Transport maximum (Tmax) Essay

1.      Vocabulary:

a.       Transport maximum (Tmax): The maximum rate of transport of a substance through active transport or facilitated diffusion, it is the transport rate at saturation. In renal physiology it’s discussed in relation to glucose and PAH.

b.      Filtration: The process through which the components of the blood like urea salts and ions with the exception of blood cells are sieved out into the Bowman’s capsule resulting in filtrate in the renal tubule.

c.       Secretion: The process of releasing a substance from a cell or gland. In renal physiology it is the transfer of materials from peritubular capillaries to renal tubular lumen and is mainly a result of active transport.

d.      Reabsorption: The process of a substance being absorbed again. Tubular reabsorbsion is the process through which solutes (glucose, salts) and water are removed from the tubular fluid and returned into the blood.

e.       GFR:  This is the volume of fluid filtered from the kidney capillaries into the Bowman’s capsule per unit time. Recorded in units of volume per time (ml/min).

f.       Osmolarity:  Is the measure of solute concentration number of solute particles per liter of solution and is expressed as Osm/L

g.      ANP: Atrial Natriuretic Peptide is a protein secreted by cardiac cells and is concerned with in homeostatic control of body water, sodium, potassium and fat. It is also a vasodilator

h.      BNP: Brain Natriuretic Peptide is a polypeptide secreted by ventricles of heart in response to stretching of cardiac muscles. It decreases central venous pressure increases natriuresis

i.        Natriuresis: This is the excretion of large amounts of sodium in the urine via action of the kidneys.  It is promoted by BNP AND ANP inhibited by aldosterone

2.      Factors that govern plasma volume: Angiotensin a protein that causes constriction hence decreased filtration and stimulates aldosterone release. Concentration of NaCl.  Renin-angiotensin-aldosterone system which causes increased Na retention and leads to reduced water loss in urine. Aldosterone.  ANP and BNP.  BNP decreases blood volume and cardiac output. Exercise decreases plasma volume. Renin enzyme mediates plasma volume. Vasopressin (ADH) affects tissue permeability role in homeostasis and regulation of water, glucose and salt in blood.

What hormones are involved?   Aldosterone increases the reabsorption of Na and water and release of k in kidneys this increases blood volume. Antiduretic hormone (ADH)

How is free water control different from serum sodium control? Free water control involves the excretion of water (that contains low or no solutes) without the influence of ADH. Serum sodium control involves hormonal regulation of Na reabsorption and excretion.

3.      When the juxtamedullary nephrons aren’t working correctly there is diminished proximal reabsorption which increases the quantities of Na and chlorine ions excreted through urine. Altered urine concentration and reabsorption and retention of water is also affected.

4.      What governs plasma osmolarity as opposed to volume? Osmolarity is controlled by the concentration of solutes while volume is controlled by reabsorption of water, and Na. Hormones regulate electrolyte concentration in plasma.

5.      What is the relationship between plasma concentration (of a drug), transport maximum, and urine concentration?  Be able to calculate.  When the plasma is saturated with drug molecules, the rate of transport of the drug is maximum and the urine concentration (of the drug) is highest.

Clearance (of the drug) C = (U x V)/P (where U is concentration, V is urine vol/time, P is plasma concentration. (U x V= urinary excretion).

6.      What ion contributes most to serum osmolarity? Sodium ions.

7.      What happens to GFR when the afferent arteriole is dilated or constricted? Dilation of the afferent arteriole increases GFR whereas its constriction decreases GFR.

How about the efferent arteriole? Constriction of the efferent arteriole increases GFR while dilation decreases GFR.

8.      How does the body respond to dehydration? Thirst is usually first response to dehydration, other physiological responses are decreased urine excretion, increased heart rate, increased respiration, decreased sweating, increased body temperature, fatigue, headaches, muscle cramps and nausea.

What hormones are activated? Antiduretic hormone and aldosterone are activated. What goes on in the kidney in regard to absorption or excretion of sodium and free water? There is increased reabsorbtion of Sodium and free water. Excretion of free water and sodium is reduced.

9.      What happens to ECF osmolarity/volume and ICF osmolarity/volume when hypotonic fluid is infused?

Increased ECF volume and decreased osmolarity. Intracellular fluid volume is in excess. Hypertonic fluid? Is a solution that contains less dissolved substances in comparison to plasma has lower concentration in comparison to plasma.

10.  What happens when a patient has too much circulating renin?

Increased levels of renin result in retention of sodium and water which lead to hypertension. How about too little?  Too little renin results in excess excretion of sodium and water.

What about aldosterone (too much/too little)? Too much aldosterone leads to inappropriate salt reabsorption; this in turn increases ECF volume and leads to symptoms commonly seen in Conn syndrome. Too little aldosterone results in low blood pressure, and symptoms commonly seen in Addison’s disease, which include fatigue, muscle pain among others.

11.  What stimulates secretion of renin?

Renin secretion is stimulated by decrease in blood pressure, blood volume, or NaCl levels.

Aldosterone?  Aldosterone secretion is stimulated by an increase in plasma concentration of Angiotensin, increased plasma potassium levels K levels, plasma acidity (increased proton concentration), decrease in blood pressure and stimulation from adrenoglomerulotropin which stimulates aldosterone secretion.

ADH? ADH secretion is stimulated by increased osmolarity of plasma, and reduced volume of extracellular fluid.

12.  How do sodium and free water permeability change as the filtrate transits the loop of Henle? Permeability increases and there is increased more is reabsorption of sodium and free water.

13.  How do loop diuretics change the answer to #12?

Loop diuretics inhibit reabsorbtion and hence the permeability decreases more water and sodium is expelled resulting in increased urine production.

References

Kuwait University, Department of Physiology, Faculty of Medicine. (2004, January 1). Renal Physiology. Retrieved May 28, 2010, from http://www.acbrown.com/renal/index.htm

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