Regulation of Blood Volume by the Kidneys

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The formation of urine begins in the same manner as the formation of tissue fluid—by filtration of plasma through capillary pores. These capillaries are known as glomeruli, and the filtrate they produce enters a system of tubules that transports and modifies the filtrate (by mechanisms discussed in chapter 17). The kidneys produce about 180 L per day of blood filtrate, but since there is only 5.5 L of blood in the body, it is clear that most of this filtrate must be returned to the vascular system and recycled. Only about 1.5 L of urine is excreted daily; 98% to 99% of the amount filtered is reabsorbed back into the vascular system.

The volume of urine excreted can be varied by changes in the reabsorption of filtrate. If 99% of the filtrate is reabsorbed, for example, 1% must be excreted. Decreasing the reabsorption by only 1%—from 99% to 98%—would double the volume of urine excreted (an increase to 2% of the amount filtered). Carrying the logic further, a doubling of urine volume from, for example, 1 to 2 liters, would result in the loss of an additional liter of blood volume. The percentage of the glomerular filtrate reabsorbed—and thus the urine volume and blood volume—is adjusted according to the needs of the body by the action of specific hormones on the kidneys. Through their effects on the kidneys, and the resulting changes in blood volume, these hormones serve important functions in the regulation of the cardiovascular system.

Regulation by Antidiuretic Hormone (ADH)

One of the major hormones involved in the regulation of blood volume is antidiuretic hormone (ADH), also known as vaso-pressin. As described in chapter 11, this hormone is produced by neurons in the hypothalamus, transported by axons into the posterior pituitary, and released from this storage gland in response to hypothalamic stimulation. The release of ADH from the posterior pituitary occurs when neurons in the hypothalamus called osmoreceptors detect an increase in plasma osmolality (osmotic pressure).

An increase in plasma osmolality occurs when the plasma becomes more concentrated (chapter 6). This can be produced either by dehydration or by excessive salt intake. Stimulation of osmoreceptors produces sensations of thirst, leading to increased water intake and an increase in the amount of ADH released from the posterior pituitary. Through mechanisms that will be discussed in conjunction with kidney physiology in chapter 17, ADH stimulates water reabsorption from the filtrate. A smaller volume of urine is thus excreted as a result of the action of ADH (fig. 14.11).

A person who is dehydrated or who consumes excessive amounts of salt thus drinks more and urinates less. This raises the blood volume and, in the process, dilutes the plasma to lower its previously elevated osmolality. The rise in blood volume that results from these mechanisms is extremely important in stabilizing the condition of a dehydrated person with low blood volume and pressure.

Photo Hypothalamus Thirst Mechanism
Osmoreceptors in hypothalamus

Posterior Thirst pituitary

Posterior Thirst pituitary t ADH

Water retention '

Water retention '

responses J Blood osmolality

■ Figure 14.11 The negative feedback control of blood volume and blood osmolality. Thirst and ADH secretion are triggered by a rise in plasma osmolality. Homeostasis is maintained by countermeasures, including drinking and conservation of water by the kidneys.

Clinical Investigation Clues

Remember that Charlie had a low urine output and that his urine had a high osmolality (concentration). What physiological mechanism could be responsible for this? What benefit does Charlie derive from this mechanism?

Drinking excessive amounts of water without excessive amounts of salt does not result in a prolonged increase in blood volume and pressure. The water does enter the blood from the intestine and momentarily raises the blood volume; at the same time, however, it dilutes the blood. Dilution of the blood decreases the plasma osmolality and thus inhibits the release of ADH. With less ADH there is less reabsorption of filtrate in the kidneys—a larger volume of urine is excreted. Water is therefore a diuretic—a substance that promotes urine formation—because it inhibits the release of antidiuretic hormone.

An increase in blood volume can thus be compensated by a fall in ADH secretion. However, expanded blood volume also

Cardiac Output, Blood Flow, and Blood Pressure stimulates stretch receptors in the left atrium of the heart, causing the increased secretion of a different hormone. This is a polypeptide known as atrial natriuretic peptide (discussed in a separate section shortly). This hormone promotes the increased excretion of salt and water in the urine, thereby helping to lower the blood volume.

