Hello everyone! This is Megan from Kenhub, and welcome to our tutorial on the kidneys in situ. Now, the first thing you may be wondering is what the term "in situ" means? In situ is the term used in anatomy to refer to viewing structures as they appear in a normal human body. For example, you may view the kidneys as an isolated structure outside the human body or you may view them in situ – in their normal environment. So, this tutorial will essentially explore this image here that shows the two kidneys in situ.
So, in this image, we see an anterior view of the kidneys with all the skin layers and muscular tube removed. In addition, we've removed some of the abdominal viscera like the stomach, the liver, the spleen and the intestines in order to have a better view of the kidneys. We can also see that there's lots of structures that are in close proximity to the kidneys such as the duodenum here and the pancreas here. We will go through several of these structures during this tutorial.
So, the first thing I want to cover is the structure and function of the kidneys. As you can see in this image, the kidneys are shaped like large beans with a major convexity and a major concavity. They are found in the upper left and right quadrants of the abdomen and they are retroperitoneal structures, meaning they are situated behind the peritoneum in the abdomen, which is being removed in this image. So, here, we can see the major concavity and the major convexity of the right kidney. Although the kidneys are similar in size and shape, the left kidney is a longer and more slender organ than the right kidney and is near to the midline.
So now let's move on to talk about the key functions of the kidneys. Their main function is to eliminate excess bodily fluid, salts and the by-products of protein metabolism. They essentially filter our blood and remove the waste to form urine, which is then diverted to the bladder. They also remove drugs from our system. However, it's often forgotten that the kidneys also play a key role in homeostasis of the body by regulating our electrolytes, acid-base balance, and blood pressure. The kidneys also produce hormones such as erythropoietin or EPO, which is involved in the production of red blood cells.
To understand the physiology of the kidney, it's important to understand its structure and the way that urine is transferred to the ureters. The main body of the kidneys can be divided functionally into an outer cortex, a renal medulla, and the hilum. So before we go on to talk about the production and transportation of urine, I just want to remind you what the hilum is.
So, a hilum refers to a depression or fissure where vessels, nerves or ducts enter a bodily organ. So, the hilum of the kidney is located here. Urine is produced in the nephrons which are located primarily here in the cortex of the kidney. The urine that emerges from the nephron then passes into a collecting tubule. The collecting tubules begin in the cortex and extend into the medulla and they eventually open into the minor calyces. The minor calyces coalesce into approximately three major calyces and the major calyces drain into the renal pelvis. The renal pelvis leaves the kidney at the hilum and continues on as the ureter. That is briefly the way that urine is produced and conveyed from the nephron to the ureters.
So now let's have a quick look at the left kidney. The left kidney is situated here on the left-hand side of the abdomen close to the tail of the pancreas and the jejunum. It is positioned between the levels of the twelfth thoracic vertebra and the third lumbar vertebra. The right kidney is also situated between these vertebral levels but sits a bit lower than the left kidney due to the size and placement of the right lobe of the liver, which is much larger than the left lobe.
So now that we've gone over the function of the kidneys and had a look at them in situ, it's a good time to bring up the clinical note in the kidneys. Kidney stones or renal stones are relatively common occurrence so are good thing to know about in clinical practice. They are characterized as a disorder that obstructs the urinary outflow. This can be a partial or complete obstruction depending on the size of the stone and its constituents. The composition of the stone includes calcium complexes, struvite, uric acid, cysteine, and other unknown substances, and their primary cause is an increased urinary concentration.
Renal stones may be present with or without symptoms no matter how large. Smaller stones can be detected if they dislodge themselves and end up blocking the ureter. The most common symptom is pain that radiates from the flank to the groin and it's known as ureteral colic. Colic is a term used to describe pain that comes and goes. Treatment nowadays usually consists of medication or ultrasonic vibration but most kidney stones are small enough to spontaneously pass out with the urine.
So now let's go over some of the structures situated around the kidney. The first structure we will look at are the adrenal glands also known as the suprarenal glands. We can see them in this image highlighted in green. These glands have a distinct golden yellow color and possess two functionally and structurally distinct areas – an outer cortex and an inner medulla.
