08 Posterior Abdominal Wall and Retroperitoneal Organs.pdf

Gastrointestinal System Posterior Abdominal Wall and Retroperitoneal Organs The official title of this lecture is the Posterior Abdominal Wall and Retroperitoneal Organs, but I plan to split this into two separate mini- lectures. The most significant organs on th...

Gastrointestinal System Posterior Abdominal Wall and Retroperitoneal Organs The official title of this lecture is the Posterior Abdominal Wall and Retroperitoneal Organs, but I plan to split this into two separate mini- lectures. The most significant organs on the posterior abdominal wall are the kidneys. We have considered the other organs already.  Gastrointestinal System Posterior Abdominal Wall In this one we will deal with the posterior abdominal wall.  Learning Outcomes After this lecture you should be able to: ▪ List the retroperitoneal organs ▪ Give an account of the muscles of the posterior abdominal wall and know their actions and nerve supply ▪ Know the symptoms of a psoas abscess ▪ Give the root values of the lumbar plexus and know the course and distribution of the ilioinguinal, iliohypogastric and genitofemoral nerves ▪ Name the branches of the abdominal aorta and list the vertebral levels at which they arise ▪ Name the tributaries of the inferior vena cava ▪ Give an account of the lymphatic drainage of the abdominal organs These are the learning outcomes for this part of the lecture.  Posterior Abdominal Wall (this is not included in the lecture presentation) Ligaments Parietal peritoneum Mesenteries Desc colon Asc colon Retro-peritoneal fat The posterior abdominal wall is formed by the structures which lie behind the peritoneal cavity. Here shaded in green is the parietal peritoneum lining the posterior abdominal wall. Behind this lies a layer of fat, and this is known as retro-peritoneal (or extra-peritoneal) fat. From the posterior abdominal wall, the peritoneum is folded forwards to form ligaments and mesenteries, and these are represented by the lines which are close together. These lines represent the cut edges of the peritoneal folds. For example, the ligaments labelled here from superior to inferior are the gastrophrenic, gastrohepatic and the splenorenal ligaments. The mesenteries labelled from superior to inferior are the transverse mesocolon, THE mesentery and the sigmoid mesocolon. Where the lines are separated, this is where the peritoneum wraps around the liver and the ascending and descending colon.  Retroperitoneal Organs Suprarenal glands Kidneys Pancreas Descending Duodenum colon Ascending Rectum colon The posterior abdominal wall contains the retroperitoneal organs. These include the duodenum and pancreas, although the 1st part of the duodenum is intraperitoneal. The ascending and descending colon are retroperitoneal and sometimes the caecum is as well, although it normally has a complete wrapping of visceral peritoneum hence is intraperitoneal. It usually lacks a mesentery however. The rectum is also retroperitoneal. Lastly, the kidneys and suprarenal (or adrenal) glands are retroperitoneal. As I’ve mentioned, we’ll cover those organs in a separate section. This region also contains muscles, blood vessels, nerves, and lymphatic vessels, so we’ll explore all of those in this section of the lecture.  Muscles of the Posterior Abdominal Wall Diaphragm Medial and lateral arcuate ligaments Quadratus lumborum Lumbar fascia Psoas major (and minor) Iliacus The space between the lumbar fascia and the vertebral column is occupied by the muscles of the posterior abdominal wall. The diaphragm forms the roof of the abdominal cavity, and also forms the upper part of the posterior abdominal wall. The diaphragm arches over the posterior abdominal wall muscles and forms the medial and lateral arcuate ligaments. The most lateral of the posterior abdominal wall muscles is the quadratus lumborum. This arises inferiorly from the posterior 1/3rd of the iliac crest and the lumbar transverse processes. It inserts superiorly onto the 12th rib. It is important in the depression of the 12th rib during breathing, which maximises the contraction of the diaphragm. It is innervated by the anterior primary rami of T12-L3. The medial muscle is the psoas major which flexes the thigh on the trunk, or trunk on thigh. It arises from the lumbar transverse processes and lumbar vertebral bodies. It attaches with the iliacus muscle (located on the iliac fossa) onto the lesser trochanter of the femur. Psoas major is supplied by the anterior primary rami of L1-L3. Occasionally there is a psoas minor present, but the details of this muscle are not important.  