During prolonged exercise, particularly on a warm day, a substantial amount of water (up to 900 ml per hour) may be lost from the body through sweating. The lowering of blood volume that results decreases the ability of the body to dissipate heat, and the consequent overheating of the body can cause ill effects and put an end to the exercise. The need for athletes to remain well hydrated is commonly recognized, but drinking pure water may not be the answer. This is because blood sodium is lost in sweat, so that a lesser amount of water is required to dilute the blood osmolality back to normal. When the blood osmolality is normal, the urge to drink is extinguished. For these reasons, athletes performing prolonged endurance exercise should drink solutions containing sodium (as well as carbohydrates for energy), and they should drink at a predetermined rate rather than at a rate determined only by thirst.

Regulation by Aldosterone

From the preceding discussion, it is clear that a certain amount of dietary salt is required to maintain blood volume and pressure. Since Na+ and Cl- are easily filtered in the kidneys, a mechanism must exist to promote the reabsorption and retention of salt when the dietary salt intake is too low. Aldosterone, a steroid hormone secreted by the adrenal cortex, stimulates the reabsorption of salt by the kidneys. Aldosterone is thus a "salt-retaining hormone." Retention of salt indirectly promotes retention of water (in part, by the action of ADH, as previously discussed). The action of aldosterone produces an increase in blood volume, but, unlike ADH, it does not produce a change in plasma osmolality. This is because aldosterone promotes the reabsorption of salt and water in proportionate amounts, whereas ADH promotes only the reabsorption of water. Thus, unlike ADH, aldosterone does not act to dilute the blood.

The secretion of aldosterone is stimulated during salt deprivation, when the blood volume and pressure are reduced. The adrenal cortex, however, is not directly stimulated to secrete al-dosterone by these conditions. Instead, a decrease in blood volume and pressure activates an intermediate mechanism, described in the next section.

Throughout most of human history, salt was in short supply and was therefore highly valued. Moorish merchants in the sixth century traded an ounce of salt for an ounce of gold, and salt cakes were used as money in Abyssinia. Part of a Roman soldier's pay was given in salt—a practice from which the word salary (sal = salt) derives. Salt was also used to purchase slaves—hence the phrase "worth his salt."


I Blood pressure I Blood flow to kidneys

Juxtaglomerular apparatus in kidneys



Angiotensin I ACE

Angiotensin II

Adrenal cortex


Vasoconstriction of arterioles

Salt and water retention by kidneys

Salt and water retention by kidneys

■ Figure 14.12 The renin-angiotensin-aldosterone system. This system helps to maintain homeostasis through the negative feedback control of blood volume and pressure. (ACE = angiotensin-converting enzyme.)

Renin-Angiotensin-Aldosterone System

When the blood flow and pressure are reduced in the renal artery (as they would be in the low-blood-volume state of salt deprivation), a group of cells in the kidneys called the juxtaglomerular apparatus secretes the enzyme renin into the blood. This enzyme cleaves a ten-amino-acid polypeptide called angiotensin I from a plasma protein called angiotensinogen. As angiotensin I passes through the capillaries of the lungs, an angiotensin-converting enzyme (ACE) removes two amino acids. This leaves an eight-amino-acid polypeptide called angiotensin II (fig. 14.12). Conditions of salt deprivation, low blood volume, and low blood pressure, in summary, cause increased production of angiotensin II in the blood.

Angiotensin II exerts numerous effects that produce a rise in blood pressure. This rise in pressure is partly due to vasoconstriction and partly to increases in blood volume. Vasoconstriction of arterioles and small muscular arteries is produced directly by the effects of angiotensin II on the smooth muscle layers of these vessels. The increased blood volume is an indirect effect of angiotensin II.

Angiotensin II promotes a rise in blood volume by means of two mechanisms: (1) thirst centers in the hypothalamus are stimulated by angiotensin II, and thus more water is ingested, and (2) secretion of aldosterone from the adrenal cortex is stimulated by angiotensin II, and higher aldosterone secretion causes more salt and water to be retained by the kidneys. The relationship between angiotensin II and aldosterone is sometimes described as the renin-angiotensin-aldosterone system.