The cortex of the adrenal gland produces three types of hormones. The first type are the mineralocorticoids with the most important one, aldosterone, regulating electrolyte and water balance. The second type are glucocorticoids with the most important one, cortisol, regulating carbohydrate balance. The third and final type of hormones that are produced from the cortex are sex hormones such as androgens and oestrogens. On the other hand, the medulla produces the catecholamines, noradrenalin and adrenaline, which function to produce a rapid response throughout the body in stressful situations and is part of the autonomic nervous system.
An important clinical entity related to the suprarenal glands is the most common tumor of the adrenal medulla – a pheochromocytoma. This is a catecholamine-secreting tumor that may precipitate life-threatening hypertension. Although sometimes this tumor is a result of genetic factors, the majority of cases have an unknown etiology. Classically, pheochromocytoma manifests as spells with the following four characteristics: headaches, palpitations, diaphoresis otherwise known as sweating, and severe hypertension.
There are specific biochemical tests for measuring the plasma and urine levels of catecholamines and metanephrines and this can help diagnose the disease. Surgical resection of the tumor is the treatment of choice and usually cures the hypertension. Careful preoperative treatment with alpha and beta blockers is required to control blood pressure and prevent inoperative hypertensive crisis.
The next structures we'll look at are the ureters. The ureters are an essential part of the urinary system and are involved in the transport of urine from the kidneys to the bladder. As we can see in this image, the ureters originate at the hilum of the kidney. They then pass downwards and along the sides of the pelvic wall until they curve forwards and enter the bladder inferiorly on the left and right sides.
The next structure we will look at is the hepatoduodenal ligament, which is part of a peritoneal fold known as the lesser omentum. This illustration includes some other structures that are located in this region such as the liver, the gallbladder and the large intestine. The hepatoduodenal ligament is highlighted in green and extends from the porta hepatis or the hilum of the liver to the superior part of the duodenum.
If we go back to the previous image, we can see the hepatoduodenal ligament contains some structures within it which are collectively known as the portal triad. These structures are the hepatic artery proper, the hepatic portal vein and the common bile duct.
The next structure we'll look at is the inferior vena cava, highlighted in this image in green. Like the kidneys, the inferior vena cava is retroperitoneal which means it's situated behind the peritoneum. This vein drains directly into the heart via the right atrium and has many tributaries or veins which drain into it. In this image, we can see the inferior vena cava piercing the diaphragm at the level of T8.
Also at the level of T8, we can see a group of veins which drain into the inferior vena cava. These are known as the hepatic veins and are highlighted here in this image. These veins carry deoxygenated blood from the liver to the heart. As we can see in this image, they're located on the anterior surface of the inferior vena cava.
Another structure you can see is the esophagus, part of which has been removed in this image. The esophagus is a tube that transports food from the pharynx to the stomach. It's also known as the food pipe and is about 20 centimeters long. Both the inferior vena cava and the esophagus must pass through a structure to get from the mediastinum to the abdominal cavity. This structure is the diaphragm and the hepatic side of the diaphragm is highlighted in green in this image.
As I mentioned previously, the inferior vena cava passes to the diaphragm through an opening known as the caval opening at the level of thoracic vertebra 8. The esophagus, however, passes through an opening known as the esophageal hiatus at about the thoracic level 10.
Another structure which passes through the diaphragm is the aorta. It passes through the aortic hiatus at the level of the thoracic vertebra 12 which is shown here on the right image. Once the aorta passes through the diaphragm, it's known as the abdominal aorta, which we can see highlighted on the left image. So here we can see the abdominal aorta which is about to divide into the right and left common iliac arteries at the level of L4.
So now let's go over some blood vessels we can see in this image. So let's begin with the arteries. All of the arteries that supply the abdominal viscera arise from the abdominal aorta. The abdominal aorta at the level of the twelfth thoracic vertebra or T12 gives off an important unpaired artery which is known as the celiac trunk or the celiac artery. The celiac trunk then gives off three branches that supply the foregut organs. These are the left gastric artery, the splenic artery and the common hepatic artery.
