The urinary system consists of the kidneys, ureters, bladder and urethra.  The kidneys perform many important functions:

 1) They regulate the concentration of many solutes  in the extracellular fluids..
 2) They eliminate waste products.
 3) They control the concentration of the urine.
 4) They play an important role in pH control.
5) They secrete hormones which regulate erythrocyte production, Vitamin D3 production     and blood pressure.

The kidneys function in the formation of urine and the ureters, bladder and urethra function for urine transport and storage. 

The kidneys are bean shaped organs and internally they are divided into six to ten renal pyramids.     The pyramids are comprised of an outer cortex and an inner medulla.   The tip of the pyramids forms the papilla which drains the urine into a minor calyx.  The minor calices unit to form a major calyx and the urine flows from the major calices into the renal pelvis.  The renal pelvis empties into the ureter which transports the urine to the bladder for storage.   The urine leaves the bladder into the urethra where it is transported out of the body.  The formation and transport of urine would be as follows:

In the kidney the urine is formed in structures termed nephrons.  Nephrons are composed of three components:

 1) the vascular component
 2) the tubular component
3) the corpuscle component

The vascular component is comprised of two arterioles and two capillary systems.  The afferent arteriole delivers blood to the first capillary system which is the glomerulus.  Blood pressure is high in the afferent arterial resulting in the glomerulus being a high pressure capillary system.  The glomerular blood empties into the efferent arteriole.  Due to the loss of blood pressure in the glomerulus, the efferent arteriole is a low pressure arteriole.  The efferent arteriole delivers blood to the peritubular capillary system which surrounds the renal tubules.  The peritubular capillary system is a low blood pressure system.  The tubular component of the nephron is comprised of the Bowman’s Capsule, Proximal convoluted tubule, Loop of Henle, and Distal convolutes tubule.  Several nephrons empty into a Collecting duct.  The Corpuscle component is comprised of the glomerulus and the Bowman’s capsule which form the Renal corpuscle.  The glomerulus resides inside the Bowman’s capsule which surrounds the glomerulus.

 
 The majority of the nephrons are situated in the cortex and referred to as cortical nephrons.  The remaining nephrons have a corpuscle which is located in the cortex next to the medulla.  These nephrons are termed juxtamedullary nephrons.  Juxtamedullary nephrons have larger corpuscles and a longer loop of Henle which extends deeply into the medulla. 

Nephrons carry out three major functions:

 1) Regulation of solutes
 2) Regulation of urine concentration
 3) Regulation of pH

Solute regulation in the nephron is carried out by three processes: filtration,  reabsorption, and secretion.

Although blood cells and proteins can not enter the glomerular filtrate, all other blood components which are small enough can pass into Bowman’s capsule ( glucose, amino acids, wastes, ions, etc.).  If these solutes in the filtrate are left in the filtrate they will be lost from the body in the urine.  Much energy was expanded in digestion, absorption and other processes to obtain many of the filtrate solutes ( glucose, amino acids, vitamins, etc.) and many of these solutes are essential for body maintenance.  To prevent the loss of these important solutes, the solutes are reabsorbed back into the body through the process of Reabsorption.

Reabsorption involves moving solutes from the tubular filtrate, through the tubular cells, into the peritubular capillaries.   Some solutes are reabsorbed by passive transport following their diffusion gradient either through the tubular cells or in some cases between the tubular cells.  Other solutes are reabsorbed by the tubular cells utilizing either primary or secondary active transport mechanisms.  Recall that solutes moved by active transport have transport maximums (Tm).  If a solute concentration exceeds the Tm, the excess solute will be excreted in the urine.  Reabsorption explains how solutes that are essential for the body are moved from the urine back into the blood.  The majority of reabsorption occurs in the proximal convoluted tubule where glucose, amino acid, ions, and other solutes are reabsorbed.  In the Loop of Henle  reabsorption of Na+, Cl-, and K+ takes place.  Under the influence of hormones the reabsorption of Na+ may take place in the distal convoluted tubule and the upper collecting duct.
 

 
 

Nephron secretion is the opposite of reabsorption in that it results in the tubular cells moving solutes from the peritubular capillary plasma into the tubular filtrate.  The proximal convoluted tubule secretes many inorganic and organic solutes (urea, catecholamines, penicillin, bile salts, histamine, etc.).
 

 

Secretion also occurs in the distal convoluted tubule and the upper collecting duct.  Secretion in the distal convoluted tubules and the collecting ducts depends on a sodium-potassium cotransport system located on the body side of the tubule cell.   This active transport system pumps Na+ out of the cell and K+ into the cell.  This results in an increased concentration of K+ inside the tubule cell.  The electrical gradients between the tubular cell (-70 mV) and the blood (0 mV) or urine in the tubule lumen ( -50 mV) favors the movement of the K+ into the lumen.   Thus, K+ is moved into the cell by active transport and then passively diffuses into the tubule lumen.

In addition to regulating solutes in the urine, the tubules also function in regulating the water concentration of the urine (urinary volume).  The principle function of the loop of Henle is establishing an osmotic gradient in the extracellular fluid surrounding the tubules and collecting ducts.  When the pores in the collecting duct are open, the osmotic gradient in the extracellular fluid creates an osmotic force which pulls the water out of the collecting duct into the peritubular capillaries.   If the collecting duct pores are closed,  the water remains in the urine and is excreted from the body.  By regulating the number of pores open, the concentration of the urine can be varied.  When the body becomes dehydrated the pores are opened and water is pulled back into the body resulting in a concentrated urine.   If the body has excess fluid the pores remain closed and the water passes out in the urine forming a very dilute urine.

