Regulation Of Water And Na+ Balance, ECF Volume, And Blood Pressure

5/3/10 9:00 AM
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What is the basic principle of body water balance?
Intake must be equal to output.
Where is ADH released from?
The posterior pituitary gland. it is stored in secretory granules in nerve endings in the posterior pituitary gland.
Where are the cell bodies of the nerves that contain ADH located?
In the hypothalamus.
What causes the release of ADH?
Depolarization of the nerve terminals, which causes an increase in Ca2+ influx into the terminal, which causes exocytosis of the granules that contain the ADH.
What are the 2 main stimuli of ADH release?
An increase in plasma osmolality (more important)Decrease in ECF volume
Increased plasma osmolality causes what 2 events?
1. ADH release2. Thirst
Large loss of ECF leads to what 2 events?
1. ADH release2. Thirst
T/F. Large loss of ECF does NOT cause ADH release and thirst if the plasma osmolality is low.
FALSE. Even if the plasma osmolality is low, a large loss of ECF will cause ADH release and thirst (this is because it will activate the baroreceptors in the aortic arch and carotid bodies, which will send signals to the hypothalamus to release ADH from the posterior pituitary nerve terminals).
What is another name for ADH?
Vasopressin, or arginine vasopressin
Why is ADH also called vasopressin?
Because when it is released in response to a large decrease in ECF volume, it also acts as a pressin to increase blood pressure.
In what causes would ADH be released when the plasma osmolality is low?
In functional hypovolemia. Even though there is not actually a decrease in ECF volume (there may even be an increase, like generalized edema), there is decreased venous return, so the body is responding to these signals as if there is a decrease in ECF. Therefore, the body will release more ADH and aldosterone to retain more Na and water, even though that is not needed.
What is functional hypovolemia?
A state in which ECF can be massively expanded, but the body is reacting as is ECF volume is significantly depleted. So ADH levels are high and the RAS system is activated, resulting in increased Na+ and water retention.
Give 4 examples of functional hypovolemia?
Congestive heart failureCirrhosisNephrotic syndromeAV shunting due to fistula or congenital defect
Describe the feedback regulation of body water in response to water deficit.
Water deficit causes increased plasma osmolality, which triggers 2 things- ADH release and thirst.Osmoreceptors in the supraoptic nucleus of the hypothalamus are activated --> send depolarizing action potentials to the posterior pituitary, which causes release of ADH --> Luminal membranes of the CNT and collecting duct become more H2O permeable --> excretion of hyperosmotic urine --> decreases the plasma osmolalityOsmoreceptors in the hypothalamic thirst centerActivation of behaviorl thirst drive --> increased water intake --> decrease the plasma osmolality
What is the main determinant of ECF volume balance?
Na+ balance in the bodyi.e. a constant amount of Na+ in the body
T/F. Plasma Na+ concentration is a good indicator of the amount of Na+ in the body.
FALSE. It does not tell you anything about the Na+ amount in the body. For example, there could be a lot of water in the ECF, which would lower PNa+, even though you had the same amt of Na+ in the plasma. This would false make you think you had less Na+ in the body than you actually do.
How is ECF volume related to Na+ content?
ECF volume is directly proportional to the amount of Na+ in the body. SO if you have a diet with increased Na+ intake, the ECF volume will be higher, and your weight will go up.IF you have a diet with decreased Na+ intake, the ECF volume will be lower, and your weight will decrease.
Why does it make sense that an increased salt intake would lead to an increased ECF and body weight?
If you're taking in additional salt, you are going to have increased reabsorption of salt from the ascending limb of the nephron. Therefore, the plasma osmolality will increase. This stimulates the release of ADH, which will cause your water reabsorption to also increase, so you get an increase in ECF volume. Therefore, you have weight gain. The reason why these people can become hypertensive on a high salt diet is that the RAS system is activated as well. angiotensin II is released, which stimulates the release of aldosterone (from the adrenal cortex), and aldosterone stimulates even more Na+ retention. Therefore, the body has to maintain a higher blood volume to keep Na+ balance, which causes the blood pressure to rise.
How fast does the body adjust the Na+ excretion rate to be equal to the increased Na+ intake?
It takes a few days for sodium excretion rate to match the new sodium intake rate, and during that time, the body is retaining the sodium and thus the ECF volume increases
What is pressure natriuresis?
All else being equal, increases in blood pressure will cause increase in Na+ and water excretion.
What is salt-sensitive hypertension?
Increase Na+ intake in some hypertensives, and will increase their blood pressure even more. Some people can increase their salt intake and not raise the blood pressure much.
What is the GT balance?
The nephron's ability to maintain a constant 2/3 reabsorption of NaCl from the PCT>
What is atrial natriuretic peptide (ANP)?
Released from the atria of the heart when stretch receptors in the walls of the atria are activated in response to an increase in ECF (i.e. high blood pressure).
What are the 3 main actions of ANP?
1. Systemic relaxation of vascular smooth muscle, decreasing TPR.2. Relaxation of afferent arterioles and constriction of efferent arterioles, which increases GFR.3. Inhibits Na+ reabsorption in the medullary collecting duct.
What are the 2 effects of sympathetic tone on the nephron?
1. Decreases filtration2. Increases peritubular reabsorptive forces
Describe the effects of sympathetic tone on the nephron.
Constricts the efferent and afferent arterioles, which decreases the GFR. This means less filtration and less flow to the peritubular capillaries. Less flow to the peritubular capillaries enhances the low hydrostatic pressure and the high osmotic pressure, which contributes to the peritubular reabsorptive forces.In a volume contracted state, you release catecholamines, which activate the SNS and cause this decreased filtration and increased reabsorption to occur. THis is waht you want to be able to retain water/Na+ to maintain your volume.
______ can stimulate the proximal convoluted tubule to absorb more salt.
Angiotensin II, catecholamines
How do angiotensin II and catecholamines stimulate the PCT to absorb more salt?
They act directly on the cells of the PCT to increase Na/H antiporters in the lumenal membrane
________ and ______ are the 2 major hormones that act to increase Na+ reabsorption.
Aldosterone, angiotensin II
Where does aldosterone increase Na+ reabsorption from?
Acts on the medullary collecting duct
Where does Angiotensin II increase Na+ reabsorption from?
Describe the response to hypovolemia (decreased ECF volume)
1. Release of renin, ADH, and NE2. Renin activates Ang II --> activates aldosterone, which increases Na+ and H2O reabsorption in the distal nephron, and directly increases Na+ and H2O reabsorption from the PCT --> decreases Na+ excretion3. ADH increases water reabsorption from the distal nephron --> decreased water excretion4. NE increases Na+ reabsorption and water reabsorption from the PCT which decreases Na+ excretion; also increases afferent and efferent arteriole tone, reducing GFR and reducing Na+ excretion

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