Adrenal cortex
The adrenal cortex is the part of the adrenal gland that is considered essential for life , but most conditions effect this part rather than the medulla . The cortex consists of three zones : the zona glomerulosa , zona fasciculata and zona reticularis ( Petroff & Greco , 2020 ). Secretion of hormones from the adrenal cortex is stimulated by ACTH ( released by the anterior pituitary ) and the hormones are known as corticosteroids . Around 40 corticosteroids are released from the adrenal cortex , but these are classified into three main groups , based on their action :
MINERALOCORTICOIDS The most important mineralocorticoid is aldosterone , and this hormone is secreted from the zona glomerulosa in response to the presence of angiotensin in the blood due to hypotension ( as part of the renin-angiotensin-aldosterone system – RAAS ). Aldosterone works by increasing the reabsorption of sodium in the kidney , which in turn draws more water into the blood , increasing blood volume and thereby increasing blood pressure .
GLUCOCORTICOIDS Produced in the zona fasciculata ( the main site of production ) and the zona reticularis , glucocorticoids are essential for life . They regulate all aspects of metabolism either directly or indirectly , and influence carbohydrate , protein and lipid metabolism , along with regulating cardiovascular function and blood pressure ( Petroff & Greco , 2020 ). Another action of glucocorticoids is suppression of inflammatory and immunological reactions and responses . The major glucocorticoid is cortisol .
SEX HORMONES Small quantities of sex hormones are produced in the zona fasciculata and zona reticularis in both males and females .
Adrenal medulla
The medulla is the outer part of the adrenal gland and secretion of hormones is influenced by the sympathetic nervous system . The two hormones from the adrenal medulla are catecholamines – adrenaline and noradrenaline – and both act in a similar way to prepare the body for dangerous / stressful situations – the ‘ fight or flight response ’. Adrenaline and noradrenaline release results in :
• Increased heart rate , ensuring oxygen and glucose are delivered to the brain and skeletal muscles ready for a rapid response .
• Increased respiratory rate and stimulation of bronchial dilation to maximise oxygen intake .
• Dilation of the blood vessels , leading to increased blood delivery to the brain and skeletal muscles . Blood supply to organs that are considered less essential at the time of fight or flight is constricted , e . g . the gastrointestinal tract .
Gonads
Gonads are the sex organs : ovaries in females and testes in males .
Ovaries
A pair of ovaries are located in the abdominal cavity caudal to the kidney and secrete two hormones :
OESTROGEN This hormone is secreted by the mature follicular cells within the ovary . As discussed earlier , secretion of oestrogen is controlled by FSH , which is released from the anterior pituitary . During puberty , oestrogen is involved in the development of female characteristics , including mammary glands . During the reproductive cycle , oestrogen influences behaviour during oestrus and prepares the reproductive tract for coitus .
‘ Progesterone prepares the reproductive tract for the fertilised ovum and thickens the uterine wall for implantation ’
PROGESTERONE This hormone is secreted by the corpus luteum in the ovary . As discussed previously , corpus luteum forms from the remaining follicle tissue post ovulation and LH , secreted from the anterior pituitary , controls the development of corpus luteum and therefore release of progesterone ( Davidson & Stabenfeldt , 2020a ). Progesterone prepares the reproductive tract for the fertilised ovum and thickens the uterine wall for implantation . Progesterone is important throughout pregnancy as it desensitises the smooth muscle in the uterine wall , so it is unable to contract and the foetus remains in place . As pregnancy comes to an end , the corpus luteum degenerates and progesterone levels fall , allowing the smooth muscle to regain the ability to contract in preparation for parturition .
Testes
The testes are male sex organs and , in adults , this pair of glands lie outside of the body cavity , within the scrotal sac . The hormone secreted by the testes is :
TESTOSTERONE This hormone is secreted from the interstitial cells within the testes and secretion is influenced by ICSH , which is released from the anterior pituitary ( Romano & Brinsko , 2020 ). Similarly to how oestrogen works in females , testosterone is responsible for development of male characteristics , for example , muscle development and penis size , while also influencing male behavioural traits including aggression , sex drive and territorial behaviour . Testosterone also stimulates spermatogenesis , the process of sperm production in the seminiferous tubules .
VOL 38 • July 2022
57