Assignment #9 - Renal Physiology

1. 
Describe, in detail, the effects that the following altered states would have on blood pressure and plasma osmolarity (if any). Be sure to include a step-by-step description of underlying processes occurring. Note that plasma osmolarity is a measure of the concentration of solutes in the blood and is most profoundly affected by plasma sodium concentrations. (8 points)
a) Increased aldosterone secretion (hyperaldosteronism)
Increased aldosterone secretion causes increased sodium reabsorption from the tubules. Since sodium has a high osmotic pressure, water will “follow” the sodium back into the blood stream, and blood pressure will increase. Even though sodium is moving into the blood stream, plasma osmolarity will not change because the water moves into the blood stream as well, keeping the overall sodium concentration the same.
b) Decreased aldosterone secretion (hypoaldosteronism)
Decreased aldosterone secretion results in less sodium being reabsorbed, and therefore less water being reabsorbed, resulting in decreased blood volume and decreased blood pressure. Osmolarity will remain the same.
c) Increased ADH secretion (syndrome of inappropriate ADH (SIADH)
When ADH binds to receptors on the outer membrane cells of the distal and collecting tubules, it activates cAMP as a second messenger, which, in turn, causes water channels in the inner membrane cells to open. The water can then freely flow through the tubule membrane cells and into the peritubular capillaries, increasing blood volume and blood pressure. Plasma osmolarity will decrease because more water is moving into the blood stream, but not sodium.
d) Decreased ADH secretion (diabetes insipidus)
When no ADH is present, the membranes of the distal and collecting tubules are impermeable to water. In diabetes insipidus, ADH secretion is lacking, resulting in fewer ADH molecules not being available to bind to their receptors and open the water channels as described in “c” above. When the tubules are less permeable to water, less water is reabsorbed. Decreased volume will lead to decreased blood pressure and increased osmolarity.
2. Describe two different mechanisms through which angiotensin converting enzyme (ACE) inhibitors work as antihypertensive
 medications to reduce blood pressure. Try to answer this without going to a drug book! (4 points)
ACE inhibitors block the conversion of angiotensin I to angiotensin II in the renin-angiotensin-aldosterone system. Blocking this conversion helps reduce blood pressure in two ways. First, angiotensin II causes arterial vasoconstriction, stimulates thirst, and increases ADH secretion. All three of these things can lead to increased blood pressure, so blocking its formation will directly diminish its effects. Second, angiotensin II stimulates the renal cortex to release aldosterone (which increases blood pressure by increasing sodium and water reabsorption in the tubules). Again, by blocking the converting enzyme that forms it in the first place, ACE inhibitors minimize the blood pressure raising effects of aldosterone.
3. Explain (in detail) why a diabetic would show the presence of glucose in the urine with a blood glucose level of 300 mg/dl. (4 points)
Reabsorption of glucose is dependent on carrier molecules in the membrane cells of the proximal convoluted tubule. Transporting glucose molecules is limited because there are a finite number of these carrier molecules. They can transport glucose molecules at a maximum rate of 375 mg/min, which correlates to a blood sugar level of 180 mg/dL. At this point, the carrier molecules are saturated and have renal threshold, or Tm, meaning that any blood sugar in excess of that amount will result in glucose molecules not being reabsorbed from the urine filtrate, and being excreted in the urine.
4. Renal failure occurs when the kidneys fail to filter (& then reabsorb) and secrete. In renal failure, describe what happens to the following clinical variables and provide a brief explanation. (4 points) a) plasma potassium levels—will increase because of decrease filtration rate and failure to secrete K+ back into filtrate
b) plasma hydrogen (i.e. plasma pH)— plasma hydrogen will increase (plasma pH will decrease) because more hydrogen ions are retained/not secreted.
c) blood volume and blood pressure—will increase because a lower glomerular filtration rate means less volume entering the urinary system, therefore more volume is staying in the cardiovascular system.
d) plasma creatinine and urea (measured as
 blood urea nitrogen) levels—will increase because of decreased filtration rate.