Clinical Investigation Clues

Remember that Charlie's urine had virtually no Na+ in it.

■ What physiological mechanism is responsible for this?

■ What benefits does Charlie derive from this mechanism?

The renin-angiotensin-aldosterone system can also work in the opposite direction: high salt intake, leading to high blood volume and pressure, normally inhibits renin secretion. With less angiotensin II formation and less aldosterone secretion, less salt is retained by the kidneys and more is excreted in the urine. Unfortunately, many people with chronically high blood pressure may have normal or even elevated levels of renin secretion. In these cases, the intake of salt must be lowered to match the impaired ability to excrete salt in the urine.

One °f the newer classes of drugs that can be used to treat hypertension (high blood pressure) are the __jTri angiotensin-converting enzyme, or ACE, inhibitors. These drugs (such as captopril) block the formation of angiotensin II, thus reducing its vasoconstrictor effect. The ACE inhibitors also increase the activity of bradykinin, a polypeptide that promotes vasodilation. The reduced formation of angiotensin II and increased action of bradykinin result in vasodila-tion, which decreases the total peripheral resistance. Because this reduces the afterload of the heart, the ACE inhibitors are also used to treat left ventricular hypertrophy and congestive heart failure. Another new class of antihypertensive drugs allows angiotensin II to be formed but selectively blocks the angiotensin II receptors.

Atrial Natriuretic Peptide

As described in the previous section, a fall in blood volume is compensated for by renal retention of fluid through activation of the renin-angiotensin-aldosterone system. An increase in blood volume, conversely, is compensated for by renal excretion of a larger volume of urine. Experiments suggest that the increase in water excretion under conditions of high blood volume is at least partly due to an increase in the excretion of Na+ in the urine, or natriuresis (natrium = sodium; uresis = making water).

Chapter Fourteen

Increased Na+ excretion (natriuresis) may be produced by a decline in aldosterone secretion, but there is evidence that there is a separate hormone that stimulates natriuresis. This natriuretic hormone would thus be antagonistic to aldosterone and would promote Na+ and water excretion in the urine in response to a rise in blood volume. A polypeptide hormone with these properties, identified as atrial natriuretic peptide (ANP), is produced by the atria of the heart. By promoting salt and water excretion in the urine, ANP can act to lower the blood volume and pressure. This is analogous to the action of diuretic drugs taken by people with hypertension, as described later in this chapter.

In addition to its stimulation of salt and water excretion by the kidneys. ANP also antagonizes various actions of angiotensin II. As a result of this action, ANP decreases the secretion of aldosterone and promotes vasodilation.

Test Yourself Before You Continue

1. Describe the composition of tissue (interstitial) fluid. Using a flow diagram, explain how tissue fluid is formed and how it is returned to the vascular system.

