Anatomía Renal - Anatomía & Fisiología

Introducción

Los riñones son órganos pares (derecho e izquierdo) que residen en el abdomen dorsal. Su función es filtrar la sangre a través de los glomérulos para formar el filtrado. Dicho filtrado es, a continuación, reabsorbido a lo largo de las nefronas, hasta que quedan los componentes innecesarios para el organismo. Algunos componentes, que se normalmente se absorben en su totalidad a nivel renal, se pueden encontrar en el organismo en exceso, en estas ocasiones los túbulos son capaces de responder, reabsorbiendo menor cantidad de dichos componentes. De este modo el riñón juega un papel importante en la homeostasis del organismo. También son capaces de interpretar el grado de hidratación del animal, excretando mayor o menos cantidad de agua según las necesidades.

Los riñones reciben el 25% del gasto cardíaco. De éste filtran el 20% del plasma, formando el filtrado, del cual se reabsorbe todo excepto el 1%. Esto supone que todo el volumen sanguíneo es filtrado y reabsorbido cada 30 minutos. En este caso, los riñones se encargan de mantener el volumen y la composición del plasma, regulan la cantidad de agua, así como los niveles de iones y pH, retienen nutrientes y excretan desechos, toxinas y excesos de electrolitos. Realizan dichas funciones vía filtración glomerular, reabsorción de solutos, secreción tubular, balance hídrico y regulación ácido-base.

A labelled saggital section of a lamb kidney(Courtesy of Donal McNally - University of Nottingham)

Anatomía Común

Histology section of a normal kidney (© RVC 2008)

Histology section of a normal renal cortex (© RVC 2008)

A prosection of the abdomen of a male dog (© UoN 2008)

A prosection of the abdomen of a female dog(© UoN 2008)

• Los riñones son parte del tracto urinario, donde la sangre es filtrada produciendo la orina.
• Son pares y se encuentran en posición retroperitoneal.
• Situados en la región caudo-dorsal del abdomen.
• Descansan en una hendidura de la fascia sublumbar. Normalmente con abundante grasa que recubre y protege a los riñones de la presión de otros órganos.
• El riñón derecho es más craneal que el izquierdo, excepto en el cerdo.
• En las especies en las que el riñón derecho es más craneal, éste se encuentra en la fosa renal del lóbulo caudado del hígado.
• Asimismo el riñón izquierdo es más móvil.
• En los primeros estadios del desarrollo todos los animales tienen un riñón multi-lobulado, pero conforme se van desarrollando van adquiriendo las características típicas de la especie.

Componentes Básicos del Riñón

Cápsula Fibrosa Externa

Una gruesa cápsula recubre el parénquima previniendo la expansión del riñón. Se separa fácilmente en un riñón sano, pero se adhiere al parénquima en caso de patologías.

Corteza Renal

• 2 Partes
• Zona externa
• Zona interna (yuxtaglomerular)
• Contiene las siguientes partes de la nefrona
  • Corpúsculo renal
  • Túbulo contorneado proximal
  • Túbulo contorneado distal
  • Rayos Medulares

Médula Renal

• Contiene las pirámides medulares
• La parte más próxima a la corteza es la base de la pirámide, que se estrecha para formar la parte interna -papila renal
• La médula se puede dividir en dos partes, la externa y la interna
• Encontraremos diferentes partes de la nefrona en dichas áreas.

Médula Externa

Se puede dividir, a su vez, en franja externa e interna.

Franja Externa

Esta sección se encuentra adyacente a la corteza y contiene las siguientes partes de la nefrona:

• Túbulos rectos proximales
• Túbulos rectos distales
• Porción delgada descendiente y porción gruesa ascendiente del Asa de Henle
• Túbulos colectores
• Vasa recta

Franja Interna

Adyacente a la franja externa, esta sección contiene:

• Porción delgada ascendente y porción gruesa descendiente del Asa de Henle
• Túbulos colectores

Diferencias

La franja externa contiene los túbulos rectos proximales y la franja interna no. Asimismo la franja interna contiene las porciones delgadas ascendentes y la externa no. La diferencia pone de manifiesto la demarcación anatómica entre ambas.