Endoabdominal and Psoas Fascia The endo-abdominal fascia lies anterior to these muscles, but of particular note is the psoas fascia which surrounds the psoas major muscle. The psoas muscle with its fascial sheath lies in close approximation to many of the abdominal viscera. Should any of these organs become inflamed or infected, there may be irritation of the psoas muscle. There are many nerves (of the lumbar plexus) that pass around and through the muscle, and enlargement of it can cause nerve compression. This is often a problem in haemophiliacs for example, where haematomas (leaked blood clots) may cause nerve damage. Infections on the posterior abdominal wall may track along the psoas fascia. This may be from tuberculosis of lumbar vertebrae. The pus from such infections collects inside the psoas sheath and this can be seen as a swelling at the top of the thigh, mimicking a femoral hernia. Patients with lumbar tuberculosis (Pott’s disease) may not only develop a swelling at the distal end of the psoas sheath, but if enough pus collects there, also demonstrate a swelling in the lower abdomen. This pus will irritate the psoas muscle and cause it to go into a spasm, producing flexion of the hip as well as giving an excruciating pain.  Psoas Abscess Here we can see a right psoas abscess in this CT scan. The muscle has enlarged and is full of pus. Here the two psoas muscles have been highlighted so you can easily compare one to the other.  Lumbar Plexus The lumbar plexus lies in the substance of psoas major and is formed from the anterior rami of L1-L4, but also usually has a contribution from T12. The branches of the lumbar plexus emerge through or around the psoas major muscle. Its usual branches are the iliohypogastric and ilioinguinal from L1, the genitofemoral from L1/2, the lateral femoral cutaneous from L2 and L3, the femoral and the obturator from L2-L4, and the lumbosacral trunk from L4 and L5. The latter is considered to be part of the sacral plexus. In addition, in about 35% of the population there is an accessory obturator nerve from L3/L4 and there are muscular branches to the adjacent psoas and quadratus lumborum muscles, mostly from L1-L3.  Lumbar Plexus The only branches that you need concern yourself with in relation to the abdomen however are the iliohypogastric, ilioinguinal and genitofemoral nerves. The iliohypogastric is cutaneous to the region of the upper lateral gluteal region and mons pubis (the rounded mass over the pubic symphysis), whilst the ilioinguinal nerve is sensory to the anterior skin of the external genitalia and upper medial thigh. Both nerves have a variable origin but mostly from L1. Both are probably motor to the muscles of the lower part of the anterior abdominal wall, but this also is probably variable. Some sources suggest that the iliohypogastric is purely sensory. There again, other sources suggest that both are purely sensory. The genitofemoral nerve arises from L1 and L2 and this nerve pierces the psoas muscle before dividing into a femoral branch containing the L1 fibres and a genital branch containing the L2 fibres. The femoral branch supplies a small region of skin over the femoral triangle, whilst the genital branch supplies the cremaster muscle in the spermatic cord as well as the dartos muscle in males. In females it supplies the skin of the labia majora.  Major Abdominal Blood Vessels Let’s turn our attention now to the blood and lymphatic vessels lying on the posterior abdominal wall. The abdominal aorta lies mostly in the midline. It enters the abdomen under the median arcuate ligament at the level of T12 and ends at the level of L4 (roughly at the level of the umbilicus) by dividing into the common iliac arteries. The inferior vena cava begins in front of the body of the 5th lumbar vertebra and ascends to the right of the aorta, to pierce the central tendon of the diaphragm.  The Abdominal Aorta As the aorta descends through the diaphragm, it becomes known as the abdominal aorta and gives off both visceral and parietal branches. The visceral branches supply the organs whilst the parietal branches supply the body wall.  The Abdominal Aorta Let’s take a closer look at its branches.  