2. Define the term edema and describe four different mechanisms that can produce this condition.

3. Describe the effects of dehydration on blood and urine volumes. What cause-and-effect mechanism is involved?

4. Explain why salt deprivation causes increased salt and water retention by the kidneys.

5. Describe the actions of atrial natriuretic peptide and explain their significance.

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  • Elias Yemane
    How ayliters of blood flow through the kidneys per day?
    8 years ago
  • eula
    Does kidney regulate blood volume?
    8 years ago
  • Donnamira
    How does adh and rennin angiotensin aldosterone system act on blood pressure and blood volume stem?
    8 years ago
  • mikko
    Does salt stimulate hypothalmic osmoreceptors?
    8 years ago
  • amanuel
    How the human kidney is involved in the regulation of blood pressure?
    8 years ago
  • fabiola
    What is blood volume it need to filtrate one liter of urine?
    8 years ago
  • Sagramor
    When the hypothalamic osmoreceptors detect a rise in blood osmolarity?
    8 years ago
  • daniel
    What hormone promotes water retention by the kidneys?
    8 years ago
    Does aldosterone affect blood volume or osmolality?
    8 years ago
    Why does water only increase urine volume?
    8 years ago
  • Lacey
    Is blood volume the opposite of.urine volume?
    8 years ago
  • Daisy
    Is most filtrate reabsorbed into the body or excreted in the urine?
    8 years ago
    Does adh compensate for low blood volume?
    8 years ago
  • elaine walker
    How can antidiuretic hormone change the total blood volume?
    8 years ago
  • gebre
    How are kidneys involved in the regulation of blood pressure?
    8 years ago
  • Tanta
    What are the various hormones involved in control of blood volume?
    8 years ago
  • Monica Keskinen
    How does water regulate blood volume?
    8 years ago
    What hormone can promote increased blood volume and increase blood pressure?
    8 years ago
  • Rufus Goldworthy
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    8 years ago
  • Kenneth
    Which hormone causes excretion of water and salt when blood pressure is high?
    8 years ago
  • claudio
    What “modifies” the plasma of blood to make urine?
    8 years ago
    What is a hormone tha reduces blood volume but increases urine volume?
    8 years ago
  • P
    How does negative feedback mechanisms operate to regulate blood volume and urine output?
    8 years ago
  • almaz
    How much blood volume is required by the kidneys?
    8 years ago
  • fiori filmon
    What drug promotes urine formation in order to reduce blood volume?
    8 years ago
  • Tombur
    How does adh control blood osmolarity through negative feedback?
    8 years ago
  • guido
    How is blood volume regulated by the hypothalamus, adrenal cortex, and atria?
    8 years ago
  • yusef
    What two mechanisms increase blood volume?
    8 years ago
    Does atrial natriuretic peptide increased water retention in the kidneys?
    8 years ago
  • Daniele
    What effect does salt deprived blood have on muscles?
    8 years ago
  • haiduc
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    8 years ago
  • ilkka
    What promotes water reabsorption at the kidneys and stimulates thirst?
    8 years ago
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    How the kidneys dilute the blood of salt?
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    8 years ago
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    8 years ago
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    Is the levelof antidiuretic hormone change in your blood if you drink a liter of water?
    8 years ago
  • elisabeth
    What stimulates Adrenocortical hormones in regulation of blood volume?
    7 years ago
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    What are the hormones that are involved in regulating sodium and blood volume?
    7 years ago
  • Sven Wexler
    Which hormone stimulates an increase in blood volume?
    7 years ago
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    When the hypothalmic osmoreceptors detect a rise in blood osmolarity?
    7 years ago
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    Which hormone promotes secretion of na into kidneys, lowers blood volume?
    7 years ago
  • Rinaldo
    How do the kidneys regulate blood volume?
    7 years ago
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    How does salt maintain blood volume?
    7 years ago
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    7 years ago
  • melanie
    What harmone promote increased blood volume and pressure?
    7 years ago
  • ted
    Which mechanism regulates blood volume and pressure in the kidneys?
    7 years ago
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    What occurs in athletes when extra blood volume dilutes iron concentrations?
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    When ADH levels rise in humans?
    7 years ago
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    What hormones are involved to increase blood volume?
    7 years ago
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    7 years ago
  • angela
    What regulates osmolarity andvolume of blood in humans?
    7 years ago
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    What process increases blood volume?
    7 years ago
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    How does the kidneys conseve blood volume?
    7 years ago
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    7 years ago
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    What hormones are involved in increasing blood flow to the kidneys?
    7 years ago
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    How come filtration rate by kidney is 180L/day if we have only 5.5L of blood?
    4 years ago
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    Why our body weight does not increase though our kidney filtrate about 180l water per day?
    4 years ago
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    How changes in blood volume regulate renin secretion?
    4 years ago
  • Linda
    How does the change in blood volum regulate renin secretion?
    4 years ago
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    How kidney maintain blood volume?
    4 years ago
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    How negative feedback control blood volume?
    3 years ago
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    3 years ago
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    3 years ago
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    3 years ago
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    How does the kidney help to control blood volume?
    3 years ago
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    How negative feedback regulates plasma volume?
    3 years ago
  • j
    How adh helps in regulate of blood volume?
    3 years ago
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    3 years ago
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    What mechanism increases blood volume?
    3 years ago
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    3 years ago
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    Does drinking maintain blood volume?
    3 years ago
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    How the nephron regulates blood volume?
    3 years ago
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    What are effects of the production of large amounts antidiuretic harmone in the human body?
    3 years ago
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    What regulatory mechanism occurs in response to a decrease blood volume or pressure?
    3 years ago
