HUMAN ORGAN EVOLUTION-KIDNEY

Human Organ Evolution-Kidney

ABSTRACT

The evolution of the kidney has had a major role in the emigration of vertebrates from the land onto dry land. The human kidney has conserved to a remarkable extent many of the molecular and functional elements of primordial apocrine kidneys that regulate fluid balance and eliminate potentially toxic endogenous and xenobiotic molecules in the urine entirely by transepithelial secretion. However, these occult secretory processes in the proximal tubules and collecting ducts of human kidneys have remained underappreciated in the last half of the twentieth century as investigators focused, to a large extent, on the mechanisms of glomerular filtration and tubule sodium chloride and fluid reabsorption. On the basis of evidence reviewed in this paper, we propose that transepithelial salt and fluid secretion mechanisms enable human renal tubules to finely regulate extracellular fluid volume and composition day to day and maintain urine formation during the cessation of glomerular filtration.

Human Organ Evolution-Kidney

Introduction

The Liver, Pancreas, Salivary and lacrimal glands secrete relatively large amounts of Na+, Cl-, K+, and HCO coupled with the isosmotic movement of land (Sonnenberg, 1976). Sweat glands exhibit similar properties and are counted among the organs of secretion. With the exception of Heidenhain, Marshall, Fine, and a few other heretics who have observed net fluid secretion in isolated tubule segments in vitro, the human kidney has not been counted among the organs that produce a land secretion.

The urine that emerges from human kidneys is widely believed to represent fluid that is left over from incomplete tubular reabsorption of glomerular filtrate. In the renal field, “secretion” is a term that is usually reserved to describe the deposition into urine of organic anions and cations, largely by proximal tubules, and of potassium, ammonium, and protons by distal tubules and collecting ducts (Sonnenberg & Chong, 1980). However, recent evidence demonstrating that isolated nonperfused outer and inner medullary collecting ducts (OMCD and IMCD, respectively) dissected from rat kidneys secrete fluid has reopened the possibility that segmental salt and fluid secretion by human renal tubules may contribute to the formation of the final urine. In light of this new information, it is not unreasonable to suppose that the secretory transport of organic and inorganic solutes into the proximal tubules, coupled with the downstream secretion of salt and fluid into the collecting ducts, contributes to the formation and the modification of urine by human kidneys. In this brief review, we reexamine the evolutionary basis of renal tubule secretory mechanisms that couple solute and fluid transport into the urine formed by human kidneys and how this process may participate in normal and pathological states (Sonnenberg & Chong, 1980).

Evidence of Renal Tubule Solute and Fluid Secretion

Protovertebrates

In his brilliant treatise, From Fish to Philosopher, Homer Smith considered how the kidney may have evolved in the land from early excretory structures. The leading electrolyte constituents in land are (in meq/l) chloride, sodium, magnesium, and calcium (Sullivan & Grantham, 1982). In this context, the secretory transport of solutes, including NaCl and organic molecules, coupled with the ...