Médula Interna

• La medulla interna solo contiene las siguientes partes:
  • Asa de Henle
  • Túbulos colectores
  • Vasa recta

Pelvis Renal

• El seno renal se localiza en hendidura que deja la cara medial del riñón
• En él se encuentra la pelvis renal
• A ésta llegan todos los túbulos renales
• A continuación la pelvis drena la orina a los uréteres
• La pelvis renal está cubierta con epitelio de transición
• Varía entre especies
• No se encuentra en bóvidos
• Contiene glándulas mucosas en équidos

Divisiones del Riñón

• The kidney is divided into renal lobes from a structural point of view.
• These can then be divided into a renal pyramid and the piece of cortex above it
• Renal Pyramids are important descriptor of part of the kidney.
  • These structures are visible grossly and are the region of the medulla which appears triangular in cross section. They are actually cone shaped and the pointed part which faces towards the renal pelvis is termed the apex and the part against the cortex the base.
• They can also be subdivided into renal lobules comprising of nephrons, a medullary ray and a collecting duct.

• In some species the kidneys are described as multipyramidal this means they have multiple lobes and pyramids. Others are described as unipyramidal meaning they only have one lobe and pyramid.

Innervation

• Sympathetic and parasympathetic fibres from solar plexus
• Travel with renal arteries
• Sympathetic fibres synapse in coeliac ganglion and cranial mesenteric ganglion

Lymphatic Drainage

Renal lymph nodes

Anatomical Species Differences

A picture showing a typical lamb and pig kidney. (Courtesy of Donal McNally - University of Nottingham

The various species have major and rather striking differences in the structure of their kidneys.

Canine, Feline, and Ovine

These species all have similar renal anatomy. Their kidneys are relatively short and thick and they are the traditional kidney bean shape. They have a smooth outer surface and have a single renal papilla. The renal pelvis is large and irregular with recesses which are finger like processes.

The kidney of the feline is relatively bigger than the other species and is quite distinctive because the sub-capsular veins which run towards the hilum are visible. The size of the kidney can be estimated by palpation or via imaging. On a ventrodorsal radiograph, normal kidney size is usually between two to three times the length of second lumbar vertebra, 3 - 4.5 centimetres.

Bovine

The kidneys of the bovine do not lose their foetal lobulation. In fact the surface of each kidney is divided into approximately 12 lobules. The right kidney is flattened and ellipsoidal where as the left kidney is thicker at the caudal end than the cranial. Each kidney is surrounded by the capsula adiposa; a layer of fat. Despite what it’s externally lobulated appearance may suggest the cortex of the bovine kidney is continuous and the kidney is of multipyramidal type. The bovine kidney has no renal pelvis but rather the Ureters enters the kidney and divide into a cranial and caudal branch. These branches then subdivide and the papilla at the apex of the pyramids open and drain into these.

The right ureter leaves the kidney and passes along the roof of the abdomen to the pelvis in a fairly standard pattern. The left ureter however moves across the dorsal surface of its kidney to return to the midline and follow a course as if the kidney was located on the left. (both kidneys in the bovine are located on the right see the anatomical landmarks section for further details)

Porcine

The kidneys are dorsoventrally flattened. The renal pelvis opens into quite a large space of two major calicyes from which bud about 10 minor calyces. These attach to one renal papillae each. The kidneys have a smooth surface.

Equine

The equine kidneys not only have very different shapes compared to the rest of the domestic species but they also each have a different shape. The right kidney is described as heart shaped whilst the left is described as being pyramidal. Each organ weighs approximately 700g and both are dorsoventrally flattened. The kidneys are basically unipyramidal and the only demarcation between what were the multiple pyramids of the foetus are the interlobar arteries. This is not always the case in the foal where it is common to be able to identify lobes and the external surface is not always smooth. The horse has a single renal papilla like the dog and its renal pelvis is large and irregular with 2 recesses (finger like processes). The cells of its pelvis secret mucin giving the urine its cloudy appearance.

Anatomical Landmarks

Each species has a slightly different orientation of the kidneys within the abdomen.

