Kidneys

Mnemonic

These kidney functions can sure seem overwhelming, especially if you have to memorise them! But here is a neat little mnemonic to help. Just remember ' A WET BED', which stands for:

• Maintaining ACID-base balance
• Maintaining WATER balance
• ELECTROLYTE balance
• TOXIN removal
• BLOOD Pressure control
• Making ERYTHROPOIETIN
• Vitamin D metabolism

External anatomy

If we wanted to examine someone’s kidneys with ultrasound, we definitely must know where to find them. Since they are located deep retroperitoneally, the easiest way to examine them is from the patient’s back. 

The kidneys are located between the transverse processes of T12-L3 vertebrae, with the left kidney typically positioned slightly more superiorly than the right. This is because the liver and the stomach offset the symmetry of the abdomen, with the liver forcing the right kidney a bit down, and the stomach forcing the left kidney a bit up. The superior poles (extremities) (T12) of both kidneys are more medially pointed towards the spine than the inferior poles (extremities) (L3). The hilum of the kidney usually projects at the level of the L2 vertebra. Thus, the ureter is seen paravertebrally starting from the L2 and going downwards.

Now let’s pay attention to the borders of the kidneys. A bean-like structure like the kidney has two borders: medial and lateral. The lateral border is directed towards the periphery, while the medial border is the one directed towards the midline. The medial border of the kidney contains a very important landmark called the hilum of the kidney, which is the entry and exit point for the kidney vessels and ureter.

The most superior vessel is the renal vein which exits the kidney, just under it is the renal artery that enters in, and under the artery is the exiting ureter. Alternatively, the anterior to posterior orientation follows the same pattern: renal vein, renal artery and ureter. It is important to remember this order of vessels and ducts since this is the only thing that will make you able to orient the kidney and differentiate the left one from the right when they are outside of the cadaver. 

The kidney tissue is protected by three layers that entirely surround the kidney: 

• The fibrous capsule (renal capsule)
• The perirenal fat body (a.k.a. perinephric fat)
• The renal fascia which besides the kidneys also encloses the suprarenal gland and its surrounding fat. 

Outside the fascia is the most superficial layer – a layer of fat tissue called the pararenal (/paranephric) fat body. This layer sits posteriorly and posterolaterally to each kidney and separates it from the muscles of the abdominal wall. 

Relations

Now that we’ve mastered the borders, it will be easier to take a closer look at the anatomical relations that the kidneys share with other abdominal structures.

Right kidney anterior surface

After looking at the overview of the kidneys in situ, it may seem as they are cluttered with all abdominal organs. Yet, the relations of the kidneys with other organs are often found in Anatomy tests. For that reason, we got you covered with this topic nicely and concisely. Let’s start with the right kidney anterior surface.

• The highest portion of the superior pole is covered with the right suprarenal gland
• The superior one-half of the anterior surface is in contact with the layer of peritoneum that separates it from the liver. This potential space that separates the liver from the right kidney is called the hepatorenal pouch of Morison. Under normal conditions, this pouch is empty, but certain pathological conditions, such as ascites or hemoperitoneum, can cause fluid to collect within the pouch. This can be visualized with ultrasound or CT.
• At exactly the center of the anterior surface, imagine a horizontal stripe that extends from the medial concavity toward the center of the lateral convexity – that is the area of the kidney that is directly touched by the retroperitoneal posterior wall of the descending duodenum
• The lateral part of the inferior pole is directly contacted with the right colic flexure (also known as the hepatic flexure) which is also retroperitoneal at this part
• The rest of the inferior pole is associated with the peritoneum of the small intestine, more precisely the jejunum

Left kidney anterior surface

Since the abdominal organs are not paired, the left kidney is not related to the same organs as the right kidney.

The anterior surface of the left kidney, has the following anatomical relations:

• Just like the right kidney, the highest part of the superior pole of the left is also covered with the left suprarenal glandF
• The inferior portion of the superior pole contacts with the peritoneum of the stomach (medially) and spleen (laterally)
• Just inferior to the stomach and spleen impression, is where the left kidney directly contacts the pancreas
• The lateral part of the inferior half of the anterior surface is directly associated with the left colic flexure (also known as the splenic flexure) and descending colon
• The medial part of the inferior half and the inferior pole are contacted by the peritoneum of the jejunum

Posterior surface relations

The posterior surfaces of both kidneys are related to certain neurovascular structures and muscles:

• 1 Artery: subcostal artery
• 2 Bones: 11th and 12th ribs
• 3 Nerves: subcostal, iliohypogastric, and ilioinguinal nerves
• 4 Muscles: diaphragm, psoas major, quadratus lumborum, transversus abdominis

You can easily remember these with the mnemonic: “1-2-3-4 All Boys Need Muscle”.

The superior half of each kidney is covered by the diaphragm, which is why the kidneys move up and down during respiration

The muscular relations of the inferior half are easy to remember by dividing the kidney surface into three vertical stripes, where the medial stripe represents the impression of the psoas major muscle, the central stripe the quadratus lumborum, and the lateral stripe the transversus abdominis muscle.

