Video: Kidneys

Ever wondered which of your organs functions similar to a washing machine? Drum roll, please… it's the kidneys! While you might dread the thought of laundry day once or twice a week, spare a thought for your kidneys, which are doing your metabolic laundry 24/7.

Just like your clothes go in dirty and come out clean from your washer, so too does our blood as it passes through the kidneys. And although you can manage with just one of them, it goes without saying that you can't live without them.

Let's get to know these organs a little better now as we explore the external and internal anatomy of the kidneys.

The kidneys are bilateral bean-shaped organs located on the posterior abdominal wall in the left and right upper abdominal quadrants. They are retroperitoneal organs, which means that they are positioned behind the peritoneum, the serous membrane which lines the abdominopelvic cavity, and a part of the urinary system. Each kidney variably weighs about 150 grams in males and 135 grams in females and they are about 11 to 14 centimeters in length, 6 centimeters wide, and 4 centimeters thick.

Let's now have a closer look at the anatomical location of the kidneys.

They are located just lateral to the transverse processes of vertebrae T12 to L3 with the left kidney typically positioned slightly more superiorly than the right. This is mainly due to the position of the largest abdominal organ, the liver, which causes the right kidney to be pushed down a little, compared to the left side.

As the kidney is bean-shaped, it has both superior and inferior extremities known as poles. The superior pole extends more medially towards the spine than its inferior counterpart and its superomedial surface is covered by the suprarenal gland, which we are going to discuss shortly. Other parts of the superior pole of the right kidney lie within the renal impression on the inferior surface of the liver while the superior pole of the left kidney is in close relation to both the stomach and the spleen.

The inferior pole is a little farther from the median line and extends to within 5 centimeters of the iliac crest. The lateral part of the inferior pole of the right kidney relates to the right colic flexure while the rest of the inferior pole is associated with the peritoneum of the jejunum. The inferior pole of the left kidney is typically associated with the left colic flexure, descending colon, and peritoneum of the jejunum.

Ready to kick off with exploring the external structure of the kidneys? Let's go!

Overall, the kidney is protected by three layers that entirely surround it. First, we have the renal capsule, also known as the fibrous capsule of the kidney. This is composed of dense irregular connective tissue which maintains the shape of the kidneys and protects them from damage. Next, we have the perirenal fat capsule covering the fibrous renal capsule. This is a protective layer of adipose tissue which in turn is enveloped by a tough renal fascia which encloses the kidneys, suprarenal gland, and perirenal fat.

The pararenal fat body, a collection of adipose tissue located superficial to the renal fascia, functions to protect the kidneys and renal blood vessels from external physical stimulation. The fascia and, to a lesser extent, the overlying peritoneum serve to firmly anchor the kidneys to the posterior abdominal wall in a retroperitoneal position.

Moving on, the kidneys have anterior and posterior surfaces. The anterior surface relates to the digestive organs overlying it. Anteriorly, the right kidney is related to the liver, duodenum, and right colic flexure of the ascending colon. The left kidney is related to the stomach, jejunum, pancreas, spleen, and descending colon. In contrast, the posterior surface faces the posterior abdominal wall. It is in close relation with the diaphragm, the medial and lateral arcuate ligaments, the muscles of the posterior abdominal wall, and two nerves -- the subcostal and iliohypogastric nerve.

These surfaces are separated by the lateral and medial borders of the kidneys. The lateral border of the kidney is relatively long, smooth, and convex. On the other hand, the medial border is shorter, concave, and steeply grooved. The center of the medial border is marked as the hilum of the kidney where structures enter and exit the kidney.

Shall we have a closer look at the structures of the renal hilum? Let's go!

The hilum usually sits around the level of vertebra L2. From anterior to posterior, the renal hilum transmits three structures. The renal vein is formed by the union of two to three renal interlobar veins in the renal sinus, which is a shallow cavity within the kidney. The renal vein emerges from the renal hilum anterior to the renal artery and drains directly into the inferior vena cava at the vertebral level L2.

The renal artery arises from the anterolateral aspect of the abdominal aorta at the vertebral level L1 to L2, slightly inferior to the origin of the superior mesenteric artery. Now the renal arteries supply the oxygenated blood to the kidneys and they enter the kidney at the hilum, as we have already seen. We'll explore their branches when we look at the internal structure of the kidney.

