Page 194 - 84 human physiolofy part-1

Chapter 10

blood volume as well as blood pressure. Simple examples of ways that this balance can be changed
include ingestion of water, dehydration, blood loss and salt ingestion.

Reabsorption of water

Direct control of water excretion in the kidneys is exercised by the anti-diuretic hormone (ADH),
released by the posterior lobe of the pituitary gland. ADH causes the insertion of water channels into
the membranes of cells lining the collecting ducts, allowing water reabsorption to occur. Without ADH,
little water is reabsorbed in the collecting ducts and dilute urine is excreted. There are several factors
that influence the secretion of ADH. The first of these happen when the blood plasma gets too
concentrated. When this occurs, special receptors in the hypothalamus release ADH. When blood
pressure falls, stretch receptors in the aorta and carotid arteries stimulate ADH secretion to increase
volume of the blood.

Reabsorption of Salt

The Kidneys also regulate the salt balance in the blood by controlling the excretion and the
reabsorption of various ions. As noted above, ADH plays a role in increasing water reabsorption in the
kidneys, thus helping to dilute bodily fluids. The kidneys also have a regulated mechanism for
reabsorbing sodium in the distal nephron. This mechanism is controlled by aldosterone, a steroid
hormone produced by the adrenal cortex. Aldosterone promotes the excretion of potassium ions and the
reabsorption of sodium ions. The release of Aldosterone is initiated by the kidneys. The
juxtaglomerular apparatus is a renal structure consisting of the macula densa, mesangial cells, and
juxtaglomerular cells. Juxtaglomerular cells (JG cells, also known as granular cells) are the site of renin
secretion. Renin is an enzyme that angiotensinogen (a large plasma protein produced by the liver) into
Angiotensin I and eventually into Angiotensin II which stimulates the adrenal cortex to produce
aldosterone. The reabsorption of sodium ions is followed by the reapsorption of water. This causes
blood pressure as well as blood volume to increase.

Atrial natriuretic hormone (ANH) is released by the atria of the heart when cardiac cells are
streatched due to increased blood volume. ANH inhibits the secretion of renin by the juxtaglomerular
apparatus and the secretion of the aldosterone by the adrenal cortex. This promotes the excretion of
sodium. When sodium is excreted so is water. This causes blood pressure and volume to decrease.

Hypernatremia

An increase in plasma sodium levels above normal is hypernatremia. Sodium is the primary
solute in the extracellular fluid. Sodium levels have a major role in osmolarity regulation. For excitable
cells the electrochemical gradient for sodium across the plasma membrane is critical for life. Water
retention and an increased blood pressure usually are signs of hypernatremia. If the plasma sodium
levels are below normal it is called hyponatremia. Signs of this are low plasma volume and
hypotension.

Diuretics

A diuretic (colloquially called a water pill) is any drug that elevates the rate of bodily urine

194 | Human Physiology