Parietal Branches inf. phrenic (T12) lumbar (4 pairs) common iliac median sacral external iliac internal iliac Parietal branches of the abdominal aorta are the left and right inferior phrenic arteries, which run under the diaphragm, four paired lumbar arteries (left and right) and the median sacral. This latter artery is unpaired. The abdominal aorta terminates at the level of the umbilicus into the right and left common iliac arteries. These in turn divide into an internal iliac that supplies the pelvis and perineum, and an external iliac that supplies the lower limb.  Paired Visceral Branches (middle) suprarenal (L1) renal (L1/L2) gonadal (L2) There are paired visceral branches to the suprarenal glands which arise at the L1 vertebral level. These are the middle suprarenal arteries. There are larger paired renal arteries arising at the L1/L2 intervertebral disc and testicular or ovarian arteries arising at L2. The renal arteries pass to the kidneys. The testicular or ovarian arteries supply the testes in the male and the ovaries in the female, respectively.  Pre-Aortic Branches coeliac trunk (T12) superior mesenteric (L1) inferior mesenteric (L3) Next, we have the unpaired branches. Each embryological section of gut takes its own blood supply. These arise from the anterior surface of the abdominal aorta and are hence termed “pre-aortic” branches. These are the coeliac trunk (coeliac axis) which supplies the foregut, the superior mesenteric artery which supplies the midgut, and the inferior mesenteric artery, which supplies the hindgut.  Vasculature of the Gut The foregut is drained principally via the splenic vein, the midgut via the superior mesenteric vein and the hindgut via the inferior mesenteric vein. Ultimately, these three veins come together to form the hepatic portal vein, which takes the blood (rich in nutrients) to the liver. The liver then sends the blood back to the systemic circulation via the hepatic veins which drain into the inferior vena cava. Visualisation of the Abdominal Aorta The abdominal aorta can be visualized using contrast angiography either using X-rays or CT. This pseudo-3D CT image on the right shows the kidneys as well as the aorta and its branches as these organs are highly vascular. Now it may just be me, but every time I see this image it reminds me of something else…..  The Inferior Vena Cava Hepatic Left Inf phrenic suprarenal suprarenal renal gonadal Left gonadal As the inferior vena cava ascends through the abdomen, it receives tributaries that correspond to the branches of the abdominal aorta. However, the left suprarenal and gonadal vessels join the IVC via the left renal vein. This is an important point to note. A clot (or thrombus) in the left renal vein will cause venous pressure not only in the left renal vein but also in these other tributaries. The patient may complain of an enlarged scrotal sac which feels hot long before they complain of a sore back from their kidney pain. Note that the IVC does not receive venous blood directly from the digestive tract, pancreas or spleen. The blood from these organs passes first via the hepatic portal vein and then through the capillaries/sinusoids of the liver before entering the IVC by way of the hepatic veins.  Ascending Lumbar Veins The four paired lumbar veins are of course tributaries of the IVC, however they also communicate with each other just lateral to the vertebral column. As each of the communicating veins link up, they form the ascending lumbar vein on each side. These veins are continuous with the azygos and hemiazygos veins in the posterior mediastinum. The azygos and hemiazygos veins begin at the junction of the ascending lumbar vein and the subcostal vein and pass deep to the left and right crus of the diaphragm, to enter the posterior mediastinum.  IVC and SVC Anastomoses You may recall that the azygos system of veins drain into the superior vena cava. Hence the ascending lumbar veins provide a channel that connects the IVC (via lumbar veins) with the SVC (via azygos system of veins). This connection is important in cases of caval blockage. Should the IVC become occluded, then blood can reach the heart (via the SVC) by using this shunt. Similarly, should the SVC become blocked, blood can flow down the azygos system towards the IVC. This is permitted, since there are no valves in these veins.  