The Gross Structure of the Kidney

The kidneys are bean-shaped organs lying against the dorsal surface of the abdominal cavity. They are surrounded by a tough capsule of connective tissue and receive oxygenated blood from the renal arteries running from the dorsal aorta. The renal veins return the blood to the inferior vena cava. The outer layer is called the cortex and this contains the filtration devices, referred to as the Malpighian bodies. These are made up of an outer double walled Bowman’s capsule surrounding a coiled capillary known as the glomerulus. These filtration devices connect to the nephrons which carry the filtrate and process it into urine. The urine enters a space called the pelvis through openings of the nephrons on the pyramids.

The Nephron:

The nephron is the basic functional component of the kidney. There are many thousands of nephrons in each kidney. The main areas of the nephron are shown on the diagram opposite. The blood capillaries which surround the nephrons are not shown on the diagram. The arteriole leaving the glomerulus divides into a network of peritubular capillaries which surround the proximal and distal convoluted tubules. These receive materials by active transport from the convoluted tubule wall cells. Other capillaries are formed which loop down into the medulla following the line of the loop of Henle, these are the vasa recta vessels. The shape of these capillaries is important in maintaining the high concentration of salt solution found in the medulla of the kidney and as they pass the collecting ducts they will carry reabsorbed water back into the general circulation. The basic functions of each of the sections of the nephron are described in the following paragraphs.

ULTRA-FILTRATION

The flow of blood out of the glomerulus is restricted by a narrowing of the efferent arteriole and this increases the hydrostatic pressure of the blood within the glomerulus. The epithelial cells of the glomerulus and the podocytes of the inner Bowman's capsule wall create a physical filtering device allowing small molecules to leave the plasma. Molecules of molecular weight greater than 68,000 are unable to leave the plasma, and cells are also prevented from leaving the glomerulus. The volume of filtrate leaving the glomerulus is determined by the difference between the

hydrostatic pressure in glomerulus which pushes fluid out and the inward force resulting from the osmotic pressure of plasma and the back-pressure of the filtrate.

SELECTIVE REABSORPTION:

The filtrate passes into the proximal convoluted tubule, the cells of which have a brush border and many mitochondria. Glucose, amino acids and vitamins diffuse into these cells and are then actively transported out of the cell into the peritubular fluids from which they diffuse into the peritubular capillaries. The movement of these materials results in an osmotic dragging out of water from within the convoluted tubule. Polypeptides are absorbed from the filtrate by pinocytosis and lysed into amino acids by the cells of this region. Most of the water reabsorbed from the filtrate will be taken back in this region of the nephron.

MAINTAINING A HIGH MEDULLARY OSMOTIC PRESSURE

The filtrate passes into the Loop of Henle, which acts as a counter-current multiplier creating a high salt concentration in the medulla of the kidney. There are two types of nephron, one type with short loops of Henle which reach only to the outer medulla and with the glomerulus in the outer cortex, and the juxtamedullary nephrons which have longer loops of Henle which reach deep into the medulla. Desert mammals are able to produce urine of much greater hypertonicity than man and have a greater proportion of juxtamedullary (long) nephrons than does man. Under normal conditions of free access to water the short nephrons perform most of the regulatory functions while in conditions of water crisis the juxtamedullary nephrons take on an important regulatory function. As filtrate passes down the descending limb of the loop of Henle water flows out into the peritubular fluids. The descending limb is relatively impermeable to sodium ions. As the filtrate moves up the wide portion of the ascending limb a sodium pump mechanism forces ions into the peritubular fluids. This results in sodium ions being trapped in the deep medulla. The high osmotic pressure developed by the action of the loop of Henle allows water to be reabsorbed from the collecting ducts, or ducts of Bellini, as they travel down from the distal convoluted tubules towards the pyramids.