    How is the heart regulates in controlling blood volume?
    3 years ago
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    How is the heart related in controlling blood volume?
    3 years ago
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    3 years ago
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    3 years ago
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    How ADH work in maintaining blood pressure and blood volume?
    3 years ago
  • lavinia
    When ADH levels increases what happens to urine blood osmolarity and blood volume?
    3 years ago
  • Bisrat
    How the kidney regulate blood content?
    2 years ago
  • gilberto
    What Control tha quantity of water in urine?
    2 years ago
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    How renin help to control low blood volume?
    2 years ago
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    What hormones do the kidneys secrete?
    2 years ago
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    How do kidney regulate blood water?
    2 years ago
  • Carmen Esposito
    2 years ago
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    2 years ago
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    2 years ago
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    2 years ago
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    2 years ago
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    2 years ago
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    2 years ago
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    How does the kidney regulates the amount of water in blood when one is very thirsty?
    2 years ago
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    How adrenal gland control blood osmotic pressure?
    2 years ago
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    What role does adh play in aintaining blood volume?
    2 years ago
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    How is adh involved in regulating blood Volume and Blood Pressure?
    2 years ago
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    What role do renin hormone play in the maintenance of blood volume and blood pressure?
    2 years ago
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    How to increase blood volume by renin angiotensin aldosterone system?
    2 years ago
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    2 years ago
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    How the antidiuretic hormone regulate blood volume?
    2 years ago
  • Matta
    How do kidneys respond to increased blood pressure?
    2 years ago
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    When amount of ADH decresses in blood,what happens ?
    2 years ago
  • LEA
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    2 years ago
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    How does the body act to maintain homoistasis when blood volume is low physiologically?
    2 years ago
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    2 years ago
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    How does the kidney restore blood volume?
    2 years ago
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    Which hormone decreases blood volume by increasing the excretion of sodium and fluids?
    2 years ago
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    How is blood plasma volume and acidity regulated?
    1 year ago
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    How does the regulation of blood plasma volume and acidity work?
    1 year ago
  • Hayden
    How do the kidneys respond to a decrease in blood volume and pressure?
    1 year ago
  • olo
    How human body regulates blood plasma volume?
    1 year ago
  • caleb
    How does angiotensin and the atrial natriuretic peptide affect urine production?
    1 year ago
    How does atrial natriuretic peptide ruduce or incease water loss in the kidneys?
    1 year ago
    Which hormones and enzymes increase blood volume?
    1 year ago
    How does the kidney regulate is someone has lost a litre of blood?
    1 year ago
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    How does atrial natriuretic peptide cause the kidneys to secrete more sodium and water?
    1 year ago
  • miikka
    Can blood loss lower ADH?
    1 year ago
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    1 year ago
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    What stage of urine production does low blood volume affect?
    1 year ago
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    How the sense of taste is involved in blood volume regulation?
    1 year ago
  • SARA
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    1 year ago
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    1 year ago
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    1 year ago
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    1 year ago
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    1 year ago
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    1 year ago
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    1 year ago
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    1 year ago
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    1 year ago
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    1 year ago
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    1 year ago
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    1 year ago
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    12 months ago
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    12 months ago
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    11 months ago
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    11 months ago
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    11 months ago
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    11 months ago
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    10 months ago
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    9 months ago
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    8 months ago
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    8 months ago
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    7 months ago
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    7 months ago
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    6 months ago
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    6 months ago
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    5 months ago
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    4 months ago
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    3 months ago
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    How blood volume and urine volume are linked.?
    3 months ago
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    How blood volume, blood ph and blood osmolality is balanced by the kidneys.?
    2 months ago
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    How does the body resond to increased loodvooume?
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    What hormone regulates blood volume?
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    1 month ago
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    What blood pressure regulatory hormone takes care of sudden blood volume increase?
    29 days ago
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    How blood volume affects your health?
    16 days ago
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    14 days ago
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    13 days ago
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    10 days ago
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    4 days ago

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