•
The endocrine
system consists of glands widely separated from each other with no physical
connections.
• Endocrine glands are groups of secretory cells
surrounded by an extensive network of capillaries that facilitates diffusion of
hormones (chemical messengers) from the secretory cells into the bloodstream.
They are commonly referred to as ductless glands because hormones diffuse
directly into the bloodstream. Hormones are then carried in the bloodstream to
target tissues and organs that may be quite distant, where they influence
cellular growth and metabolism.
•
Homeostasis
of the internal environment is maintained partly by the autonomic nervous
system and partly by the endocrine system.
Anterior pituitary
Some of the hormones secreted by the
anterior lobe stimulate or inhibit secretion by other endocrine glands (target
glands) while others have a direct effect on target tissues. Table 9.1
summarises the main relationships between the hormones of the hypothalamus, the
anterior pituitary and target glands or tissues.
Growth hormone (GH)
• This is the most abundant hormone synthesised by the
anterior pituitary.
• It stimulates growth and division of most body cells
but especially those in the bones and skeletal muscles. Body growth in response
to the secretion of GH is evident during childhood and adolescence, and
thereafter secretion of GH maintains the mass of bones and skeletal muscles.
• It also regulates aspects of metabolism in many
organs, e.g. liver, intestines and pancreas; stimulates protein synthesis,
especially tissue growth and repair; promotes breakdown of fats; and increases
blood glucose levels
Thyroid stimulating hormone (TSH)
• This hormone is synthesised by the anterior pituitary
and its release is stimulated by thyrotrophin releasing hormone (TRH) from the
hypothalamus. It stimulates growth and activity of the thyroid gland, which
secretes the hormones thyroxine (T4) and tri-iodothyronine (T3).
Anterior Pituitary Hormones
Adrenocorticotrophic hormone (ACTH, corticotrophin)
•
Corticotrophin
releasing hormone (CRH) from the hypothalamus promotes the synthesis and
release of ACTH by the anterior pituitary. This increases the concentration of
cholesterol and steroids within the adrenal cortex and the output of steroid
hormones, especially cortisol.
Prolactin
• This hormone is secreted during pregnancy to prepare
the breasts for lactation (milk production) after childbirth.
Gonadotrophins
Just before puberty two gonadotrophins (sex hormones) are secreted in
gradually increasing amounts by the anterior pituitary in response to luteinising
hormone releasing hormone (LHRH), also known as gonadotrophin releasing
hormone (GnRH). At puberty secretion increases, further enabling normal
adult functioning of the reproductive organs; in both males and females the
hormones responsible are:
• follicle stimulating hormone (FSH)
• luteinising hormone (LH).
Antidiuretic hormone (ADH, vasopressin)
The main effect of antidiuretic hormone
is to reduce urine output (diuresis is the production of a large volume of
urine). ADH acts on the distal convoluted and collecting ducts of the nephrons
of the kidneys, increasing their permeability to water. As a result, the
reabsorption of water from the glomerular filtrate is increased. The amount of
ADH secreted is determined by the osmotic pressure of the blood circulating to
the osmoreceptors in the hypothalamus.
Thyroid Gland – Thyroxine and tri-iodothyronine
•
T3 and
T4 are essential for normal growth and development, especially of
the skeleton and nervous system.
• Most other organs and systems are also influenced by
thyroid hormones – physiological effects of T3 and T4 on
the heart, skeletal muscles, skin, digestive and reproductive systems are more
evident when there is underactivity or overactivity of the thyroid gland.
Calcitonin
•
This hormone
is secreted by the parafollicular or C-cells in the thyroid gland.
• It acts on bone
cells and the kidneys to reduce blood calcium (Ca2+) levels when
they are raised.
Parathyroid Glands
•
There are
four small parathyroid glands, two embedded in the posterior surface of each
lobe of the thyroid gland. They are surrounded by fine connective tissue
capsules. The cells forming the glands are spherical in shape and are arranged
in columns with sinusoids containing blood between them.