Carnivores

The kidneys in these species are very mobile. Especially the left one and especially in the feline. These anatomical landmarks describe the most common locations. The right kidney is more cranial than the left and tends to lie beneath L1 - L3 where as the left tends to lie under L2-L4. The reason the right kidney is less mobile than the left is that it tends to be almost entirely enclosed within the renal fossa of the caudate lobe of the liver. The kidneys are enclosed within a layer of fat and this increases with the obesity of the animal.

Medially to the right kidney you will find the caudal vena cava and the right adrenal gland can be found dorsolateral to this structure but still medially to the kidney. Ventrally can be found the Descending duodenum with the right pancreatic limb more towards the ventromedial aspect. In females the right ovary can be found caudoventrally.

The Cranial pole of the left kidney contacts the greater curvature of stomach it also contacts the spleen on its dorsomedial aspect as well as potentially contacting the left limb of the pancreas. The left adrenal gland can be found at the medial aspect of the cranial pole. The Caudal pole of the left kidney contacts the Small intestine and the Descending colon. At its caudoventral aspect in females is found the left Ovary.

Ruminants

Due to the rumen taking up most of the left side of the abdomen it is normal in the bovine to find both of the kidneys on the right side. In the ovine the kidneys are surrounded by very thick masses of fat which reduce the impact of the rumen on their location. The kidneys in the living animal vary their position substantially with respiration and with the pressure of the other organs.

The right kidney has a retroperitoneal attachment to the "sublumbar" musculature. Cranially it touches the liver. In the dead specimen the right kidney occupies the region between the last rib and somewhere between the 2nd and 3rd lumbar vertebrae’s transverse process. The right kidney usually contacts pancreas, duodenum, adrenal glands and colon as well as the afore mentioned liver.

The left kidney in the bovine is to be found caudoventrally to the right one usually in the region between the 2nd and 4th lumbar vertebrae

Porcine

The left and right kidneys are more or less aligned in the pig unlike other species. The kidneys of the pig are embedded in lots of fat and lie against the psoas muscle. They span the distance between the last rib cranially and the 4th lumber vertebrae caudally. The right kidney does not touch the liver unlike other species and is related ventrally to the descending duodenum and jejunum. The left kidney is ventrally related to the ascending colon, the base of the caecum and the pancreas.

Equine

The kidneys of the horse are both enclosed in a fat capsule. Dorsally they rest against the psoas muscle and against the diaphragm.

The right kidney is to be found ventrally to and between the last 2 ribs and first lumbar transverse process. Cranially it touches the liver and caudally it is attached to the pancreas and the base of caecum. The duodenum winds around its lateral and then ventral surfaces. Medially is the caudal vena cava and adrenal gland.

The left kidney is between the last rib and 3rd transverse process. Its ventral surface is almost completely covered by the peritoneum and contacts the small intestine and and small colon. The spleen contacts it cranioventrally. Medially is the left adrenal gland and aorta.

Renal Blood Supply

• Supplied by renal arteries
• Arise from aorta
• 3-4mm diameter - dog
• Often divide into dorsal and ventral branches before entering kidney
• Common to find double the normal number - dog

Left Renal Vessels

• Left renal artery
  • Originates 2cm caudal to the right renal artery
  • ~3-4cm long - dog

• Left renal vein
  • Immediately ventral to artery
  • ~3-4cm long - dog

Right Renal Vessels

• Right Renal Artery
  • 2cm cranial to left renal artery
  • 4 cm caudal to cranial mesenteric artery
  • Passes dorsal over the caudal vena cava
  • ~4-5cm long - dog

• Right Renal Vein
  • Ventral to artery
  • ~4-5cm long - dog

Internal Vascularisation

Once the renal artery enters the kidney is divides into the interlobar arteries. These pass through the gaps between the renal pyramids as the reach the junctions between the cortex and medullar they branch into the arcuate arteries which move over the base of the pyramids. From these come the interlobular arteries which supply the individual lobules of the cortex. These arteries then branch many times to supply individual glomeruli. The capillaries of the glomerulus then rejoin to form one vessel which then forms the peritubular capillaries of that nephron. The interlobular arteries are examples of end arterioles and there are few anastomoses. Obstruction of one of these arterioles causes ischaemic damage in the kidneys. This is also potentially the case with the interlobar arteries.