Question

Ashley drinks 0.75 L of hypertonic saline. Describe, in detail, how different physiological systems in Ashley’s body will respond after drinking 0.75 L of hypertonic saline in order to try to achieve homeostasis.

Answer 1

Hypertonic saline has higher concentration of saline or NaCl. It will thus increase the plasma osmolarity as osmolarity of plasma is directly dependent on sodium ions. It also increases mean arterial pressure due to increase in osmolarity. Hence, water will tend to move out of cells in an attempt to equalize osmolarity. As a result, cells will shrivel. Osmoreceptors neurons are present in vasculosum termanalis and supraoptic and paraventricular nuclei of thalamus. These neurons get stretched and will depolarize. This causes release of antidiuretic hormone or ADH. ADH acts on collecting duct cells and bind to V2 receptors. It increases intracellular CAMP, causing induction in expression of aquaporin. Aquaporin channels are inserted on apical side of plasma membrane, leading to aquaporin channels. Water is reabsorbed from urine filtrate via these aquaporin channels. This will increase plasma osmolarity, and achieve homeostasis.

When blood pressure increases in afferent arteriole of kidneys, there is stretching of vascular smooth muscle. Inward directed ion channel get activated, depolarized and contract. This will prevent the increase in blood flow and prevent damage to kidneys. There is also a decrease in rennin secretion due to increased stretching of smooth muscle when blood pressure increases significantly. As renin is not secreted, angiotensin I is not formed from angiotensinogen, nor is converted to angiotensin II. Aldosterone secretion from adrenal glands decreases and there will be no reabsorption of sodium from urine. Instead, sodium is secreted and increase potassium retention, thereby maintaining GFR to normal levels. Plasma osmolarity will decrease due to increased sodium loss through urine.

Atrial natriuretic peptide production is increased due to cardiac distension and glomerular filtration rate is increased. This will shut down the renin-angiotensin-aldosterone system. As a result, there is decreased sodium reabsorption. It also increased water reabsorption. It also inhibits a sodium channel in the apical membrane of tubules, decreasing sodium reabsorption. Thus, sodium excretion is increased, thereby decreasing the plasma osmolarity.

Glucocorticoid levels are also increased in response to high salt intake. Glucocorticoids cause energy utilization in muscle and liver. This leads to production of urea. Urea is used by kidneys to conserve water.