SALT REABSORPTION

The distal convoluted tubule is responsible for the reabsorption and secretion of salts to maintain a mineral ion and acid-base balance. The excretion of salts in the urine can be controlled by the activity of the distal convoluted tubule cells under the influence of the hormone aldosterone coming from the adrenal cortex. The substance renin is released from a group of cells in the afferent glomerular arteriole, known as the juxtaglomerular apparatus, in response to low sodium levels. The renin causes a blood globulin (protein) to become modified into angiotensin which stimulates the release of aldosterone from the adrenal cortex. This then passes in the blood to stimulate the active uptake of sodium salts and secretion of these into the peritubular capillaries by the distal convoluted tubule cells, while allowing potassium ions to escape. Thus restoring the sodium-potassium balance.

OSMOTIC REABSORPTION OF WATER

The reabsorption of water in the ducts of Bellini is controlled by the action of Anti diuretic Hormone secreted from the posterior pituitary. When present in the blood the ADH attaches to the cell membranes of the duct wall cells, activating a mechanism to increase the permeability of the cells to water, probably by causing the uncapping of transmembrane protein pores. This allows water to be withdrawn from the filtrate by osmosis, and also increases the flow of urea from the tubule into the peritubular fluid. The water is taken up by the capillaries of the vasa recta system which are arranged in a counter-current flow pattern in the medulla. The counter-current arrangement prevents escape of sodium from the medulla and thus prevents a lowering of the osmotic pressure in the medulla. This osmotic reabsorption system can regulate the flow of liquid from the nephrons so that the urine produced ranges from isotonic to hypertonic. The release of ADH is the result of nervous stimulation of the pituitary by the hypothalamus in which osmoreceptors are situated. These are constantly sensing the osmotic pressure (now called water potential) of the blood.

ACID-BASE BALANCE

The predominant buffering system in mammal fluids results from the action of plasma proteins of the albumen group, hydrogen carbonate and phosphate ions. The albumens are composed of amino acids which are zwitterions, they have a basic and acidic groups on their molecules and can therefore act as efficient acid and base buffering agents. If the acid-base balance is significantly disturbed then the distal convoluted tubules of the kidney nephrons can secrete either hydrogen or hydrogen carbonate ions, whilst retaining the complementary ion to restore the balance. The secretion of hydrogen carbonate ions together with calcium ions over a long period can give rise to the development of kidney stones as a chalky deposit develops in the pelvis of the kidney.

EXCRETION IN OTHER ANIMALS

In insects the small body volume combined with the impermeable body surface necessitates a reduction in water losses. The flushing function of water in the removal of toxic nitrogenous waste cannot be afforded by these creatures hence they use uric acid as their excretory product. Uric acid has a negligible solubility in water and therefore has a very low toxicity and can be removed from the body in a crystalline form. Reptiles and birds both reproduce using shelled eggs which means that their nitrogenous waste will accumulate in the egg during development. Uric acid is used as the excretory product in these vertebrates during the egg developmental phase and this is retained into adult life.

Fish have a problem with their salt/water balance according to their particular environment and line of evolution. The cartilaginous fish, the sharks, evolved in freshwater and moved into a marine environment later, as did the bony fish. This means that the body fluid concentrations are lower than the surrounding seawater which poses problems for them. The shark types solved the problem by producing urea and becoming highly tolerant of it, they have levels of 2% in the blood, some 65 times the human level. The increased solute content of the blood thus makes them isotonic with the surrounding seawater which reduces their water control problem. The bony fish living in seawater is forced to swallow seawater and absorb it through the gut and then pump sodium chloride out through its gills. The water stress on these fish necessitates their adoption of trimethylamine as the excretory product as this is much less toxic than ammonia as a waste product. Freshwater fish have the problem of being more concentrated than their environment, their hypertonic nature means that they have a constant influx of water by osmosis and a loss of solutes. Their gills pump ions into the blood from the very dilute surrounding water, whilst their kidneys produce huge quantities of extremely dilute hypotonic urine. The constant flushing effect of this water disposal system makes it possible for them to use ammonia as a waste product. It should be remembered that creating a waste product is an energy consuming function with ammonia as the least costly, trimethylamine more costly, urea costlier still and uric acid most costly. The balance between energy cost, the value of water for survival and the evolutionary starting point of the group concerned will determine the final solution adopted.