Question

1. An individual has a higher-than-normal blood volume, due to excess fluids—not high enough to cause symptoms, but enough to cause the body to attempt a restoration to normal levels. Also, it is stipulated that there are no ion imbalances present. Describe all processes by which the body induces the decrease of blood volume. Your response should include a review of all body cells and structures capable of detecting high blood volume, the body responses that stimulate fluid excretion, and all processes for the removal of fluids from the body.

2. An individual has a higher-than-normal blood volume, solely due to excess water—not high enough to cause symptoms, but enough to cause the body to attempt a restoration to normal levels. Describe all processes by which the body induces the decrease of blood volume. Your response should include a review of all body cells and structures capable of detecting high water levels (whether directly or indirectly), the body responses that stimulate water excretion, and all processes for the removal of water from the body. Please note that while some parts of your response should be similar to that of response #3, there should be many more differences than similarities.

Answer 1

1.Blood volume is necessary to maintain adequate perfusion to all of the tissues in the body. Nearly all cells in the body require replenishment of nutrients and a removal system for waste, both of which the blood provides.

The renal system, and more specifically the kidney, is primarily responsible for regulating blood volume. The kidney's primary function is to modify the solutes and water content of the blood through filtration, reabsorption, and secretion. As blood passes through the glomerulus of the kidney, solutes and water are filtered out depending on a variety of signaling molecules. Then, as the filtrate passes through the tubules, some of the filtrates are reabsorbed along with water. The amount of water and solute reabsorbed is what primarily regulates blood volume. If blood volume is too low, more filtrate reabsorbs; if blood volume is too high, less filtrate reabsorbs. The kidney is also responsible for the secretion of erythropoietin. Erythropoietin is the protein that signals the bone marrow to produce red blood cells. Therefore, the kidney is responsible for both the regulation and partial production of blood volume.

While blood is a component of the cardiovascular system, this system is hardly responsible for its regulation. Instead, the cardiovascular system maintains arterial pressure for the adequate perfusion of all bodily tissues. This system detects changes in blood volume and reflects it through increasing or decreasing arterial pressure. Reduced blood volume leads to collapsing vessels, reduced pressure, and subsequently reduced perfusion pressure. The cardiovascular system combats low blood volume by constricting blood vessels until the body reaches a blood pressure that restores proper perfusion pressure. Blood volume and blood pressure are interconnected through the renal and circulatory system, specifically the renin-angiotensin-aldosterone system .

As mentioned earlier, the skeletal system is responsible for the production of blood cells which make up blood volume. When signaled by erythropoietin, the bone marrow creates erythrocytes which are eventually released into circulation. Leukocytes, which form a small portion of total blood volume, are also produced by the bone marrow when stimulated by colony stimulating factors released from mature leukocytes. Lastly, the nervous system aids in regulating blood volume by interacting with all three other systems. It is responsible for some of the stimulus at the level of the glomerulus as well as the constriction of blood vessels through sympathetic nerve activity.

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Blood volume also functions in the maintenance of body osmolality. Osmolality refers to the balance of solutes and water within a solution, in this case, the blood. A properly functioning system maintains an osmolality of 275 to 295 mOsm/kg of water through water and sodium manipulation primarily at the kidney.[5] When one of these two varies from the standard range, plasma osmolality changes and may increase or decrease plasma volume. Changing plasma osmolality results in an imbalance between intracellular and extracellular compartments. This imbalance can cause water entry or exit from cells. Overall, it may greatly increase or decrease blood volume. Increased blood volume is called hypervolemia and decreased blood volume is called hypovolemia.

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Hypervolemia occurs when blood volume is increased and can occur through renal failure, congestive heart failure, liver failure, excessive sodium intake, or any other dysfunction of sodium regulation.[8] Furthermore, prolonged hypertension may result in renal damage eventually culminating in fluid imbalance. When sodium is retained in the body, water is as well. This retention results in increased plasma and subsequently increased blood volume. Uncontrolled hypervolemia results in fluid accumulation in various body cavities and other extracellular spaces. Ascites, pulmonary edema, and edema are all possible outcomes of hypervolemia.

Regulation of Water I ntake

Drinking water is considered voluntary. So how is water intake regulated by the body? Consider someone who is experiencing dehydration, a net loss of water that results in insufficient water in blood and other tissues. The water that leaves the body, as exhaled air, sweat, or urine, is ultimately extracted from blood plasma. As the blood becomes more concentrated, the thirst response—a sequence of physiological processes—is triggered. Osmoreceptors are sensory receptors in the thirst center in the hypothalamus that monitor the concentration of solutes (osmolality) of the blood. If blood osmolality increases above its ideal value, the hypothalamus transmits signals that result in a conscious awareness of thirst.

Regulation of Water Output

Water loss from the body occurs predominantly through the renal system. A person produces an average of 1.5 liters (1.6 quarts) of urine per day. Although the volume of urine varies in response to hydration levels, there is a minimum volume of urine production required for proper bodily functions. The kidney excretes 100 to 1200 milliosmoles of solutes per day to rid the body of a variety of excess salts and other water-soluble chemical wastes, most notably creatinine, urea, and uric acid. Failure to produce the minimum volume of urine means that metabolic wastes cannot be effectively removed from the body, a situation that can impair organ function. The minimum level of urine production necessary to maintain normal function is about 0.47 liters (0.5 quarts) per day.

The kidneys also must make adjustments in the event of ingestion of too much fluid. Diuresis, which is the production of urine in excess of normal levels, begins about 30 minutes after drinking a large quantity of fluid. Diuresis reaches a peak after about 1 hour, and normal urine production is reestablished after about 3 hours.