Comparative Kidney Structure And Function In Vertebrates.

The Kidney is the principal excretory organ in a vertebrate and they are two present. In general, each consists of a mass of coelomoducts opening into a collecting duct. Kidneys are short and posterior in all vertebrates but in Fishes and Salamanders, they extend along most of the body cavity. Indeed, comparatively there are three basic types of kidneys in vertebrates.

1. Pronephros or Head kidney
2. Mesonephros or Mid kidney
3. Metanephros or Hind kidney

The kidneys of lower vertebrates from Cyclostomes to Amphibians and the embryonic kidneys of the higher groups develop segmentally, a pair per body somite (Pronephros, Mesonephros). Some of the tubules have Nephrostomes opening to the coelom, thus somewhat resembling the Nephridia of earthworms.

In Reptiles, Birds, and Mammals adult kidneys (Metanephros) are non segmental and drain wastes only from the blood (see below).

Table 1;Types of kidneys in vertebrates

Generally, from each kidney of whatever type, a common collecting duct, the Ureter, carries the waste posteriorly. In Amphibians, Reptiles and Birds the Cloaca, to which Urinary bladder connects in Amphibians and some Reptiles. Normally, the waste or Urine is always fluid except in Reptiles and Birds, where the semisolid excretions (Uric acid) are voided as a white paste (Guano) with the faeces.

In most mammals the Ureters connect directly to the bladder, while a median duct, the Urethra discharges to the exterior, passing through the Penis in the males or Vulva in females. Hence, the interrelated excretory and reproductive systems of vertebrates are commonly termed the Urogenital system.

The kidney in health elaborates a urine appropriate in volume and composition to the needs of the body at all times. Physiologic anatomy of the kidney shows that each human kidney for instance, consists of an inner Medulla and an outer Cortex, the latter containing about a million minute excretory units, or Nephrons. A Nephron is made up of:

• Malpighian or Renal Corpuscles which compose of a globular double-walled Bowman capsule around a clump of arterioles, or Glomerulus
• Tubule both convoluted and straight, surrounded by blood capillaries

The corpuscle is about 0.2mm in diameter by 50-60mm long. The 2 million nephrons of a man, end to end, would extend for nearly 50 miles.

All the tubules discharge into a central cavity (Pelvis) of the kidney that connects to the Ureter.

Kidney Functions in Vertebrates

The kidney performs two main functions in vertebrates.

• Homeostasis
• Excretion of end products of bodily metabolism.

The prime function of the kidney in vertebrates is to maintain homeostasis, i.e. the constancy of the internal environment. The kidney is the main executive organ of the body in this respect, acting in some direction under the influence of the endocrine system, in others through complex physical and chemical regulatory mechanisms.

These various regulatory functions include volume control of body water, osmotic control of the extra cellular fluid, acid-base balance, body concentration of individual electrolytes, arterial blood pressure and possibly erythropoiesis

The excretion of waste products such as urea, although an essential role of the kidney, is of minor biological importance compared with its contribution to homeostasis.

However, according to the generally accepted theory of kidney function (by Cushy), Protein-free fluid passes from the arterioles in the Glomerulus through the Bowman capsule by ultra-filtration. This process takes place because of the pressure of the blood; the fluid in the capsule has the same percentage composition as blood Plasma minus the colloids to which the membranes are impermeable.

The second step is differential reabsorption by cells of the tubules-mostly in the proximal convoluted tubule but also in the loop of Henley and the distal convoluted tubule. Reabsorption involves work with the use of oxygen and the expenditure of energy. Materials such as Glucose that are present in the blood and in the Glomerular filtrate but not in normal Urine illustrate this. Such substances are reabsorbed completely and in a direction opposite from that expected by simple diffusion.

The most remarkable feature of reabsorption by the tubules is its selectivity. For example, about 1,140grams of salt (NaCl) passes from the Glomerulus’s into the tubules each day ,but normally only 4 to 8 grams of this leaves the body in the Urine. The rest is reabsorbed into the blood stream. Urea, on the other hand, is constantly being removed; it is about half (30g daily) of all solids in the urine, where it is in much higher concentration than in the blood Plasma.(See table of comparative action of human kidney below);

Table 2; Concentrating action of the Human kidney.