Lymphatic Drainage of Abdomen The lymphatic drainage of the abdomen follows the pattern of the arteries. There are groups of lymph nodes arranged around the origin of the arteries of the gut, and these are known as pre-aortic nodes. Individually, these are the coeliac nodes (shown in yellow), superior mesenteric nodes (shown in light blue) and inferior mesenteric nodes (shown in light green). Enlargement of pre-aortic nodes signals that there is infection or cancer in the gut. The paired organs have lymph which drains to nodes alongside the abdominal aorta, and these are known as para-aortic nodes (shown in dark green), as they lie alongside the aorta. They can also be referred to as right and left lumbar nodes. Actually, there is a lot more complexity to the naming of these, particularly around the inferior vena cava, but the description I have given you here is all that you need to make sense of it.  Lymphatic Drainage of Abdomen The lymphatic vessels from the para-aortic lymph nodes coalesce to form the left and left lumbar trunks and these terminate in the cysterna chyli, a large lymph sac near the aortic opening in the diaphragm. This also receives the intestinal trunk from the pre-aortic nodes. From here, the lymph travels through the aortic opening via the thoracic duct.  Urinary System The Kidneys This is the second part of the lecture on the posterior abdominal wall and kidneys focussing on the latter, but really it deserves it own slot. Indeed, it should be seen as the beginning of a couple of lectures on the anatomy of the urinary system.  Learning Outcomes After this lecture you should be able to: ▪ Give an account of the basic development of the kidney ▪ Describe the congenital conditions that can arise from maldevelopment ▪ Give an account of the anatomical relations of the kidneys ▪ Describe the internal structure of the kidney ▪ Describe the arterial supply and venous drainage of the kidneys ▪ Describe the lymphatic drainage of the kidneys ▪ List the nerve supply of the kidneys and discuss the role of those nerves. Here are the learning outcomes for the Urinary part of the lecture.  The Kidneys (this is not included in the lecture presentation) ▪ Paired solid organs ▪ 10-13cm x 6cm x 3cm ▪ Function: » Urinary excretion » Fluid, electrolyte and acid- base balance » Vitamin D metabolism » Renin and erythropoietin production The kidneys are paired solid organs, yet we only need one to survive. Actually, you can survive on about 75% of a single kidney. Indeed some people are born with only one kidney and they may not even know. This is renal agenesis. Even if there are two, only one might be working. This is renal dysplasia. They are 10-13cms in length, and on average 6cm deep and 3cm wide. Rather oddly, the left one is slightly larger than the right and of course female kidneys are smaller than male ones on average. The kidneys perform a number of vital functions, not least of which is urinary excretion. The kidneys take 25% of the cardiac output and filter the blood plasma removing about 1% of it as urine. As you can see from the list shown here, there are plenty of other functions of these organs, but I don’t plan to go through the details here. Chronic kidney disease (CKD) is a common problem, affecting around 10% of the adult population. CKD becomes more common with age, affecting between 20-25% of people aged 65 to 74. And many of those individuals will seek surgical help.  Development of the Kidneys The development of the kidneys is fascinating. The urinary system develops from a ridge composed of intermediate mesoderm. This extends along the posterior wall of the abdominal cavity and grows in a cranial to caudal direction. The distal end eventually enters a cavity called the cloaca. Cloaca means sewer in Latin, so this ultimately is where the waste products will be disposed of. In fact, there are 3 kidney systems that develop with one disappearing as another begins. Between day 20 to 25 of intrauterine life there is the formation of the pronephros. On day 25, as the pronephric duct proceeds in cranial- caudal direction it induces intermediate mesoderm in the thoracolumbar area to become mesonephric tubules. From day 35 the metanephros develops. This becomes the final functional renal system.  Formation of the Ureteric Bud The metanephric diverticulum arises close to the distal end of the mesonephric duct before it enters the cloaca. This diverticulum is then surrounded by the metanephric cap. The metanephric diverticulum gives rise to the ureter, renal pelvis, major and minor calyces and the collecting tubules that we will discuss later.  