Function
•
The
parathyroid glands secrete parathyroid hormone (PTH, parathormone).
Secretion is regulated by blood calcium levels. When they fall, secretion of
PTH is increased and vice versa.
• The main function of PTH is to increase the blood
calcium level when it is low.)
Adrenal cortex
The adrenal cortex produces three groups of steroid hormones from
cholesterol. They are collectively called adrenocorticocoids
(corticosteroids). They are:
•
glucocorticoids
•
mineralocorticoids
•
sex hormones
(androgens).
The hormones in each group have
different characteristic actions but due to their structural similarity their
actions may overlap.
Glucocorticoids
• Cortisol (hydrocortisone) is the main
glucocorticoid but small amounts of corticosterone and cortisone are also
produced. They are essential for life, regulating metabolism and responses to
stress. Secretion is controlled through a negative feedback system involving
the hypothalamus and anterior pituitary.
Mineralocorticoids (aldosterone)
• Aldosterone is the main mineralocorticoid. Its
functions are associated with the maintenance of water and electrolyte balance
in the body. Through a negative feedback system it stimulates the reabsorption
of sodium (Na+) by the renal tubules and excretion of potassium (K+)
in the urine.
Pancreatic Islets – Insulin
Insulin is a polypeptide consisting of about 50 amino acids. Its main
function is to lower raised blood nutrient levels, not only glucose but also
amino acids and fatty acids. These effects are described as anabolic, i.e. they
promote storage of nutrients. When these nutrients, especially glucose, are in
excess of immediate needs insulin promotes their storage by:
•
acting on
cell membranes and stimulating uptake and use of glucose by muscle and
connective tissue cells
•
increasing
conversion of glucose to glycogen (glycogenesis), especially in the liver and
skeletal muscles
•
accelerating
uptake of amino acids by cells, and the synthesis of protein
•
promoting
synthesis of fatty acids and storage of fat in adipose tissue (lipogenesis)
•
decreasing glycogenolysis
(breakdown of glycogen into glucose)
• preventing the breakdown of protein and fat, and gluconeogenesis
(formation of new sugar from, e.g., protein).
Glucagon and Somatostatin (GHRIH)
The effects of glucagon increase blood glucose levels by stimulating:
•
conversion of
glycogen to glucose in the liver and skeletal muscles (glycogenolysis)
•
gluconeogenesis.
Secretion of glucagon is stimulated by a low blood glucose level and
exercise, and decreased by somatostatin and insulin.
Somatostatin (GHRIH)
The effect of this hormone, also produced by the hypothalamus, is to
inhibit the secretion of both insulin and glucagon in addition to inhibiting
the secretion of GH from the anterior pituitary
=======================
•
Mouth
• Pharynx
• Oesophagus
• Stomach
• Small intestine
• Large intestine
•
Rectum and
anal canal
Accessory Organs
•
Three pairs
of salivary glands
• The pancreas
•
The liver and
biliary tract.
The walls of the alimentary tract
•
Adventitia or
serosa – outer covering
• Muscle layer
• Submucosa
•
Mucosa –
lining
Nerve Supply
The parasympathetic supply
One pair of cranial nerves, the vagus nerves, supplies most of the
alimentary canal and the accessory organs. Sacral nerves supply the most distal
part of the tract. The effects of parasympathetic stimulation are:
• increased muscular activity, especially peristalsis,
through increased activity of the myenteric plexus
•
increased
glandular secretion, through increased activity of the submucosal plexus.
Salivary Glands
Parotid glands
• These are situated one on each side of the face just
below the external acoustic. Each gland has a parotid duct opening into the
mouth at the level of the second upper molar tooth.
Submandibular glands
• These lie one on each side of the face under the angle
of the jaw. The two submandibular ducts open on the floor of the mouth, one on
each side of the frenulum of the tongue.
