Question

Which would be expected to decrease the osmolarity of a mammal's body fluids? a. an increase in the plasma aldosterone concentration b. a decrease in the plasma ADH concentration c. dehydration d. loss of blood due to injury e. an increase in the plasma ADH concentration

Short answer

The expected action that would decrease the osmolarity of a mammal's body fluids is an increase in the plasma ADH concentration.

Step-by-step solution

Evaluating the First Option

An increase in the plasma aldosterone concentration would increase the sodium retention in the body, thus increasing the osmolarity. So, this option does not decrease the osmolarity.

Evaluating the Second Option

Decreasing the plasma ADH concentration would lead to less water being reabsorbed in the kidneys. This would result in a more dilute urine and a relative increase in the osmolarity of body fluids. Therefore, this option does not decrease the osmolarity, but rather increases it.

Evaluating the Third Option

Dehydration, by definition, means losing more water than you take in, which increases the concentration of solutes in the body, therefore increasing the osmolarity. So, this option does not decrease the osmolarity.

Evaluating the Fourth Option

Loss of blood due to injury could potentially decrease the osmolarity. However, blood is not only made up of water, it also contains solutes. So, if blood is lost, both fluids and solutes are lost, which does not lead to a decrease in osmolarity.

Evaluating the Last Option

An increase in the plasma ADH concentration results in more water being reabsorbed into the body. As a result, the concentration relative to solutes decreases, thereby decreasing osmolarity. Hence, this option would decrease osmolarity.

Key concepts

Plasma Aldosterone Concentration

Aldosterone, a hormone produced by the adrenal glands, plays a critical role in regulating the balance of salts and water in our body. It instructs the kidneys to retain sodium and excrete potassium. When the plasma aldosterone concentration increases, kidneys reabsorb more sodium leading to water retention. This scenario causes an increase in the body’s fluid volume without a corresponding increase in solute, which raises the overall fluid osmolarity.

This mechanism is part of a complex system our body uses to control blood pressure and maintain homeostasis. It is worth noting that increased aldosterone can lead to high blood pressure due to the extra fluid being held in the body.

Plasma ADH Concentration

The antidiuretic hormone (ADH), also known as vasopressin, is a peptide hormone secreted by the pituitary gland. Plasma ADH concentration dictates the balance of water in the body by controlling the amount of water reabsorbed by the kidneys. When ADH levels rise, more water is reabsorbed into the bloodstream, reducing urine output and decreasing the osmolarity of body fluids.

• If the body is dehydrated, ADH release increases to conserve water.
• Conversely, with an excess of water, ADH levels drop to allow more water excretion.

ADH is a key player in preventing dehydration and maintaining the proper osmotic balance within the body.

Dehydration Effect on Osmolarity

Dehydration occurs when the amount of water leaving the body is greater than the amount being taken in. The state of dehydration thus has a direct impact on the osmolarity of body fluids. As the body loses water, the concentration of solutes such as sodium and chloride in the blood remains unchanged or may slightly increase due to continued metabolic activity.

This results in an increase in the osmolarity of body fluids. High osmolarity triggers the sensation of thirst and the release of ADH to promote water reabsorption, a compensatory response aiming to dilute the concentration of solutes and return osmolarity to normal.

Blood Loss Osmolarity

The loss of blood due to an injury often triggers concerns about potential changes in the osmolarity of body fluids. Following blood loss, the body may experience a reduction in overall fluid volume which can impact osmolarity. However, because blood contains both water and solutes, loss of blood will consist of these two components proportionally.

Despite a reduction in volume, the loss does not drastically alter the ratio of solutes to water in the remaining fluid, which means that blood loss, in itself, does not necessary lead to a decrease in osmolarity. It is vital to replenish fluids quickly during blood loss to maintain homeostasis and stabilize osmolarity.