The Pelvic Kidneys The final metanephric kidneys then begin their development within the pelvis. With fetal growth the kidneys migrate upwards within the abdomen during week 6 to week 10. The mesonephros is regressing from its cranial end. Meanwhile the metanephric diverticulum splits from the mesonephric duct to enter into the cloaca separately. The arterial supply to the kidney starts in pelvis, fed by branches of the common iliac arteries.  Migration During migration, the aorta forms new branches at an appropriate level, and these replace the more caudal branches. The mesonephros has now completely disappeared, but the mesonephric duct has been utilised by the gonad. Migration halts on reaching the suprarenal glands. The mesonephric artery at that point remains as the renal artery.  Relocation of the Kidneys This all happens during the 6th to 9th week of intrauterine life. Let’s watch this short animation of the ascent of the kidneys towards the suprarenal glands.  Duplex Kidney (this is not included in the lecture presentation) Duplex kidney is the most common of renal anomalies with an incidence of 1 in 125 in the general population. The ureteric bud may divide low down so that the kidney has two hila (bifid pelvis), the ureters uniting at any point between the pelvi-ureteric junction and the bladder. If there are two ureteric buds, there will be double pelves and ureters, the normally placed ureter opening into the bladder somewhere near the normal position and the other (ectopic) ureter opening below it. The normal ureter drains the lower part of the kidney. Ectopic ureteric openings have the disadvantage that the normal valvular action at the orifice is disturbed, and vesico-ureteric reflux may occur. The gross appearance of the kidney makes it look as though it is a single kidney, but in reality, there are two kidneys, each with its own ureter.  Polycystic Kidney (this is not included in the lecture presentation) Polycystic disease is a relatively common condition occurring in about 1 in 1000 of the population. One or both kidneys are grossly enlarged and 'bubbly’ in appearance due to the presence of an enormous number of cysts. These are local dilatations of tubules. Any part of the nephron or the collecting duct may be involved. This is mostly caused via a genetic defect, which affects the union of the metanephric cap with the ureteric bud. Too many cysts can interfere with kidney function, and renal failure may occur. If this happens, then a kidney transplant will be required. If the condition only affects one kidney, then surgery is unnecessary unless it results in pain or obstructs other organs or tissues.  Horseshoe Kidney (this is not included in the lecture presentation) Because the kidneys are close together in the pelvis, their lower poles may touch each other and fuse. The bridge that is created is called the isthmus. If this happens, normal ascent is impossible as the isthmus cannot get past the inferior mesenteric artery. This artery, as well as the two ureters, thus lies in front of the isthmus and ureteric obstruction may occur. Horseshoe kidney occurs in 1:400 births and is more common in males. It is not fully understood why this is the case, but of course males utilise the mesonephric ducts and this may be related. The anomaly represents a failure of separation of the embryologic metanephric ridges.  Position of Kidneys and Ureters The kidneys lie on the posterior abdominal wall and are therefore retroperitoneal in location. The kidneys lie in the deep furrows (gutters) at the side of the vertebral column adjacent to the T12 to L3 vertebrae. Since they are connected to vessels that are more centrally located, they are tilted forwards, so that the hilum is more anteriorly placed than the body of the kidney. Each kidney has an anterior and posterior surface, with a lateral margin and a medial hilum. The kidneys also have a superior and inferior pole. Even these are tilted somewhat at an angle, since the superior pole is more medially placed. You needn’t concern yourself too much with all of this tilting, it doesn’t have much bearing on clinical practice. I just thought you might like to know! Relation of Kidneys to Ribs and Vertebrae The upper parts of the kidneys are protected by the ribs, but the right one is lower than the left, such that the right only reaches as high as the 12th rib, whilst the left reaches the 11th rib. This is an effect of having a liver on the right, preventing it from getting any higher. The right kidney has its lower end opposite a plane drawn through the spine of the L3 vertebra. The ureters descend on the posterior wall anterior to the psoas major muscle. On contrast X-ray, the ureters can be seen close to the tips of the transverse processes of the lumbar vertebrae.  