Unused amino acids derived from proteins are not stored. Instead they are deaminated-the Amino group (NH2) is removed by an enzyme (Deaminase) and converted to Ammonia, a rather toxic material. In the Liver of mammals the Ornithine cycle combines Ammonia with Carbon dioxide to yield the less toxic Urea which is excreted by the kidney. Three Amino acids in the Liver serve in a repeating cycle, aided by the enzyme Arginase. As shown below:

Arginase: CO2 NH3

↓

Ornithine

↓ NH3

↓

Citrulline

↓ H2O

↓

Argenine

↓

Urea

The overall reaction is thus:

2NH3 + CO2 + H2O → CO (NH2)2 [= Urea) + 2H2O.

The residue from deaminated amino acids is converted to fat or Carbohydrate for further use. Materials like Glucose, Sodium and Calcium are called “high-threshold substances” because they are reabsorbed in considerable quantities, little or none being left to pass out in normal Urine; those reabsorbed in small quantities (Urea, Uric acid, etc.) are termed “low-threshold substances.” In addition to Glomerular filtration and reabsorbtion, there is some direct tubular excretion of waste products difficult to metabolize in the body.

The capacity of the human kidneys is truly remarkable. They comprise scarcely 1/200 (0.5%) of the total body weight; yet receive 20% of the blood volume pumped by the Heart. A significant amount of blood (about 1,700 quarts) flows through the kidneys each day, but only about 180 quarts of fluid is being filtered, and of this 178quarts is reabsorbed, thus only 1 or 2 quarts passes off as Urine. In other words, an amount equal to the entire volume of blood is filtered by the tubules about 30 to 36 times per day.

Urinary output is controlled in several ways. Filtration, the diffusion of fluids from the blood Plasma through the Glomerular membranes to the Capsular space, is directly influenced by blood pressure. This, in turn, is affected by Epinephrine from the Adrenal medulla which constricts the blood vessel of the Glomerulus’s.

Another hormone, Cortin, from the Adrenal Cortex, affects reabsorption of Sodium and Chloride in the kidney tubules and the elimination of Potassium, but the reabsorption of water is the most basic process of kidney function as previously stated. About 80% of water in the tubular fluid is probably recaptured by direct diffusion into the Capillaries and then to the venous system as a result of attraction by Colloids in the blood. Additional water is taken up in the Loop of Henley and Distal part of the tubule, a process controlled by the Pituitary gland.

Indeed, normal kidney function is essential to health, and any irregularity or disease in the kidney is serious. Certain salts, especially, Oxalates may crystallize to form Kidney stone in the Pelvis of the kidney and sometimes requires removal by surgery. Thus, the content of the Urine may be altered by other abnormal conditions.

Urinalysis, therefore may give useful clues to the general state of bodily function healthy or otherwise. Abnormal constituents the Urine may be Albumin, excess glucose, Acetone bodies, Cell casts, Pus, Blood, or Bile pigments. It is a remarkable fact that removal of one kidney, and even part of the second kidney, does not entirely hinder the total excretory process in man. An artificial kidney has been devised and is now available in many hospitals for cases of acute kidney failure or blood poisoning. Blood is diverted from an artery through a cellophane tubing set in a circulating bath fluid and then back to a Vein. The cellophane has pores of about the same size as the Glomerular Capillaries so substances will diffuse in or out depending on the concentration of each in the bath and in the blood. By adjusting the concentration of substances in the bath, it is possible to add or remove elements from the blood as desired.

Summarily, be that as it may, though kidney exist in a unique form, the comparative kidney structures and functions among the vertebrates show that for a healthy and sound living, an effective and efficient kidneys are unavoidable since it is the chief Osmoregulator for the body. Hence, in man preventing some of the organisms that would impair their functions as much as possible should protect our kidneys.