 
                                                            pH and ACID-BASE BALANCE

A third function carried out by the kidney is pH regulation.  PH is a measurement of the acidity of a solution. The pH of a solution is determined by the concentration of hydrogen ions (H+) in solution.

When an acid (HA) is added to water it dissociates into free H+and the base it is
associated with (A-).   A base is a substance which can combine with free H+ and remove it from a solution. This can be expressed as HA <-----> H+ + A-.   The pH of water, pH 7,  is neutral with the H+ equaling the OH- (the base A-).   As the pH becomes < 7, the H+ concentration increases, the OH- concentration decreases and the solution becomes more acidic.

 
 As the pH becomes > 7, the H+ concentration decreases,  the OH- concentration increases and the solution becomes more basic.  The illustration above diagrams a pH scale and gives some examples of solutions with different pHs.

How much of an acid dissociated when in solution is always constant for a specific acid.  The dissociation constant (K) represents the degree of dissociation for a particular acid.   The dissociation constant can be expressed as follows:

     Ka =      [H+} x [A-] / HA

The pH of the body fluids is regulated by several systems: chemical buffering systems; the respiratory system; and the kidneys.  There are three major chemical buffering systems in our body which function to regulate pH:

 1) Bicarbonate buffers     2) Phosphate buffers   3) Protein buffers

Of these three,  the bicarbonate buffer system is especially important in pH regulation.  The bicarbonate buffer system would be as follows:

  CO2 + H2O <-----> H2CO3 <-----> H+ + HCO3-

The respiratory system can neutralize two to three times as much acid as the chemical buffers.  The action of the respiratory system involves the same equation as the bicarbonate buffer presented above.  In the tissues CO2 is produced by the cells, enters the red blood cell in the plasma where it combines with water to form carbonic acid which then dissociates to the bicarbonate ion and the hydrogen ion.   The bicarbonate ion leaves the red blood cell and enter the plasma where it is transported to the lungs.  The hydrogen ion is buffered by the hemoglobin in the red blood cell.
In the lungs the carbon dioxide level is low, it is being expired from the body, and the reaction reverses itself with the hydrogen combining with bicarbonate ion to form carbonic acid which then dissociates to water and carbon dioxide.  The carbon dioxide is then exhaled from the lungs.  An increased respiration rate will increase carbon dioxide removal and result in a decrease in blood hydrogen resulting in an increase alkalinity (> pH) of the blood.  In contrast,  a decreased respiratory rate will result in an increase in carbon dioxide resulting in an increase in hydrogen and an increased acidity (< pH) of the blood.

The kidney can neutralize more acid or base than either the chemical buffering systems or the respiratory system.  Recall that renal tubules secrete H+ into the tubular fluid where it can pass out in the urine.  Thus, the kidney has the ability to directly remove hydrogen ions from the blood rather than simply buffer them.  However, the free hydrogen ion in the tubular fluid cannot exceed a concentration of pH 4.5.  If the pH of the tubular fluid falls below 4.5, there is not a sufficient concentration gradient for the hydrogen ion between the tubular cell and the tubular fluid,  so hydrogen ion secretion stops.  It is therefore essential that the free hydrogen ion in the tubular fluid be buffered to prevent the tubular fluid pH from dropping below 4.5.  There are three buffering system in the tubular fluid: bicarbonate, phosphate and ammonia.

Bicarbonate is the primary buffering system in the tubular fluid.   The bicarbonate ion enters the tubular fluid from the blood as sodiumbicarbonate.   The hydrogen ion secreted into the tubular fluid displaces the sodium and combines with the bicarbonate to form carbonic acid.  The carbonic acid then dissociates into carbon dioxide and water.  The carbon dioxide reenters the tubular cell and the hydrogen ion is excreted in the water molecule.  For every carbon dioxide reabsorbed into the tubular cell a bicarbonate ion is formed and passes into plasma of the peritubular capillary.  The sodium which dissociates from the bicarbonate ion is actively moved from the fluid into the tubular cell.   The bicarbonate buffering system in the tubular fluid is illustrated below:

 
As shown above, if the hydrogen ion concentration exceeds the bicarbonate ion concentration, the excess hydrogen ion will accumulate in the tubular fluid and the pH will begin to drop.   At this point the phosphate and ammonia buffers come into play to prevent an excess of free hydrogen ions and a lowering of the pH.   In cases where the blood becomes alkaline (excess bicarbonate ion), the bicarbonate ion concentration in the tubular fluid will exceed the hydrogen ion concentration..  The tubular cell membrane is not permeable to bicarbonate ions so the excess bicarbonate ions will be excreted in the urine.  Excretion of the bicarbonate ions results in a decrease in the blood pH ( > acidity).

In all three pH regulating systems - chemical buffers, respiration and the kidneys- the control of pH has involved the bicarbonate buffer.  The two major factors in this relationship are the bicarbonate ion concentration and the carbon dioxide concentration.  These two variables play a major role in determining the body fluid pH.   The relationship between these two factors can be expressed in the Henderson-Hasselbalch equation.  This relationship is expressed as follows:
 
   pH =pKa + log   [ HCO3-] / [CO2]

This equation demonstrates that the pH of the extracellular fluid changes whenever the concentration of bicarbonate ion or carbon dioxide changes.  Increased bicarbonate ion will result in an increase in pH, an increased alkalinity.   The bicarbonate ion concentration is regulated primarily by the kidneys.  In contrast, an increase in carbon dioxide lowers the pH, an increase in acidity.  The carbon dioxide concentration is controlled primarily by the respiratory system. Acid-base disturbances which occur due to changes in the bicarbonate ion concentration are termed metabolic acid-base disorders, whereas disturbances due to carbon dioxide concentration changes are termed respiratory acid-base disorders.