Relation of Kidneys to the Rib Cage L1 The centres of the hila of the two kidneys lie about 5 cm from the median plane, the left and right respectively above and below the transpyloric plane. This plane is midway between the jugular notch of the sternum and the pubic symphysis. It passes through the tips of the 9th costal cartilage at the costal margin. More specifically, it is a plane that transects the pylorus of the stomach (not that this is easy to see on a living patient).  Relation of the Kidneys to the Peritoneum Bare area of Liver Parietal peritoneum The kidneys lie on the posterior abdominal wall surrounded by a fat and fascia. In front of them lies the parietal peritoneum. The superior pole of the right kidney is in direct contact with the liver as there is no peritoneum over the bare area of the liver. The lower pole of the right kidney is associated with the hepatic flexure of the colon. The left kidney is associated with the splenic flexure of the colon and with the splenorenal ligament. Above of course, lies the spleen itself. Relation of the Kidneys to Other Organs ▪ Kidneys » Suprarenal glands » Duodenum (R kidney) » Transverse mesocolon » Splenorenal lig. (L Kidney) » Tail of Pancreas (L Kidney) On the superior poles of each kidney are the suprarenal glands. The right one is triangular in shape and the left one is crescent shaped. These can also be called the adrenal glands, although this is a much less accurate term. Adrenal simply means near the kidney, whilst suprarenal specifies that it lies above. The duodenum is also a right-sided retroperitoneal structure, and hence is in close contact with the hilum of the right kidney. We have already mentioned that the right kidney is closely associated with the liver and hepatic flexure of the colon, and the left kidney is closely related to the splenic flexure of the colon at either end of the transverse mesocolon. The superior pole of the left kidney is associated with the spleen and splenorenal ligament. Lastly, the left kidney is in contact with the tail of the pancreas.  Anterior - Right Kidney (this is not included in the lecture presentation) Upper Pole Right Suprarenal Gland Anterior border Liver Medial border Descending Duodenum Inferior Pole Right Colic Flexure Here then is a table which summarises the relations of the right kidney.  Anterior - Left Kidney (this is not included in the lecture presentation) Upper Pole Left Suprarenal Gland Anterior border Stomach, Spleen Medial border Tail of Pancreas Inferior Pole Left Colic Flexure And here is one to summarise the relations of the left kidney.  Relation to the Ribs (this is not included in the lecture presentation) Posterior relations Whilst we are revising what we know so far, here is a summary of the relations of the kidneys to the ribs posteriorly. The left kidney is related to the 11th and 12th rib, whilst the right one is related to the 12th rib only.  Relation to the Diaphragm (this is not included in the lecture presentation) Posterior relations As well as being related to the rib cage and the organs of the abdomen, the kidneys are also related to muscle. The superior poles are related to the diaphragm. Indeed, the kidneys move on respiration. Relation to the Muscles of the Body Wall (this is not included in the lecture presentation) Posterior relations They are also related to the muscles of the abdominal wall. Laterally, they lie on the transversus abdominis muscle. This is highlighted in orange. The middle part of the kidney sits on the quadratus lumborum muscle. This is highlighted in yellow. Medially, the kidney, renal pelvis and ureter are associated with the psoas major muscle. This is highlighted in lilac.  Relations In Situ (this is not included in the lecture presentation) Here we can see those muscles highlighted together, viewed from the front. And now magically we can put the kidneys in situ to see those relations more clearly. Relation of Kidneys to the Retroperitoneum Parietal peritoneum Renal fascia This cross-section through the abdomen illustrates how the kidneys sit in the loins, with their medial side tilted forwards. The kidneys are posterior to the parietal peritoneum and anterior to the posterior abdominal wall muscles. The point of entry and exit of the renal vessels is via the renal hilum. The veins lie anterior to the arteries, and the ureters lie posterior to both of those. They are not shown on this illustration. We can see from this section that the kidneys are surrounded by renal fascia. This fascia crosses the midline to permit easy access to the vessels.  