So, the first of these three arteries that we're going to see is the left gastric artery. In this image on the left, we can only see a small portion of the artery and then it cuts a coronal plane. But here in the right image, we can see the left gastric artery as it originates from the celiac trunk and we can see that it supplies the superior portion of the lesser curvature of the stomach. It also supplies the lower esophagus. So, just to reiterate, the abdominal aorta gives off the celiac trunk which then gives off one of its three branches, the left gastric artery.
So, the second branch of the celiac trunk we can see is the splenic artery which is highlighted in green in these images. The splenic artery is the largest branch of the celiac trunk and it takes a torturous course to the left along the superior aspect of the pancreas and then divides into smaller branches which enter the hilum of the spleen. So, again to reiterate, the abdominal aorta gives off the celiac trunk which then gives off the splenic artery.
The third and final branch of the celiac trunk is the common hepatic artery. This artery as you can see in the image on the right gives off several branches that supply the liver, the stomach, the duodenum and the pancreas. So the celiac trunk is located on the anterior portion of the abdominal aorta and its third and final branch is the common hepatic artery.
Immediately below the celiac trunk at the level of the first lumbar vertebra, the abdominal aorta gives off another single artery, the superior mesenteric artery. As we can see in this image on the left, it passes underneath the neck of the pancreas and, in this image on the right, we can see that it continues downwards giving off numerous branches that supply the pancreas, the duodenum, all of the small intestine and the ascending and transverse colon.
Below the superior mesenteric artery, a pair of arteries arise from the abdominal aorta at the level of the first and second lumbar vertebra. These are the renal arteries which supply the two kidneys. In this image, we can see the left renal artery highlighted in green which passes behind the pancreas and the duodenum and enters the hilum of the left kidney. The right renal supplies the right kidney but cannot be seen in this image. So not only does the abdominal aorta give off the celiac trunk and the superior mesenteric artery, it also gives rise to the left and right renal arteries.
So now let's move on to talk about some of the veins which are visible when we see the kidneys in situ. The first one we'll look at is the superior mesenteric vein. As we can see in this image, it runs under the neck of the pancreas parallel to the superior mesenteric artery. This vein receives blood from numerous tributaries that come from the duodenum, the pancreas, the small intestine and the ascending and transverse colon. The superior mesenteric vein joins with the splenic vein to form the hepatic portal vein.
The next veins we will look at are the colic veins. The left colic vein follows the path of the left colic artery. It drains the descending colon and it drains into the inferior mesenteric vein. The inferior mesenteric vein then drains into the splenic vein. The splenic vein joins with the superior mesenteric vein to form the hepatic portal vein which is part of the portal triad we talked about earlier. There's also a middle colic vein and a right colic vein but they aren't shown in this image. They drain the transverse colon and the ascending colon respectively and drain into the superior mesenteric vein.
All of the veins that we've mentioned so far drain blood from the gastrointestinal tract and the spleen into one large vein which is known as the hepatic portal vein and is now highlighted in green. As I mentioned previously, this structure is part of the portal triad which also consists of the hepatic artery proper and the common bile duct. This vessel carries nutrient-rich blood from the gastrointestinal tract and the spleen to the liver. One very important thing that you have to remember is that the hepatic portal vein is not a true vein as it conducts blood to the liver and not directly to the heart. Blood from the gastrointestinal tract contains both nutrients and toxins from food and the liver is, therefore, responsible for removing these components from the blood. The hepatic portal vein divides into the left and right portal veins just before entering the liver.
The final veins that we can see when we look at the kidneys in situ are, of course, the renal veins. Here, we can see part of the left renal vein. It drains the left kidney and carries blood purified by the kidney to the inferior vena cava. There's also a right renal vein but our view of it is obscured by the duodenum. It drains blood purified by the right kidney to the inferior vena cava.
And before we move on to look at some other organs surrounding the kidneys, let me show you these final vessels which are located inside the hepatoduodenal ligament. These are the right and left hepatic ducts which unite to form the common hepatic duct.
