65-70% of our body contains water. Despite such a great fluid composition, our body often suffers from dehydration due to several reasons. Fluid maintenance is an inclusive phenomenon constantly under the influence of multiple factors. ADH or antidiuretic hormone is primarily responsible to manage these consequences by keeping a constant check on the abnormal fluid status. ADH is an endocrinal hormone, also called arginine vasopressin (AVP) released by the hypothalamus. It is one of the most influential hormones necessary for the maintenance of electrolyte balance and the body`s hydration. Nevertheless, the hormone is also an inseparable part of many other body systems including the cardiovascular and renal system. The whole mechanism played by the hormone lies in the regulation of the body’s osmotic balance which can eventually manage our blood pressure and the excretory function of kidneys.
Synthesis and Storage
The ADH is produced as peptide prohormone by the neurons found in the supraoptic and paraventricular nuclei of the hypothalamus. The hormone is converted into AVP before it is released into the target cells. Followed by its synthesis, the hormone is temporarily stored in the hypothalamus and then continues to ravels down the axons which ultimately lead it to the posterior pituitary. The posterior pituitary helps to identify the necessary signals before releasing the hormone into the circulation. The Pituitary hormone releases ADH only when the hypertonicity in the extracellular spaces of the body cells rises which results in the hyperosmolality- a major signal to the pituitary. As the hypertonicity in the ECS rises, the blood capillaries are prepared to respond by dilating themselves, which helps in the free solute movement during the fluid balance.
Functions of ADH
1. ADH Effects on the kidney
ADH can directly influence on the kidney and vascular system. The hormone helps to increase the water permeability of proximal collecting tubules and the inner medullary collecting ducts of the kidney. The collective efforts of these two structures help the kidneys to reabsorb the maximum quantity of solute free water. ADH also increases the ionic concentrations of some minerals in our body; especially calcium. Sodium, an essential electrolyte responsible to hold more water composition is constantly under the influence of ADH. ADH retains the maximum amount of sodium thus bringing the fluids into hyperosmolality. Hyperosmolality increases the tonicity of the fluids, as a result, the fluids become hypertonic in the blood.
2. ADH and cardiovascular system
Vasopressin regulates the blood pressure by influencing on the inner walls of arteries while the most important underlying mechanism of ADH being the regulation of the osmotic pressure which affects the plasma osmolality. The plasma osmolality in our body is affected by the total volume of plasma in the circulatory system. As the volume of plasma rise, the amount of fluid loss through kidney decreases and vice -versa. Moreover, the retention of plasma helps to catch more Na+ within the body. Consequently, water is being pushed from intracellular space to the blood plasma due to the pressure gradient. Excess of blood plasma can stimulate also the osmoreceptors cells to contract which thereby resulting in the stimulation of hypothalamus to release more ADH.
3. Role of ADH in blood pressure regulation
A rise in the ADH levels makes DCT and collecting ducts less permeable to water, as a result, urine output decreases while the body`s plasma level increases. On the contrary, if the plasma osmolality drops, it is the indication of more fluid is being drained out through the kidneys. ADH can influence the diameter of peripheral blood vessels to increase peripheral vascular resistance which in turn help in maintaining blood pressure in our body.
4. Role of ADH in managing dehydration
Dehydration and ADH secretion are directly proportional to each other. When a person becomes dehydrated, the brain stimulates the hypothalamus to release more Vasopressin. Having already said that ADH signals the kidneys to recover water from urine with the help of their tubules which can effectively manage the hydration status of our body by rising the blood plasma.
Miscellaneous functions of ADH
1. Vasopressin on pain management
Recent studies concluded that vasopressin may have analgesic (pain killer) effects. However, the studies have limited data which supports this phenomenon in human beings. Laboratory mice have responded well when vasopressin was injected into their blood.
2. ADH and emotions
The concept of whether vasopressin regulates emotions is highly speculative because the hormone is not primarily linked to all the emotions. However, studies provided a partial data on the fact that emotions including, love and aggressions have raised when ADH increased. On the other hand, temperature regulation centre is located in the hypothalamus, which provides some clues about the relationship about the body temperature and ADH levels.
3 Major mechanisms of action of ADH
No matter which organ is involved in the process, the fundamental basis of ADH action is the retention of more fluid. However, the 3 principal ways by which ADH acts on our body are:
Helping to raise the tubular reabsorption of solute free water into blood.
Constricting the arterioles to stimulate peripheral blood vessels to raise BP.
Managing some emotions including sexuality, love, anger, aggression and social behaviours.
Effects of ADH imbalance on our body
The normal ADH level in the adults is 1-5 picograms per millilitre (pg/mL) but it may vary slightly under different laboratory settings. Increase or decrease in the serum ADH levels affects our body functions in many ways. An imbalance in this hormone is linked with a number of systemic disorders which can seriously affect our body. The 2 major conditions associated with the imbalance of ADH are SIADH and Diabetes insipidus.
1. SIADH (syndrome of inappropriate antidiuretic hormone)
Increased levels of ADH is called SIADH, a condition closely linked with bladder cancer, cancer of the blood (leukaemia and lymphoma), brain tumours, lung cancer and cancer of pancreatic gland. It is mainly caused by the head injury, due to which the hypothalamic area is damaged, as a consequence causing an imbalance in the ADH. The illness is identified by symptoms such as fluid imbalance, excessive thirst, seizures and swelling in the brain (cerebral oedema). In some cases, signs and symptoms of kidney diseases, heart failure, signs of general sickness, feeling of vomiting (Nausea) cramps or tremors in the legs, depressed mood, impaired memory, irritability, personality changes, confusion and coma are evident, however proper clinical correlation with the help of many laboratory findings is essential.
2. Diabetes insipidus
Diabetes insipidus is an ADH deficiency disease where the kidneys fail to retain fluids. It is connected with lower plasma levels of ADH. Diabetes insipidus is distinguished by excessive thirst, frequent urination, weakness, passing colourless ( dilute) urine instead of pale yellow coloured, frequent waking through the night, dry skin, constipation, weak muscles and bed-wetting.