Relations of the Kidneys Pararenal fat Renal fascia Perirenal fat Capsule of kidney Renal hilum The kidneys are contained in a thin, transparent but tough capsule (renal capsule). Suprarenal glands are contained in the same capsule as the kidneys. The kidneys and suprarenal glands are also surrounded by a thick layer of fat, known as the perinephric or perirenal fat, that helps to retain them in position. This, in turn, is enclosed in the renal fascia. This is a membranous condensation of extraperitoneal tissue that splits to enclose each kidney and suprarenal gland, but the anterior and posterior layers remain separate below the kidney. The renal fascia can be seen on CT scans in most patients except those with very little fat. Outside of the renal fascia is yet more fat; the retroperitoneal fat. This is also referred to as pararenal fat. Hence, they are well-cushioned organs. The fascial layering is important clinically, as it limits the spread of infection. It will also contain the flow of blood from a ruptured kidney.  Internal Structure of Kidney If a kidney is dissected open, the cortex and medulla can be recognized, as can the division of the kidney substance into lobes. Each unit or lobe of cortex and medulla is separated by a deep prolongation of cortical substance called the renal column. Each unit of medullary substance is called a pyramid and the free part is the papilla. This projects into one of the minor calyces of the pelvis and these unite to form major calyces and, finally, the renal pelvis itself.  The Calyces, Pelvis and Hilum The cup-shaped outline of the normal calyx can be seen clearly in an intravenous urogram. The hilum is where vessels, nerves and lymphatics enter or leave the kidney, although occasionally aberrant arteries enter at the poles. The ureter begins as a continuation of the renal pelvis which also leaves the kidney at the hilum. Around the renal pelvis there is a space within the kidney known as the renal sinus, and this contains fat, nerves and vessels. The nerves are sympathetic in type and are vasomotor to the renal vessels. There are also sensory nerves that convey pain signals back to T10-T12 segments of the spinal cord. There are parasympathetic nerves from the vagus nerve in the renal plexus, but there is little evidence that these extend beyond the ureter, where they supply the ureteric smooth muscle.  Structural Unit of Kidney Here we can see a single unit of cortex and medullary pyramid. The cortex contains the Bowman’s capsules, ana proximal and distal tubules. The pyramids contain the loops of Henle and the collecting ducts. The urine is fed into the renal pelvis before it is transported through the ureters.  Renal and Suprarenal Arteries Superior suprarenal artery (from inferior phrenic artery) Middle suprarenal artery (from aorta) Inferior suprarenal artery (from renal artery) The left renal vein passes anterior to the aorta below the superior mesenteric artery. In this position it can be occluded by a change in the angle of the superior mesenteric artery or by atherosclerosis in that vessel. The suprarenal gland is highly vascular. It receives three sources of arterial blood. The first of these is via the superior suprarenal artery, and this arises from the inferior phrenic artery. The second branch is from the aorta, and this is the middle suprarenal artery. The third branch ascends from the renal artery, and this is the inferior suprarenal artery.  Arterial Supply of Kidney The renal artery divides in the hilum, or perhaps before, usually into five segmental arteries. These supply corresponding segments of the kidney. The segmental arteries divide into interlobar, arcuate and cortical radiate arteries, before forming the afferent arterioles for the glomeruli. It is important to understand that each of the segmental arteries is an end- artery, i.e. there is no anastomosis between it and the branches of adjacent segmental arteries. This has surgical implications. One can isolate a segment of kidney and perform partial nephrectomy without worrying about anastomoses. One must be careful though as ligation of any branch will result in necrosis of the tissue supplied as it has no collateral supply. Aberrant renal arteries are common, and they pass across to enter the kidney substance directly into a pole rather than through the hilum. The accessory arteries are also anatomical end-arteries.  