Endocrine system
The endocrine system consists of glands widely separated from each other with no physical connections. Endocrine glands are a group of secretary cells surrounded by an extensive network of capillaries that facilitates diffusion of hormones from the secretary cells into the bloodstream. They are also referred to as ductless gland because hormones diffuse directly into the bloodstream.
Overview of hormones action
When a hormone arrive at its target cell, it binds to a specific receptor, where it acts as a switch influencing chemical or metabolic reactions inside the cell. Receptors for peptide hormones are situated on the cell membrane and those for lipid-based hormones are located inside cells. Examples : Steroids
| Lipd based hormone | Peptide hormone |
|---|---|
| Glucocorticoid | Adrenaline |
| Mineralocorticoid | Noradranaline |
| Thyroid hormone | Insulin,Glucagon |
The level of hormone in the blood is variable and self-regulating within its normal range. A hormone is released in response to a specific and usually its action reverses or negates the stimulus through a negative feedback mechanism. This may be controlled either indirectly through the release of hormones by the hypothalamus and the anterior pituitary glands, e.g. steroid and thyroid hormones, or directly by blood levels of the hypothalamus, e.g. insulin and glucagon and determined by plasma glucose level.
The effect of a positive feedback mechanism is amplification of the stimulus and increasing release of the hormone until a particular process is complete and the stimulus ceases , e.g. release of oxytocin during labour.
Pituitary gland and hypothalamus
The pituitary gland and the hypothalamus acts as a unit, regulating the activity of most of the other endocrine glands. The endocrine gland lies in the hypophyseal fossa of the sphenoid bone below the hypothalamus, to which it is attached by a stalk. It is the size of a pea, weighs about 500mg and consists of the two main parts that originate from different types of cell. The anterior pituitary is an upgrowth of glandular epithelium from the pharynx and the posterior pituitary a down growth of nervous tissue from the brain. There is a network of nerve fibres between the hypothalamus and posterior pituitary.
Blood supply
Arterial blood-this is from branches of the internal carotid artery. Anterior lobe is supplied indirectly by blood that has already passed through a capillary bed in the hypothalamus but the posterior lobe is supplied directly.
Venous drainage -containing hormones from both lobes, venous blood leaves the gland in short veins that enter the venous sinuses between the layers of dura mater.
The influence of the hypothalamus on the pituitary gland
It consists of the following terms-
- Anterior pituitary
- Posterior pituitary
Anterior pituitary
The anterior pituitary is supplied indirectly with arterial blood that has already passed through a capillary bed in the hypothalamus. This network of blood vessels forms part of the pituitary postal system , which transport blood from the hypothalamus to the anterior pituitary where it enters thin-walled sinusoids that are in close contact with the secretary cells.
Growth Hormone
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, intestine and pancreas; stimulates protein synthesis, especially tissue growth and repair; promotes breakdown of fats and increased blood glucose levels. Its release is stimulated by growth hormone releasing hormone and suppressed by growth hormone release inhibiting hormone(GHRIH), also known as tomato-statin, both of which are secreted by the hypothalamus . Secretion by hypoglycaemia , excessive and anxiety. Secretion peaks in adolescence and then decline with age.
Thyroid stimulating hormone{TSH}
The release of this hormone 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 triiodothyronine(T3). Release is lowest in the early evening and highest during the night. Secretion is regulated by a negative feedback mechanism, I.e. when the blood level of the thyroid hormones is high, secretion of TSH is reduced, and vice versa.
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 steroids hormones, especially cortisol
ACTH levels are highest at about 8 a.m . And fall to their lowest about midnight, although high levels sometimes occur at midday and 6 p.. this circadian rhythm is maintained throughout life. It associated with the sleep pattern and adjustment to changes takes several days, e.g. following changing work shifts, travelling to a different time zone.
Secretion is also regulated by a negative feedback mechanism, being suppressed when blood level of ACTH rises. Other factors that stimulate secretion include hypoglycaemia , exercise and other stressors, e.g. emotional states and fever.
Prolactin
This hormone is secreted during pregnancy to prepare the breast for lactation after childbirth. The blood level of protection is stimulated by prolactin releasing hormone (PRH) released from the hypothalamus and it is lowered by prolactin inhibiting hormone (PIH , dopamine ) and by increased blood level of protection. Immediately after birth, suckling stimulates protection secretion and lactation. The resultant high blood level of prolactin secretion and lactation. The resultant high blood level is a factor in reducing the incidence of conception during lactation.
Posterior pituitary
The posterior pituitary is formed from nervous tissue and consists of nerve cells surrounded by supporting glial cells called pituicytes.These neurones have their cell bodies in the supraoptic and paraventricular nuclei of the hypothalamus and their axons form a bundle known as the Hypothalamohypophyseal tract Posterior pituitary hormones are synthesised in the nerve cell bodies, transported along the axons and stored in vesicles within the axon terminals in the posterior pituitary.
Oxytocin
Oxytocin stimulates two target tissue during and after childbirth: uterine smooth muscle and the muscle cells of the lactating breast.during childbirth increasing amounts of oxytocin are released from the posterior pituitary into the bloodstream in response to increasing stimulation of sensory stretch receptors in the uterine cervix as the baby's head progressively dilates it. Sensory impulses are generated and travel to the control centre in the hypothalamus, stimulating the posterior pituitary to release more oxytocin. In turn this stimulates more forceful uterine contractions and greater stretching of the uterine cervix as the baby's head is forced further downwards. This is an example of a positive feedback mechanism which stops soon after the baby is delivered when distention of the uterine cervix is greatly reduced.
