Question

Draw a diagram of the hypothalamus and its connection to the posterior pituitary (2). Include ONLY the neurosecretory cells that secrete an antidiuretic hormone (ADH). The cells that secrete ADH are osmoreceptors that respond to high blood osmolarity by secreting ADH. Given the information about the target organ and action of ADH, explain the homeostatic mechanism that involves ADH that acts when blood osmolarity is too high and be sure to explain how the original stimulus is reversed. (3)

Answer 1

• Antidiuretic hormone is secreted mainly by the supraoptic nucleus and small amount by the paraventricular nucleus of the hypothalamus.
• This hormone is transported from hypothalamus to the posterior pituitary through the nerve fibers of hypothalamo- hypophyseal tract . While anterior pituitary hormones transported to the anterior pituitary by hypothalamo- hypophysial portal blood vessels.
• The main function of the antidiuretic hormone is retention of water by acting on kidneys. This hormone increases the water reabsorption from the distal convoluted tubule and collecting duct in the kidneys.
• When there is absence of this hormone,the distal convoluted tubule and collecting duct are impermeable to water . Hence , reabsorption of water does not takes place in the renal tubules and dilute urine is secreted .This will lead to loss of large amount of water through urine and this condition is known as diabetes insipidus.
• The secretion of the antidiuretic hormone is totally depends upon the volume of body fluid and osmolarity of body fluid.
• When there is loss of body fluids occur , antidiuretic hormone is released immediately in large quantities . This may be due to increased osmolality of the body fluids . The osmoreceptors give response to change in the osmolar concentration of the blood.
• These osmolar receptors are situated in the hypothalamus near supraoptic and paraventricular nucleus. These receptors get activated when there is increased osmolar concentration of blood.
• These osmolar receptors stimulates supraoptic and paraventricular nucleus which then releases antidiuretic hormone.
• Then antidiuretic hormone causes reabsorption of water from renal tubules and maintains the normal osmolarity of body fluids.
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