The last and posteriormost structure exiting the renal hilum is the renal pelvis which is an expanded funnel-shaped area through which urine is strained from the kidney. The apex of the renal pelvis extends away from the kidney and is continuous with the superior end of the ureter.

Let's now take a closer look at the suprarenal gland, also commonly known as the adrenal gland. This is the last structure we will explore as part of the external anatomy of the kidney.

The suprarenal glands are paired endocrine glands situated over the superomedial aspect of the superior pole of each kidney. The suprarenal cortex secrete steroid hormones involved in growth, metabolism, and reproduction such as cortisol, aldosterone, and precursors to the sex hormones, testosterone and estrogen, while the suprarenal medulla secretes catecholamines, or stress hormones, such as norepinephrine and epinephrine directly into the blood.

The suprarenal glands are highly vascular. They receive their vascular supply from three different sets of arteries. The first one is a group of arteries known as the superior suprarenal arteries. There are usually numerous small arteries arising from the inferior phrenic artery, a branch of the abdominal aorta. The middle and inferior suprarenal arteries also provide additional blood supply to the glands through small branches from the aorta and renal artery, respectively.

Now that we've discussed the external anatomy of the kidney, it's time to inspect its internal anatomy.

Functionally, the kidney can be divided into three major regions which consist of the renal cortex, renal medulla, and the renal sinus. The renal cortex is the outer subcapsular portion of the kidney. It houses a large portion of the nephron, which is the basic functional unit of the kidney, as well as a dense network of blood vessels which supply them. Specific structures found in the renal cortex include the renal corpuscles, convoluted parts of renal tubules, and the initial portions of the collecting ducts.

The renal corpuscle is the filtration unit of the nephron, which is the functional unit of the kidney. It consists of a knot of capillaries, the glomerulus, surrounded by a double-walled glomerular capsule, commonly known as Bowman's capsule, that opens into a renal tubule which is responsible for reabsorption of substances back into the blood.

The specific structures found within the renal cortex are covered in more detail in our tutorial focusing on the microanatomy of the kidney.

Ultrafiltration, a membrane filtration process, takes place in the renal cortex. This process allows for the removal of water, sodium, metabolic waste, and excess substances in the blood to be removed from the blood under high pressure. After filtration, the renal cortex is involved in the reabsorption of useful substances like water, glucose, and electrolytes back into the bloodstream as needed to maintain balance for homeostasis. Additionally, erythropoietin, a hormone necessary for the synthesis of new red blood cells, is also produced in the renal cortex.

Let's move on to the renal medulla, which is the inner region of the parenchyma of the kidney. The renal medulla is characterized by multiple cone-shaped tissue masses which are called renal pyramids and contain the nephron loop of each nephron as well as the medullary portion of the collecting ducts and papillary ducts and blood vessels. Collectively, a renal pyramid and the overlying renal cortex form a renal lobe. In humans, on average there are 7 to 18 renal lobes.

The base of the renal pyramids contacts the renal cortex and its apex, the renal sinus. Extensions of the renal cortex, known as renal columns, separate the renal pyramids and mark the boundaries of the renal lobes. The apices of the pyramids, also known as renal papillae, converge towards the centralmost part of the kidney known as the renal sinus.

Each papilla contains several papillary ducts which empty urine into a small chamber known as a minor renal calix. A number of minor calices then merge to form a major calix, of which there are usually two to three in the kidney. The major renal calices then unite to form the renal pelvis. The renal pelvis exits the kidney via the renal hilum, continuing on as the ureter.

Okay, time for a recap about the functions of the kidneys.

We learned that the kidneys filter the blood to remove excess water, minerals, and waste products of protein metabolism producing urine in the process. Additionally, the kidneys maintain homeostasis by controlling the amount of water, ions, and other substances in the blood, regulating the volume and osmolality of the extracellular fluid. This is primarily achieved by altering the amount of sodium and water excreted.

Healthy kidneys also respond to aldosterone, a steroid hormone secreted by the suprarenal glands, which helps regulate blood pressure by managing the levels of sodium and potassium in the blood. We've also seen that kidneys play a role in red blood cell synthesis due to the production of the hormone responsible for it.

That brings us to the end of our anatomy tutorial. Today, we looked at the external and internal anatomy of the kidney and its functions.

See you next time.