So now we're going to move on to talk about some organs that are in close proximity to the kidneys. The first one we'll talk about is the large intestine. The part of the large intestine that is more clearly seen is the transverse colon, however, we've dissected some of this away in this illustration. As we can see in this image, it sits superficial to the inferior aspect of the right kidney and the superior aspect of left kidney. The transverse colon finally turns downward forming this curvature here which is located inferior to the spleen and, for this reason, it's called the splenic flexure or the left colic flexure.
The last part of the large intestine we can see is the descending colon. So as well as the large intestine, the kidneys are also closely related to the small intestine. The first part of the small intestine that we'll look at is the duodenum. The duodenum connects the stomach to the next part of the small intestine – the jejunum. So, the duodenum has four parts. The first one as you can see here is the part that lies immediately after the stomach and it's called the superior part of the duodenum. In this first part of the duodenum, there is a dilation which is called the ampulla or the duodenal cap, and this is the region where most duodenal ulcers occur.
The next part of the duodenum is the one we can now see highlighted in green. It begins at the superior duodenal flexure and descends downwards; therefore, it's known as the descending part of the duodenum. This part is important from a clinical perspective because the hepatopancreatic ampulla or the ampulla of Vater is located in this part of the duodenum. Through this ampulla, pancreatic enzymes and bile are released into the duodenum in order to aid digestion.
The horizontal part of the duodenum is the third segment of the duodenum. It begins here just after the inferior duodenal flexure and continues until the ascending part of the duodenum. This part of the duodenum is the longest aspect. The fourth and final segment of the duodenum is the ascending part. This part of the duodenum passes upward to the left of the aorta extending from the horizontal part of the duodenum up to the duodenojejunal flexure. So that leads us on to another organ we see – the jejunum.
The jejunum is the second part of the small intestine that starts immediately after the duodenum at the duodenojejunal flexure. The first part of the jejunum is highlighted here in this image and its main function is to absorb nutrients which have been digested by the duodenum.
The final organ we're going to talk about is the pancreas. This organ measures approximately 15 centimeters in length and is retroperitoneal except for a small part its tail. It lies mostly posterior to the stomach in the duodenal loop and partly behind the omental bursa. The pancreas is both an endocrine and exocrine gland that can be divided anatomically into a head, a body and a tail. Functioning as an exocrine gland, the pancreas secretes enzymes into the duodenum in order to break down proteins, lipids, carbohydrates and nucleic acids during digestion. However, functioning as an endocrine gland, the pancreas secretes the hormones insulin and glucagon into the blood in order to control blood sugar levels throughout the day.
If you look here at the anterior surface of the pancreas, you can see a line that crosses nearly all of the anterior surface. This is the route of the transverse mesocolon. The transverse mesocolon is a peritoneal fold that is part of the mesenteries and connects the transverse colon to the posterior abdominal wall. As we can see in the next illustration, its two layers leave the posterior abdominal wall from the anterior surface of the body and head of the pancreas and extend outwards to encircle the transverse colon. These layers contain blood vessel, nerves and lymphatics that relate to the transverse colon.
Another important peritoneal fold is the mesentery proper which provides attachment for the intraperitoneal small intestines. The root of this membranous tissue – the root of the mesentery – extends from the posterior wall of the abdominal cavity, from the duodenojejunal flexure to the right iliac fossa.
So to finish off this tutorial, let's look at the parietal peritoneum. You may remember that I mentioned before that the kidneys are retroperitoneal structures and this means that they are located behind the parietal peritoneum which is highlighted in green in this image on the left. A mnemonic that can be used to remember the retroperitoneal structures in the abdomen is SADPUCKER. S stands for the suprarenal glands otherwise known as the adrenal glands, A is for aorta and inferior vena cava, and D is for the duodenum. It's important to note that only the second and third parts of the duodenum are retroperitoneal. P stands for pancreas excluding the tail, U stands for the ureters and C stands for the ascending and descending colon. K is for the kidneys, E for esophagus and R for rectum.
So that concludes our tutorial on the kidneys in situ. I hope you enjoyed it and thank you for listening.
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Now, good luck everyone, and I will see you next time.