyroid, parathyroid** and **adrenal glands,**
    the **pancreas, ovaries** and **testes**.
-   The **hypothalamus** is situated under the **cerebrum** of the
    brain. It produces or controls many of the hormones released by the
    pituitary gland lying adjacent to it.
-   The **pituitary gland** is divided into two parts: the **anterior
    pituitary** and the **posterior pituitary**.
-   The **anterior pituitary** produces:

:\* **Growth hormone** that stimulates body growth

:\* **Prolactin** that initiates milk production

:\* **Follicle stimulating hormone** (**FSH**) that stimulates the
development of **ova**

:\* **Luteinising hormone (LH**) that stimulates the development of the
**corpus luteum**

:\* Plus several other hormones

-   The **posterior pituitary** releases:

:\* **Antidiuretic hormone** (ADH) that regulates **water loss** and
raises **blood pressure**

:\* **Oxytocin** that stimulates milk "let down".

-   The **pineal gland** in the brain produces **melatonin** that
    influences **sexual development** and **breeding cycles**.
-   The **thyroid gland** located in the neck, produces thyroxine, which
    influences the **rate of growth** and **development** of young
    animals. Thyroxine consists of 60% **iodine**. Lack of iodine leads
    to **goitre**.
-   The **parathyroid glands** situated adjacent to the thyroid glands
    in the neck produce **parathormone** that regulates blood
    **calcium** levels and the excretion of **phosphates**.
-   The **adrenal gland** located adjacent to the kidneys is divided
    into the outer **cortex** and the inner **medulla**.
-   The **adrenal cortex** produces:

:\***Aldosterone** that regulates the blood concentration of **sodium
and potassium**

:\* **Cortisone** and **hydro-cortisone** that affect **glucose,
protein** and **fat** metabolism

:\* Male and female **sex hormones**

-   The **adrenal medulla** produces **adrenalin** responsible for the
    **flight, fright, fight** response that prepares animals for
    emergencies.
-   The **pancreas** that lies in the first bend of the small intestine
    produces**insulin** that regulates blood **glucose** levels.
-   The **ovaries** are located in the lower abdomen produce 2 important
    sex hormones:

:\* The **follicle cells** of the developing ova produce **estrogen**,
which controls the development of the **mammary glands** and prepares
the uterus for pregnancy.

:\* The **corpus luteum** that develops in the empty **follicle** after
ovulation produces **progesterone**. This hormone further prepares the
**uterus** for pregnancy and maintains the pregnancy.

-   The **testes** produce **testosterone** that stimulates the
    development of the **male reproductive system** and **sexual
    characteristics**.

## Homeostasis and Feedback Control

Animals can only survive if the environment within their bodies and
their cells is kept constant and independent of the changing conditions
in the external environment. As mentioned in module 1.6, the process by
which this stability is maintained is called homeostasis. The body
achieves this stability by constantly monitoring the internal conditions
and if they deviate from the norm initiating processes that bring them
back to it. This mechanism is called feedback control. For example, to
maintain a constant body temperature the hypothalamus monitors the blood
temperature and initiates processes that increase or decrease heat
production by the body and loss from the skin so the optimum temperature
is always maintained. The processes involved in the control of body
temperature, water balance, blood loss and acid/base balance are
summarized below.

## Summary of Homeostatic Mechanisms

### 1. Temperature control

The biochemical and physiological processes in the cell are sensitive to
temperature. The optimum body temperature is about 37 C \[99 F\] for
mammals, and about 40 C \[104 F\] for birds. Biochemical processes in
the cells, particularly in muscles and the liver, produce heat. The heat
is distributed through the body by the blood and is lost mainly through
the skin surface. The production of this heat and its loss through the
skin is controlled by the hypothalamus in the brain which acts rather
like a thermostat on an electric heater. .

\(a\) When the body temperature rises above the optimum, a decrease in
temperature is achieved by:

-   Sweating and panting to increase heat loss by evaporation.

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-   Expansion of the blood vessels near the skin surface so heat is lost
    to the air.

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-   Reducing muscle exertion to the minimum.

\(b\) When the body temperature falls below the optimum, an increase in
temperature can be achieved by:

-   Moving to a heat source e.g. in the sun, out of the wind.

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-   Increasing muscular activity

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-   Shivering

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-   Making the hair stand on end by contraction of the hair erector
    muscles or fluffing of the feathers so there is an insulating layer
    of air around the body

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-   Constricting the blood vessels near the skin surface so heat loss to
    the air is decreased

### 2. Water balance

The concentration of the body fluids remains relatively constant
irrespective of the diet or the quantity of water taken into the body by
the animal. Water is lost from the body by many routes (see module 1.6)
but the kidney is the main organ that influences the quantity that is
lost. Again it is the hypothalamus that monitors the concentration of
the blood and initiates the release of hormones from the posterior
pituitary gland. These act on the kidney tubules to influence the amount
of water (and sodium ions) absorbed from the fluid flowing along them.

\(a\) When the body fluids become too concentrated and the osmotic
pressure too high, water retention in the kidney tubules can be achieved
by:

-   An increased production of anti-diuretic hormone (ADH) from the
    posterior pituitary gland, which causes more water to be reabsorbed
    from the kidney tubules.

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-   A decreased blood pressure in the glomerulus of the kidney results
    in less fluid filtering through into the kidney tubules so less
    urine is produced.

\(b\) When the body fluids become too dilute and the osmotic pressure
too low, water loss in the urine can be achieved by:

-   A decrease in the secretion of ADH, so less water is reabsorbed from
    the kidney tubules and more diluted urine is produced.