Antidiuretic hormons
The main effect of antidiuretic hormone is to reduce urine output . ADH acts on the distal convoluted tubules and collecting ducts of the nephrons of the kidneys. It increase their permeability to water and more of the glomerular filtrate is reabsorbed. ADH secretion is determined by the osmotic pressure of the blood circulating to the osmoreceptorin the hypothalamus.
Thyroid gland
The thyroid gland is situated in the neck in front of the larynx and trachea at the level of the 5th, 6th and 7th cervical and 1st thoracic vertebrae. It is highly vascular gland and weighs about 25g and is surrounded by a fibrous capsule. It resembles a butterfly in shape, consisting of two lobes, one on either side of the thyroid cartilage and upper cartilaginous rings of the trachea. The lobes are joined by a narrow isthmus, lying in front of the trachea. The gland is composed of largely spherical follicles formed from cuboidal epithelium. These secrete and store colloid, a thick sticky protein material.
It consists of the following terms-
Thyroxine and tri-iodothyronine
Iodine is essential for the formation of the thyroid hormones, thyroxine and triiodothyronine , so numbered as these molecules contain four and three atoms of the element iodine respectively. The main dietary sources of iodine are seafood, vegetable grown in iodine-rich soil and iodinated table salt. Thyroid hormone are synthesised as large precursor molecules called thyroglobulin, the major constituents of colloid. The plasma release of T3 and T4 into the blood is stimulated by thyroid stimulating hormone (TSH) from the hypothalamus and secretion of TSH is stimulated by exercise, stress, malnutrition, low plasma glucose levels and sleep.
T3T4 affect most cells of the body by:
- Increasing the basal metabolic rate and heat production
- Regulating metabolism of carbohydrate, protein and fats.
Calcitonin
This hormone is secreted by the parafollicular or C-cells in the thyroid gland. Calcitonin lowers raised blood calcium levels. It does this by acting on:
- Bone cells promoting their storage of calcium.
- Kidney tubules inhibiting the reabsorption of calcium.
Adernal glands
The two adrenal glands are situated on the upper pole of each kidney enclosed within the renal fascia . They are about 4 cm long and 3 cm thick.
Blood supply
The arterial blood supply is by branches from the abdominal aorta and renal artery. The venous return is by suprarenal veins. The right glands drains into the inferior vena cava and the left into the left renal vein.
Adernal cortex
Adernal cortex produces three groups of steroid hormones from cholesterol . They are collectively called adrenocorticoids. The groups are :
- Glucocorticoid
- Mineralocorticoid
- Sex hormones
Glucocorticoids
Cortisol is the main glucocorticoid but small amount of corticosterone and cortisone are also produced. Commonly these are collectively known as 'steroids'; 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 .
In pathological and pharmacological quantities glucocorticoids also have other effects including;
- Anti-inflammatory
- Suppression of immune response
- Delayed wound healing
Mineralocorticoid
Aldosterone is the main mineralocorticoid. It is involved in maintaining water and electrolytes balance. Through a negative feedback system it stimulates the reabsorption of sodium by the renal tubules and excretion of potassium in the urine. Sodium reabsorption is also accompanied by retention of water and therefore aldosterone is involved in the regulation of blood volume and blood pressure too.
Sex hormones
Sex hormones secreted by the adrenal cortex are mainly androgen although the amounts produced are insignificant compared with those secreted by the testes and ovaries in late puberty and adulthood.
Adernal medulla
The medulla is completely surrounded by the adrenal cortex. It develops from nervous system. When stimulated by extensive sympathetic nerve supply, the glands release the hormones adrenaline and noradrenaline.
Adrenaline and noradrenaline
Noradrenaline is the postganglionic neurotransmitter of the sympathetic division of the autonomic and some noradrenaline are released into the blood from the adrenal medulla during stimulation of the sympathetic nervous system. The action of these hormones prolongs and augments stimulation of the sympathetic nervous system. Structurally they are very similar, which explains their similar effects. Together they potentiate the fight or flight response by :
- Increasing heart rate
- increasing blood pressure
- Increasing metabolic rate
- Dilating the pupils
Local hormones
It consists of the following terms-
- Histamine
- Serotonine
- Prostaglandins
Histamine
This is synthesised and stored by mast cells in the tissues and basophils in blood. It is released as part of the inflammatory responses, especially when caused by allergy, increasing capillary permeability and causing vasodilation. It also acts as a neurotransmitter, cause contraction of smooth muscle of the bronchi and alimentary tract, and stimulates the secretion of gastric juice.
Serotonine
This is present in platelets in the brain and in the intestinal wall. It causes intestinal secretion and contraction of smooth muscle and its role in haemostasis . It is a neurotransmitter in the CNS and is known to influence mood.
Prostaglandins
These are lipid substances found in most tissues. They act on neighbouring cells but their actions are short-lived as they are quickly metabolised. Prostaglandins have potent and wide-ranging physiological effects as ;
- The inflammatory response
- Potentiating pain
- Fever
- Regulating blood pressure
- Blood clotting
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