Excretory Products and Elimination

Prostaglandins are powerful vasodilators and there is a normal tonic level of vasodilation in the renal blood vessels, which is caused by the prostaglandins.

It is the metanephros which goes on to become the adult kidney.

For nephrons which are in the outer cortex and do not have vasa recta or long loops of Henle, the efferent arterioles supply the peritubular capillaries. Flow occurs when the pressure at one end of a tube is made greater than that at the other end.

Active transport can move charged or uncharged molecules across a membrane and so can establish concentration gradients and if the molecules are charged, electrical gradients. However, if the charges are of opposite sign, then one side of the membrane will become more positive and the other side will become more negative.

The ureteric bud forms the collecting system and if it splits before it enters the kidney, the result, which is not uncommon, is that the kidney will have two ureters.

Erythropoietin is a glycoprotein hormone that controls erythropoiesis, or red blood cell production.

The osmolality of filtered fluid entering the proximal tubule from the glomerulus is essentially that of plasma from which it is produced. Reabsorption in the proximal tubule is iso-osmotic, which means that the osmolality of the tubular fluid does not change because solutes and water are removed together in the same ratio at which they are present in the tubules. For this reason, the concentration of urine (i.e. its osmolality) in the proximal tubule is not altered.

As erythropoietin is a protein, it cannot be given orally as it would be digested in the gut. It is usually given subcutaneously or less commonly, intravenously.

The main hormone regulating water reabsorption is anti-diuretic hormone (ADH or vasopressin). There are indeed osmoreceptors in the hypothalamus, which detect an increase in plasma osmolality. In response to input from these receptors and other stimuli, cells in the posterior pituitary gland release ADH.

Potassium is freely filtered and then most of the filtered potassium is reabsorbed. Potassium excretion is then controlled by the secretion of potassium by the principal cells in the collecting ducts.

If drugs are given which inhibit sodium reabsorption, then there is more sodium in the tubules and this means that more water stays in the tubules with it because of the osmotic effect of the sodium. Such drugs are known as diuretics because they increase the amount of water excretion. Diuretics such as furosemide (frusemide) inhibit sodium reabsorption in the loop of henle, drugs such as the thiazides oppose sodium reabsorption in the distal tubule and drugs such as amiloride oppose sodium reabsorption in the collecting ducts.

Overall, the body is a net producer of acid and a major function of the kidney is to excrete this acid load. A large change in body pH cannot be tolerated as it will have adverse effects on cellular function.

The common causes of urinary tract infection are Escherichia coli, Klebsiella and Proteus species. Staphylococci are not common causes of urinary tract infection.

There are buffers in the urine, including sodium phosphate. In addition, hydrogen ions can be excreted in the form of ammonium ions (NH4⁺) in the urine. The buffers are necessary because urine cannot be made very acidic because below a certain pH, the hydrogen ion gradient across the tubule wall cannot be maintained.

Sodium phosphate can exist as Na₂HPO₄ or NaH₂PO₄ and the two form a buffer pair. The form NaHPO₄ is a weak base which can accept a hydrogen ion to become NaH₂PO₄. Similarly NaH₂PO₄ is a weak acid which can donate a hydrogen ion to become Na₂HPO₄.