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-   An increase in the blood pressure in the glomerulus so more fluid
    filters into the kidney tubule and more urine is produced.

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-   An increase in sweating or panting that also increases the amount of
    water lost.

Another hormone, aldosterone, secreted by the cortex of the adrenal
gland, also affects water balance indirectly. It does this by increasing
the absorption of sodium ions (Na-) from the kidney tubules. This
increases water retention since it increases the osmotic pressure of the
fluids around the tubules and water therefore flows out of them by
osmosis.

### 3. Maintenance of blood volume after moderate blood loss

Loss of blood or body fluids leads to decreased blood volume and hence
decreased blood pressure. The result is that the blood system fails to
deliver enough oxygen and nutrients to the cells, which stop functioning
properly and may die. Cells of the brain are particularly vulnerable.
This condition is known as shock.

If blood loss is not extreme, various mechanisms come into play to
compensate and ensure permanent tissue damage does not occur. These
mechanisms include:

-   Increased thirst and drinking increases blood volume.

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-   Blood vessels in the skin and kidneys constrict to reduce the total
    volume of the blood system and hence retain blood pressure.

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-   Heart rate increases. This also increases blood pressure.

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-   Antidiuretic hormone (ADH) is released by the posterior pituitary
    gland. This increases water re-absorption in the collecting ducts of
    the kidney tubules so concentrated urine is produced and water loss
    is reduced. This helps maintain blood volume.

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-   Loss of fluid causes an increase in osmotic pressure of the blood.
    Proteins, mainly albumin, released into the blood by the liver
    further increase the osmotic pressure causing fluid from the tissues
    to be drawn into the blood by osmosis. This increases blood volume.

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-   Aldosterone, secreted by the adrenal cortex, increases the
    absorption of sodium ions (Na+) and water from the kidney tubules.
    This increases urine concentration and helps retain blood volume.

If blood or fluid loss is extreme and the blood volume falls by more
than 15-25%, the above mechanisms are unable to compensate and the
condition of the animal progressively deteriorates. The animal will die
unless a vet administers fluid or blood.

### 4. Acid/ base balance

Biochemical reactions within the body are very sensitive to even small
changes in acidity or alkalinity (i.e. pH) and any departure from the
narrow limits disrupts the functioning of the cells. It is therefore
important that the blood contains balanced quantities of acids and
bases.

The normal pH of blood is in the range 7.35 to 7.45 and there are a
number of mechanisms that operate to maintain the pH in this range.
Breathing is one of these mechanisms.

Much of the carbon dioxide produced by respiration in cells is carried
in the blood as carbonic acid. As the amount of carbon dioxide in the
blood increases the blood becomes more acidic and the pH decreases. This
is called acidosis and when severe can cause coma and death. On the
other hand, alkalosis (blood that is too alkaline) causes over
stimulation of the nervous system and when severe can lead to
convulsions and death.

\(a\) When vigorous activity generating large quantities of carbon
dioxide causes the blood to becomes too acidic it can be counteracted in
two ways:

-   By the rapid removal of carbon dioxide from the blood by deep,
    panting breaths

`   By the secretion of hydrogen ions (H+) into the urine by the kidney tubules. `

\(b\) When over breathing or hyperventilation results in low levels of
carbon dioxide in the blood and the blood is too alkaline, various
mechanisms come into play to bring the pH back to within the normal
range. These include:

-   A slower rate of breathing
-   A reduction in the amount of hydrogen ions (H+) secreted into the
    urine.

### SUMMARY

Homeostasis is the maintenance of constant conditions within a cell or
animal's body despite changes in the external environment.

The body temperature of mammals and birds is maintained at an optimum
level by a variety of heat regulation mechanisms. These include:

-   Seeking out warm areas,
-   Adjusting activity levels,

blood vessels on the body surface,

-   Contraction of the erector muscles so hairs and feathers stand up to
    form an insulating layer,
-   Shivering,
-   Sweating and panting in dogs.

Animals maintain water balance by:

-   adjusting level of antidiuretic hormone(ADH)
-   adjusting level of aldosterone,
-   adjusting blood flow to the kidneys
-   adjusting the amount of water lost through sweating or panting.

Animals maintain blood volume after moderate blood loss by:

-   Drinking,
-   Constriction of blood vessels in the skin and kidneys,
-   increasing heart rate,
-   secretion of anti-diuretic hormone
-   secretion of aldosterone
-   drawing fluid from the tissues into the blood by increasing the
    osmotic pressure of the blood.

Animals maintain the acid/base balance or pH of the blood by:

-   Adjusting the rate of breathing and hence the amount of CO2 removed
    from the blood.
-   Adjusting the secretion of hydrogen ions into the urine.

## Worksheet

Endocrine System
Worksheet

## Test Yourself

1\. What is Homeostasis?

2\. Give 2 examples of homeostasis

3\. List 3 ways in which animals keep their body temperature constant
when the weather is hot

4\. How does the kidney compensate when an animal is deprived of water
to drink

5\. After moderate blood loss, several mechanisms come into play to
increase blood pressure and make up blood volume. 3 of these mechanisms
are:

6\. Describe how panting helps to reduce the acidity of the blood

/Test Yourself Answers/

## Websites

-   <http://www.zerobio.com/drag_oa/endo.htm> A drag and drop hormone
    and endocrine organ matching exercise.

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-   <http://en.wikipedia.org/wiki/Endocrine_system> Wikipedia. Much,
    much more than you ever need to know about hormones and the
    endocrine system but with a bit of discipline you can glean lots of
    useful information from this site.

## Glossary

-   Link to
    Glossary

