5. It is the summer of your big family reunion, and you are happy to reconnect w

Question

5. It is the summer of your big family reunion, and you are happy to reconnect with your cousin Meg,

whom you haven’t seen since childhood. She confides that life has had its emotional struggles lately,

but she’s hopeful. You talk well into the afternoon, until a friendly family softball game gets started

in the ballpark across the way. “C’mon, let’s get in on it,” Meg says, but no sooner than you both

stand up from the picnic table, her eyes roll back in her head, and she falls crashing into the bench,

then flat on the ground.

At the emergency room, the doctor says that Meg experienced a strong case of orthostatic

hypotension, an uncorrected drop in blood pressure upon sudden standing. “Normally the body

compensates, and doesn’t let this happen. Apparently, an increased likelihood of postural hypotension

is one side effect of a new antidepressant drug Meg has been taking. Well, she’s had to get a few

stitches from the laceration on her forehead, and should definitely consult with her physician, but she

ought to be just fine.”

“Yes,” you add, “but I feel bad because it’s hot out, and I knew she wasn’t drinking any water but we

just kept talking. That could definitely be a contributing factor to a hypotensive episode.”

“Whoa, I’m impressed!” says your Dad. “Why don’t you share some more of that knowledge you

picked up in your Physiology class?”

How do the nervous and cardiovascular systems accomplish homeostatic control of blood pressure in a

way that normally prevents a hypotensive episode like Meg experienced? In your answer explain how

dehydration may have contributed to Meg’s orthostatic hypotension. Include specific consideration of:

- the baroreceptor reflex

- other CNS roles in responding to dehydration (specifically detection of hypertonic plasma)

- ways in which the kidneys detect and respond to a dehydrated condition

Explanation / Answer

The human organism consists of trillions of cells all working together for the maintenance of the entire organism. While cells may perform very different functions, all the cells are quite similar in their metabolic requirements. Maintaining a constant internal environment with all that the cells need to survive (oxygen, glucose, mineral ions, waste removal, and so forth) is necessary for the well-being of individual cells and the well-being of the entire body. The varied processes by which the body regulates its internal environment are collectively referred to as homeostasis. Homeostasis in a general sense refers to stability or balance in a system. It is the body's attempt to maintain a constant internal environment. Maintaining a stable internal environment requires constant monitoring and adjustments as conditions change. This adjusting of physiological systems within the body is called homeostatic regulation.Dehydration is the excessive loss of body fluid, usually accompanied by an excessive loss of electrolytes. Baroreceptors are sensory neurons that monitor arterial blood pressure. Major baroreceptors are located in the carotid sinus (an enlarged area of the carotid artery just above its separation from the aorta), the aortic arch, and the right atrium.

The symptoms of dehydration include; headaches, cramps, dizziness, fainting and raised blood pressure (blood becomes thicker as its volume decreases requiring more force to pump it around the body).Dehydration or a loss of body fluid (through sweat, urination, bleeding etc) results in an increase in ‘blood tonicity’ (the concentration of substances within the blood) and a decrease in blood volume. Where as hyperhydration or a gain in body fluid (intake of water) usually results in a reduction of blood tonicity and an increase in blood volume.

Any change in blood tonicity and volume is detected by the kidneys and osmoreceptors in the hypothalamus.

Osmoreceptors are specialist receptors that detect changes in the dilution of the blood. Essentially they detect if we are hydrated (diluted blood) or dehydrated (less diluted blood).

antidiuretic hormone (ADH): also known as vasopressin, a hormone that increases the volume of water reabsorbed from the collecting tubules of the kidney

dehydration: state of containing insufficient water in blood and other tissues

diuresis: excess production of urine

plasma osmolality: ratio of solutes to a volume of solvent in the plasma; plasma osmolality reflects a person’s state of hydration.

Homeostasis requires that water intake and output be balanced. Most water intake comes through the digestive tract via liquids and food, but roughly 10 percent of water available to the body is generated at the end of aerobic respiration during cellular metabolism. Urine produced by the kidneys accounts for the largest amount of water leaving the body. The kidneys can adjust the concentration of the urine to reflect the body’s water needs, conserving water if the body is dehydrated or making urine more dilute to expel excess water when necessary. ADH is a hormone that helps the body to retain water by increasing water reabsorption by the kidneys.

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