In the superior and posterior part of our abdomen, we find the two most important organs of the urinary apparatus. They are the kidneys, settled one on each side of the spine. They work everyday as real harmful waste collectors for the body. At a glance, the kidneys are small viscera, no more than 12 centimeters long and three centimeters in girth. Their appearance is similar to that of a bean, with a weight that fluctuates between 150 and 300 grams. But, despite their size, they are the only ones capable of performing a vital function: filtering blood and retaining the chemical residues generated by cellular work. Their mission is so important that, if a disorder seriously affects them, life will surely be at risk.
In their superior part, leaning on them, we find some small glands that make up a part of the endocrine system. These are the suprarenal glands, whose function is independent to the kidneys and are focused on the hormonal production of corticoids, adrenaline and noradrenaline. The kidneys are covered by a layer of conjunctive tissue and another layer of fatty tissue.
The first of them receives the name of renal capsule and is found strongly adhered to the surface of the kidney. The other, known as renal fascia, or Gerota’s fascia, not only contains the renal organ, but also the suprarenal gland and perirenal fat.In their inner edge we find the hilum, which is the entry and exit point of both the renal arteries and veins and of the excretory ducts, called ureters.
Renal circulation
The kidneys have an extraordinary network of blood vessels that ease its purifying job. As a matter of fact, we only consider the blood that circulates and is filtered by them to be equivalent to approximately 25% of the supply that leaves the heart (some 72 liters an hour).
Blood enters the kidney through its concave part (renal hilum) through the renal artery (coming from the descending aorta). This important duct branches into the renal cortex through the interlobar branches, which, upon reaching the renal pyramid (triangular shaped structure lodged in the renal medulla) also split into smaller vessels called arcuate or arciform arteries.
These last ones counter the base of the pyramids, becoming interlobular arteries. Each one of them will give place to the afferent arterioles, of short length and which are capillarized, forming the renal glomerulus (a real tuft of blood vessels).The efferent arteriole of the glomerulus forms the peritubular plexus, which surrounds the uriniferous tubes like a mesh of tissue. The return trip of the blood begins here, through the venous interlobular territory, which will give way to the arciform and interlobar veins, and finally, the renal vein. This important duct discharges into the inferior vena cava.
The nephron
Each kidney is made up of over a million small filtration systems, formed by an intricate network of conduits (ducts) and blood vessels: the nephron. This way, inside the renal pyramids, the nephron extracts and sends back a series of substances that travel through the bloodstream. The waste that will be eliminated through urine is selected here, and the substances that are necessary for the body will be reabsorbed, like salt, glucose and water. Because of this, its tasks can be defined by three concepts: filtration, secretion and re-absorption.
The nephron is made up of two main parts: the renal corpuscle and renal tubules (proximal convoluted tubule, nephron loop and distal convoluted tubule).Although they are located all over the kidney, most of them (nearly 80%) are found in the renal cortex, receiving the name of cortical nephrons. The remaining 20% are located near the renal medulla and are called juxtamedullary nephrons.
Renal corpuscle
In the superior part of the nephron we find a sphere-shaped structure (expansion of the renal tube’s wall) called renal or Malpighian corpuscle.Each corpuscle is formed by the glomerular or Bowman’s corpuscle, which contains the renal glomerulus. The renal capsule is the structure that wraps and contains the renal glomerulus. It has two layers: a parietal one and a visceral one. The first of them is made up by the flat epithelium and conjunctive tissue, while the second one is comprised of epithelial cells called podocytes.
The renal glomerulus originates from an afferent arteriole in charge of delivering the continuous blood supply. It enters the capsule and splits into a series of capillaries that finally discharge into another blood vessel (the efferent arteriole), which leaves the capsule in order to continue its way through the uriniferous ducts.A station with its entrances and exits perfectly pointed out, where one of the first and most important selections of substances dissolved in the blood is performed.
Sphere-like shape (expansion of the renal tube’s wall) called renal or Malpighian corpuscle. Each corpuscle is formed by the glomerular or Bowman’s capsule, which contains the renal glomerulus. The renal capsule is the structure that wraps and contains the renal glomerulus. It has two layers: a parietal one and a visceral one. The first of them is made up by the flat epithelium and conjunctive tissue, while the second one is comprised of epithelial cells called podocytes.