Internal anatomy

The parenchyma of the kidney consists of the outer renal cortex, and inner renal medulla.

The main unit of the medulla is the renal pyramid. There are 8-18 renal pyramids in each kidney, that on the coronal section look like triangles lined next to each other with their bases directed toward the cortex and apex to the hilum. The apex of the pyramid projects medially toward the renal sinus. This apical projection is called the renal papilla and it opens to a minor calix. Several minor calices unite to form a major calix. Usually, there are two to three major calices in the kidney (superior, middle, and inferior), which again unite to form the renal pelvis from which the ureter emerges and leaves the kidney through the hilum. The pyramids are separated by extensions of the cortex called the renal columns.

The pyramids contain the functional units of the kidney, the nephrons, which filter blood in order to produce urine which then is transported through a system of the structures called calices which then transport the urine to the ureter. So the pyramids represent the functional tissue that creates urine, whereas the calices are the beginning of the ureter and transport the urine to it.

Nephron

Each time a professor says 'nephron', a student gets a headache. For most of the students, the nephron is a mystical complexed structure that may be hard to understand. It doesn't have to be that way. Let's see what is nephron and how it is structured, so you can remember it for good.

Ultrastructurally, the nephron is the functional representative of the kidney. Each nephron contains arenal corpuscle, which is the initial component that filters the blood, and a renal tubule that processes and carries the filtered fluid to the system of calices. The renal corpuscle has two components: the glomerular (Bowman’s) capsule in which sits the glomerulus.

The glomerulus is actually a web of arterioles and capillaries, with a special filter which filters the blood that runs through the capillaries, the glomerular membrane. The vessel which brings blood into the glomerulus is the afferent arteriole, whereas the vessel that carries the rest of the blood out that hasn’t been filtered out of the glomerulus is called the efferent arteriole. 

The glomerular membrane is designed in a way in which it is not permeable for big and important molecules in blood, such as plasma proteins, but it is permeable to the smaller substances such as sodium, potassium, amino acids and many others. It is also permeable for the products of the metabolism, such are creatinine and drug metabolites.

So in the filtered fluid that goes to the renal tubule, we have both necessary and unnecessary substances. Because of this, the tubules are designed in a way that they reabsorb the necessary substances, (sodium, potassium, and amino acids as mentioned before) and carries them back to the blood; whereas they do not absorb but rather secrete unnecessary substances such as creatinine and drug metabolites for excretion from the body.

In this way, the consistency of blood is preserved and no important substances are lost. On the other hand, the products of cellular metabolism and drug metabolites are eliminated from the blood which prevents their depositing in the body and potential toxicity. This is why the kidney is essential for the circulatory hemostasis.

Learn more about the nephron in the following study unit or take our custom quiz to see what you know already:

Arteries

Each kidney is supplied by a single renal artery, which is a direct lateral branch of the abdominal aorta. Both renal arteries, left and right, arise just below the superior mesenteric artery, with the left renal artery positioned slightly superiorly to the right one. The left artery has a short way to the left kidney, whereas the right has to go behind the inferior vena cava in order to reach the right kidney. In addition to the renal artery, accessory renal arteries are present too. They are branches of the abdominal aorta and all together are called the extrahilar renal arteries.

When the renal arteries enter the kidney through the hilum, they split into anterior and posterior branches. The posterior branch supplies the posterior part of the kidney, whereas the anterior branch arborizes into five segmental arteries, each supplying a different renal segment. The segmental arteries then branch into the interlobar arteries, which further branch into the arcuate arteries. Finally, the arcuate arteries branch into the interlobular arteries which branch off even further by giving afferent arterioles to run blood past the glomerulus for blood filtration. It is notable that the kidney has a very rich blood supply.

You can test yourself on the renal arteries with our quiz.

Veins and lymphatics

Each kidney has a single renal vein which conducts the blood out of the kidney and is positioned anterior to the artery. The renal veins empty to the inferior vena cava, so the right vein is shorter because the inferior vena cava runs closer to the right kidney. The left renal vein passes anteriorly to the aorta just below the trunk of the superior mesenteric artery, which is risky because it can be compressed by one of those two. This is called the nutcracker phenomenon. Concerning lymphatic drainage, each kidney drains into the lateral aortic (lumbar) lymph nodes, which are placed around the origin of the renal artery.

Note that the left renal vein receives blood from the left suprarenal and left testicular veins. The left testicular vein must ascend higher and it drains to the left renal vein at a right angle, unlike the right testicular vein which joins the inferior vena cava directly. This can cause varicocele of the left testicle because gravity works against the column of the blood in the left testicular vein.

Furthermore, since the left renal vein passes between the superior mesenteric artery and the abdominal aorta, an enlargement of the superior mesenteric artery can compress the left renal vein and cause an obstruction of drainage from all three structures that use the left renal vein for drainage (left suprarenal gland, left kidney, and left testicle). This significantly affects the testicle, since an obstruction of drainage causes an obstruction of fresh arterial blood inflow, which can result in the infarction of testicular tissue. This specific condition is called the nutcracker phenomenon.