Sublingual glands
•
These glands
lie under the mucous membrane of the floor of the mouth in front of the
submandibular glands. They have numerous small ducts that open into the floor
of the mouth.
Functions of Saliva
•
Chemical
digestion of Polysaccharides
• Lubrication of food
• Cleaning and lubricating the mouth
• Non-specific defence
•
Taste
Stomach
The stomach is continuous with the oesophagus at the cardiac sphincter
and with the duodenum at the pyloric sphincter.
Muscle Layer - This consists of three layers of smooth muscle fibres:
• an outer layer of longitudinal fibres
• a middle layer of circular fibres
•
an inner
layer of oblique fibres.
Functions of Gastric Juices
•
Water further
liquefies the food swallowed.
• Hydrochloric acid:
• acidifies the food and stops the action of salivary
amylase
• kills ingested microbes
• provides the acid environment needed for effective
digestion by pepsins.
• Pepsinogens are activated to pepsins by hydrochloric
acid and by pepsins already present in the stomach. These enzymes begin the
digestion of proteins, breaking them into smaller molecules. Pepsins act most
effectively at a very low pH, between 1.5 and 3.5.
• Intrinsic factor (a protein) is necessary for the
absorption of vitamin B12 from the ileum.
•
Mucus
prevents mechanical injury to the stomach wall by lubricating the contents.
Functions of Small Intestine
•
Onward
movement of its contents by peristalsis, which is increased by parasympathetic
stimulation
• Secretion of intestinal juice, also increased by
parasympathetic stimulation
• Completion of chemical digestion of carbohydrates,
protein and fats in the enterocytes of the villi
• Protection against infection by microbes that have
survived the antimicrobial action of the hydrochloric acid in the stomach, by
the solitary lymph follicles and aggregated lymph follicles
• Secretion of the hormones cholecystokinin (CCK) and
secretin
•
Absorption of
nutrients.
Chemical Digestion in the Small Intestine
Pancreatic Juice
Pancreatic juice is secreted by the exocrine pancreas and enters the
duodenum at the duodenal papilla. It consists of:
• water
• mineral salts
• enzymes:
•
amylase
•
lipase
•
nucleases
that digest DNA and RNA
• inactive enzyme precursors including:
• trypsinogen
•
chymotrypsinogen.
Bile
Bile has a pH of around 8 and between 500 and
1000 ml are secreted daily. It consists of:
• water
• mineral salts
• mucus
• bile salts
• bile pigments, mainly bilirubin
•
cholesterol.
Functions of the Bile
•
The bile
salts emulsify fats in the Small Intestine
• Fatty acids are insoluble in water and bile salts make
cholesterol and fatty acids soluble so enable both these and fat-soluble
vitamins (A, D, E and K) to be readily absorbed
• The bile pigment bilirubin is the waste product of the
breakdown of red blood cells
• Stercobilin colours and deodorises the faeces
Intestinal Secretions
The principal constituents of intestinal
secretions are water, mucus and mineral salts.
Most of the digestive enzymes in the small
intestine are contained in the enterocytes of the walls of the villi. Digestion
of carbohydrate, protein and fat is completed by direct contact between these
nutrients and the microvilli and within the enterocytes. The enzymes that
complete chemical digestion of food at the surface of the enterocytes are:
• peptidases
• lipase
•
sucrase,
maltase and lactase.
Functions of the Large Intestine
•
Absorption of
water, mineral salts, vitamins and some drugs
• Microbial activity – bacterial growth help in the
synthesis of vitamin K and folic acid
• Mass movement
•
Defecation
Functions of the Liver
•
Carbohydrate,
fat and protein metabolism
• Synthesis of plasma proteins and clotting factors
•
Detoxification of drugs and toxic substances
• Inactivation of hormones (e.g. insulin)
• Storage (e.g. glycogen)
• Secretion of bile