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Suprarenal gland
The suprarenal (adrenal) glands lie immediately superior and slightly anterior to the superior pole of each kidney (see Figure 72.5, Figure 61.8 ). The glands are golden yellow in colour and are enclosed within the renal fascia, where they are separated from the kidneys by a small amount of loose connective tissue. Each gland possesses two functionally and structurally distinct areas: an outer cortex and an inner medulla derived from mesoderm and neural crest cells, respectively.
Macroscopically, the glands are slightly different in external appearance ( Fig. 70.1 ). The right suprarenal gland is pyramidal in shape and has two well-developed lower projections (limbs), giving a cross-sectional appearance similar to a three-pointed star. The left is more semilunar in shape, flattened in the anteroposterior plane, and marginally larger than the right. The bulk of the right suprarenal gland sits on the superior pole of the right kidney and the bulk of the left suprarenal gland lies on the anteromedial aspect of the superior pole of the left kidney.
The suprarenal glands. A , Anterior aspect. B , Posterior aspect.
The dimensions of the suprarenal glands in adults have been measured in vivo from axial computed tomography (CT) ( ). The mean maximum width of the suprarenal gland is approximately 50 mm, and the mean width of each limb of the gland (medial and lateral) is approximately 30-40 mm ( ). No individual limb should measure more than 5 mm in transverse section. In adults, each suprarenal gland weighs approximately 5 g (the medulla contributes about one-tenth of the total weight) and has a volume of approximately 3–6 cm 3 ( ).
At birth, the suprarenal glands are proportionately larger and are approximately one-third the size of the ipsilateral kidney. The cortex of each gland is expanded by a well-developed inner ‘fetal zone’, responsible for producing dehydroepiandrosterone (DHEA), the substrate for placental oestrogen in the fetus. The suprarenal cortex, specifically the fetal zone, shrinks immediately after birth and the medulla grows comparatively little. By the end of the second postnatal month, the weight of the suprarenal gland has decreased by more than 50%. The glands begin to grow again by the end of the second year of life and regain their weight at birth by puberty. There is little further weight increase in adult life.
Small accessory nodules composed mainly of cortical tissue, also known as ‘adrenal rests’, can occur in the areolar tissue near the suprarenal glands. They are also occasionally found in the spermatic cord, epididymis or testis ( ) and the ovary or broad ligament of the uterus. Ectopic suprarenal gland tissue can cause diagnostic confusion and is rarely the site of neoplastic change.
Right suprarenal gland
The right suprarenal gland lies posterior to the inferior vena cava, separated from it by only a thin layer of renal fascia and connective tissue. It also lies posterior to the right lobe of the liver and anterior to the right crus of the respiratory diaphragm and superior pole of the right kidney ( Fig. 70.2 ). The renal surface of the right suprarenal gland overlaps the anterosuperior aspect of the superior pole of the right kidney, the two lower projections (limbs) of the gland straddling the kidney. The anterior surface has two distinct facets: a narrow medial facet that lies posterior to the inferior vena cava and a triangular lateral facet that lies in contact with the bare area of the liver. The most inferior part of the anterior surface may be covered by the peritoneal reflection of the coronary ligament, which also represents the upper aspect of the hepatorenal recess. Here, the gland lies posterior to the lateral border of the descending part of the duodenum. Below the superior border, near the anterior surface of the gland, the right suprarenal vein emerges from the hilum to join the inferior vena cava. This vein is short, which makes surgical resection of the gland or mobilization of the inferior vena cava potentially hazardous. It can be inadvertently avulsed from the inferior vena cava during surgery or, occasionally, by high-energy deceleration injuries. The posterior surface of the gland is divided into upper and lower areas by a curved transverse ridge. The larger upper area is slightly convex and abuts the respiratory diaphragm, whereas the smaller lower area is concave and lies in contact with the superior aspect of the superior pole of the right kidney. The medial border of the gland is thin and lies lateral to the right coeliac ganglion and the right inferior phrenic artery where the artery runs over the right crus of the respiratory diaphragm.
Left suprarenal gland
The left suprarenal gland lies close to the left crus of the respiratory diaphragm ( Fig. 70.2 ). Its medial aspect is convex longitudinally, whereas the lateral aspect is concave because it is moulded by the medial surface of the superior pole of the left kidney. The superior border is sharply defined while the renal surface is more rounded. The upper part of the anterior surface is covered by the peritoneum of the posterior wall of the omental bursa, which separates it from the stomach and sometimes from the inferior border of the spleen. A small lower part of the anterior surface is not covered by peritoneum and lies adjacent to the pancreas and splenic artery. The hilum faces inferiorly from the lower part of the medial aspect. The left suprarenal vein emerges from the hilum and runs inferomedially to join the left renal vein. The posterior surface of the gland is divided by a ridge into a lateral area adjoining the kidney and a smaller medial area that lies in contact with the left crus of the respiratory diaphragm. The medial border lies lateral to the left coeliac ganglion and the left inferior phrenic and left gastric arteries, which ascend on the left crus of the diaphragm.
Vascular supply and lymphatic drainage
Arteries
The suprarenal glands have one of the highest arterial flow rates per gram of tissue (up to 5 ml/g/min) ( ). Each gland is supplied by superior, middle and inferior suprarenal arteries, the main branches of which can be multiple ( Fig. 70.3 ). These vessels ramify over the capsule before penetrating the gland to form a subcapsular arterial plexus, from which fenestrated sinusoids pass around clustered glomerulosa cells and between the columns of the zona fasciculata to a deep plexus in the zona reticularis. Venules from this plexus pass between medullary chromaffin cells to the central vein, which they enter between prominent bundles of smooth muscle fibres. Some relatively large arteries bypass this indirect route and pass directly to the medulla ( Fig. 70.4 ). The arterial supply of the suprarenal gland is prone to considerable variation ( , ) and only the more common variants will be described here.
Superior suprarenal arteries
The superior suprarenal artery usually arises from the ipsilateral inferior phrenic artery and passes to the gland as four or five small branches. It occasionally arises from the abdominal aorta.
Middle suprarenal arteries
The middle suprarenal artery is usually single, but can be multiple or absent. It often arises from the lateral aspect of the abdominal aorta at around the level of the superior mesenteric artery and ascends slightly over the crura of the respiratory diaphragm to anastomose with the other suprarenal arteries on the surface of the suprarenal gland. The right middle suprarenal artery passes behind the inferior vena cava close to the right coeliac ganglion, whereas the left middle suprarenal artery passes close to the left coeliac ganglion, splenic artery and the superior border of the pancreas. The middle suprarenal artery can originate from either the ipsilateral inferior phrenic or renal artery.
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