Renal physiology

Renal physiology is the study of the physiology of the kidneys.

Functions of the kidney

Filtering wastes from the bloodstream

Wastes are filtered out from the blood in the glomeruli via the process of ultrafiltration.

The ultrafiltrate is passed through, in turn, the proximal convoluted tubules, the loop of Henle, the distal convoluted tubules and is then collected by the collecting ducts to form urine.

The renal collecting ducts open into the renal pelvis and drain into the ureters which pass on the urine to the bladder.

Secretion of hormones

• Secretion of erythropoietin, which regulates red blood cell production in the bone marrow.
• Secretion of renin
• Secretion of the active form of vitamin D - 1,25-dihydroxycholecalciferol (calcitriol) and prostaglandins.

Maintaining body sodium and water balance

There is a stable balance of sodium and water in the body. The major homeostatic control point for maintaining this stable balance is renal excretion.

The kidney is directed to excrete or retain sodium via the action of aldosterone, ADH (anti-diuretic hormone a.k.a. vasopressin), ANP (atrial natriuretic peptide) and other hormones.

Cl- always follows Na+.

Adic-base homeostasis

Metabolic reactions are very sensitive to the pH level (H+ or hydronium ion concentration) of the fluid in which they occur. This is because hydronium ions can influence enzyme function.

The kidneys maintain blood plasma acid-base homeostasis by hydronium regulation. Gain and loss of hydronium must be balanced. Sources of hydrogen-ion gain:

1. Carbon dioxide
2. Production of nonvolatile acids from the metabolism of proteins and other organic molecules.
3. Gain in hydrogen ions due to loss of bicarbonate in diarrhea or other nongastric GI fluids.
4. Gain in hydrogen ions due to loss of bicarbonate in the urine.
5. Hypoventilation

Sources of hydrogen ion loss:

1. Use of hydrogen ions in the metabolism of various organic anions.
2. Loss of hydrogen ions in vomitus.
3. Loss of hydrogen ions in the urine.
4. Hyperventilation.

When hydrogen ion loss exceeds gain, alkalosis occurs. When gain exceeds loss acidosis occurs. There are various renal responses to acidosis and alkalosis:

Responses to acidosis:

1. Bicarbonate is added to the blood plasma by tubular cells.
   • This is caused by sufficient hydrogen ion secretion from the tubular epithelial cells.
   • Extra hydrogen ion secretion will bind to nonbicarbonate urinary buffers and this will lead to more new bicarbonate in the blood plasma.
   • This is also caused by increased glutamine metabolism and ammonia excretion.

Responses to alkalosis:

1. Excretion of bicarbonate in urine.
   • This is caused by lowered rate of hydrogen ion secretion from the tubular epithelial cells.
   • This is also caused by lowered rates of glutamine metabolism and ammonia excretion.

Hydronium ions and carbon dioxide

1. carbon dioxide + water + carbonic anhydrase (catalyst) ↔ carbonic acid ↔ bicarbonate + hydronium ion

Buffering of hydrogen ions

Any substance that can reversible bind hydrogen ions is called a buffer. Hydrogen ions are buffered by extracellular and intracellular buffers.

Homeostatic controls

Hydrogen-ion gain and loss must be balanced to maintain a relatively stable concentration.

Glomerular filtration

Glomerular filtration is the bulk flow of an essentially protein-free plasma from renal globerular capillaries into Bowman's capsule. This is the first step in the formation of urine.

Glomerular filtration is caused by Starling forces.

Tubular reabsorption

This is one of the second steps in the formation of urine.

As the filtrate moves through the renal tubules certain substances are reabsorbed. Reabsorption rates are high for nutrients, water, and ions but lower for waste products.

Reabsorption is caused by Starling forces, diffusion, and active transport.

Sodium reabsorption is an active process dependent upon sodium potassium pumps. Sodium reabsorption creates an osmotic difference across the tubule which drives water reabsorption through water channels. Water reabsorption also occurs passively by osmosis.

Substances to which the tubular epithelium is permeable are reabsorbed by diffusion because water reabsorption creates a concentration difference between the tubule and the interstitial fluid.

Bicarbonate reabsorption is an active process but does not depend on "bicarbonate pumps". It depends instead on the secretion of hydrogen ions from the tubular epithelial cells. In these cells the following reaction occurs:

water + carbon dioxide → carbonic acid → hydrogen ion + bicarbonate

The bicarbonate enters the interstitial fluid. The hydrogen ions are actively transported into the lumen. These hydrogens ions then combine with filtered bicarbonate and generate carbon dioxide and water. The overall result is that the bicarbonate is lost from the tubular lumen and gained in the interstitial fluid. This is a kind of indirect reabsorption.

Some key regulatory hormones for reabsorption:

• Aldosterone stimulates active NaCl reabsorption and indirectly stimulates passive water reabsorption
• Antidiuretic hormone stimulates passive water reabsorption by inserting more aquaporins in both membranes.

Tubular secretion

Tubular secretion is the transfer of materials from peritubular capillaries to renal tubule lumen. This is one of the second steps in the formation of urine.

Tubular secretion is caused mainly by active transport.

Usually only a few substances are secreted. These substances are present in great excess, or are natural poisens.

Secretion of bicarbonate into the blood plasma is achieved by tubular excretion of hydronium. It is also achieved by renal metabolism of glutamine.

Measurement of renal function

See also renal function

A simple means of estimating renal function is to measure blood urea nitrogen, creatinine and basic electrolytes (sodium and potassium). As the kidney is the most important organ in controlling these values, any derangement in these values would suggest renal impairment.

A more formal test of renal function would be to measure the glomerular filtration rate; usually a creatinine clearance test is performed.

Para-amino-hippuric acid is a renal analysis tool.