Renal Veins The venous pattern matches the arterial one, but there are no segmental veins. The interlobar veins form the renal veins, and unlike the arteries, there are free anastomoses between them. The renal veins leave the renal hilum in front of the arteries and terminate into the inferior vena cava. The left renal vein runs under the origin of the superior mesenteric artery, which is an artery of the midgut. Despite having a rich arterial supply, there is only a single vein from each suprarenal gland. On the right this drains into the inferior vena cava and on the left, it drains into the left renal vein. The left renal vein also receives the left gonadal vein. Hence blockage of the left renal vein can affect these organ systems.  Renal Lymphatics LEFT KIDNEY LEFT PARA-AORTIC LYMPH NODES RIGHT KIDNEY INTER-AORTOCAVAL AND PARACAVAL LYMPH NODES The principal lymphatic drainage of the left kidney is to the left paraaortic lymph nodes. The right kidney drains to the interaortocaval and paracaval lymph nodes. Ultimately, all lymphatics from the kidneys and ureter pass to the thoracic duct. At least some of the lymphatic drainage bypasses any of lymph nodes and joins the thoracic duct directly.  Nerve Supply of the Kidneys The kidneys and ureters, like most other organs, are said to have a dual autonomic innervation. The illustrations shown here show this. However, there is very little evidence that there are parasympathetic efferent fibres in the kidney. So the diagrams are incorrect. There may be some afferent ones, but it is unclear what role they play. It can be said with more confidence that parasympathetics are absent from the suprarenal gland. The sympathetic supply to the kidney is from the T10- T12 preganglionic neurons which synapse in both the coeliac ganglion and the aorticorenal ganglion. These control blood vessels in the organ and regulate the filtration rate. The sympathetic efferents are responsible for stimulating Beta-1-adrenergic receptors in the juxtaglomerular cells of the kidney. This stimulation leads to the activation of the renin-angiotensin-aldosterone-system (RAAS), contributing to the kidney's ability to regulate systemic blood pressure. Pain afferents from the kidney are also carried via sympathetic nerves.  Renal Stones (this is not included in the lecture presentation) One of the most common surgical interventions is to remove kidney stones. Substances can precipitate out of urine while it is still in the body - anywhere between the minor calyces and the urinary bladder. They form stones of different sizes, shapes and consistencies depending on their composition (usually calcium salts). The formation of stones is promoted by chronic dehydration, renal infections, and prolonged immobilisation which causes resorption of bone calcium. The presenting symptoms of renal and ureteric calculi are usually abdominal pain with or without signs of purulent infection.  Renal Colic (this is not included in the lecture presentation) Although many diseases of the kidneys are relatively or completely painless, spasm of the smooth muscle of the renal pelvis and/or ureter is extremely painful. Renal pain is fixed: it is commonly located in the "renal angle” (angle between back muscles – the erector spinae and 12th rib). Pain from the kidney is via the sympathetic nerves (T10-T12) of the renal plexus, most notably via the least splanchnic nerve (T12). As in the gut, the parasympathetic innervation promotes peristaltic movements of the ureter and the sympathetics inhibit it. It is these waves, of course, which move small stones, blood clots and other debris along the ureters, and around such detritus the muscle may temporarily go into spasm before shifting the material a little further. The resulting renal and ureteric colic is an intense and sickening pain focussed anywhere from the loin to the groin, depending on the exact location of the spasm at any one time. If related to the renal pelvis or uretero-pelvic junction, the location is as for renal pain, but the lower in the ureter the problem, the more the pain radiates from the loin, through the groin and into the external genitalia. Stones can become permanently lodged (usually at one of the narrow parts of the ureter) and require surgical removal. Pain from the ureters is via sympathetic nerves (T11-L1/L2).  Urinary System The Kidneys And that concludes this lecture on the kidneys. In the next lecture, we will consider the rest of the urinary tract.