Anatomy and Physiology of Animals/Endocrine System



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.



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).



Diagram 16.2 - The position of the pituitary gland and hypothalamus



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.



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.



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.



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.



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.

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.


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


 * 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.


 * Increasing muscular activity


 * Shivering


 * 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


 * 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.


 * 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.


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


 * 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.


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


 * Heart rate increases. This also increases blood pressure.


 * 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.


 * 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.


 * 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: blood vessels on the body surface,
 * Seeking out warm areas,
 * Adjusting activity levels,
 * 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.


 * 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