7. Dr. Spanky Weaselsnot is a 54-year-old math professor who maintains a healthy

Question

7. Dr. Spanky Weaselsnot is a 54-year-old math professor who maintains a healthy lifestyle. He exercises regularly, doesn't smoke or drink alcohol, and keeps his weight within the normal range for his age. Recently however, he has experienced generalized muscle weakness and recurring headaches. Spanky assumed the headaches were due to the stress of grading math homework, but over-the-counter pain medications didn't help. As these symptoms persisted, Spanky became concerned and made an appointment to see his primary care physician. On physical examination, Spanky appeared healthy. However, his blood pressure was significantly elevated at 180/120 (both supine and standing). His physician ordered some laboratory tests on his blood and urine. The results are in Table 1 below. Table 1 -Laboratory results on the blood and urine chemistry of Professor Weaselsnot. Test Result Normal Value Test Arterial Blood pH pCO2 7.4 40 mm Hg 7.50 48 mm Hg Venous Blood Na" K* ???, 142 mEq/L 2.0 mEq/L 36 mEq/L 140 mEq/L 4.5 mEq/L 24 mEq/L Urine (normal) (elevated) 200 mEq/L per 24 hr Na* K* 1350 mEq/L per 24 hr The physician suspected that Professor Weaselsnot's hypertension was caused by an abnormality in the RAA pathway. He ordered additional tests, including a plasma renin activity and serum aldosterone levels, which yielded the information shown in Table 2 Table 2- Additional laboratory results for Professor Weaselsnot. Test Plasma renin activity Serum aldosterone levels Results Decreased Increased (a) Explain how the RAA pathway is regulated, and two ways that this pathway can affect blood pressure. (6 points) (b) Based on Professor Weaselsnot's test results, he is experiencing hypokalemia (low blood potassium). What explanation can you give for this? (3 points) (c) Explain Professor Weaselsnot's muscle weakness based on his severe hypokalemia. (Think about the RMP of skeletal muscle.) (5 points) (d) Using the test results in Table 1, what acid-base disorder was Professor Weaselsnot suffering from? What test results lead you to this conclusion? (5 points)

Explanation / Answer

a)Renin angiotensin aldosterone pathway:

It plays key role in regulating systemic blood pressure. It also regulates renal blood flow and glomerular filtration rate.

When systemic hypotension or low blood pressure happens it is sensed by baroreceptor cells in blood vessels , the juxtoglomerular cells in kidney near afferent ,efferent and distal tubule secrete renin. Renin converts angiotensinogen which is a precursor of protein made in liver. Renin acts as catalyser in converting angiotensinogen to angiotensin -1. Angiotensin 1 is coverted to angiotensin-2 by angiotensin coverting enzyme secreted by lungs. Angiotensin - 2 is a most powerful vasoconstrictor.

Vasoconstriction causes the blood pressure to increase .The adrenal cortex secretes aldosterone in response to stimulation made by pituitary because of poor perfusion or increasing serun osmolality. Now the blood pressure increased so the baroreceptors sense it and stops renin secretion. Then blood pressure starts coming down.

When there is failure in renin angiotensin aldosterone pathway hypertension occurs.

b) due to Renin angiotensin aldosterone action aldosterone is secreted. Aldosterone causes kidneys to excrete potassium and regulate excretion of sodium volume . When Aldosterone level is increased in blood it causes renal reabsorption of sodium and more excretion of potassium .

So hypokalemia is experienced with hypernatremia.

c) Hypokalemia is the reason for muscle weakness. Resting membrane potential of skeletal muscle is -90mV . Because of decreased potassium it affects the polarization in muscles leading to decreased excitability .

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