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Kidney Structure and Function

Your kidneys are paired organs found on each side of the back portion of the abdominal cavity. The larger left kidney is located a bit higher than the right kidney. Unlike other organs found in the abdomen, the kidneys are located behind the lining (peritoneum) of the abdominal cavity, thus they are considered retroperitoneal organs. These bean-shaped organs are protected by the back muscles and the ribs, as well as the fat (adipose tissue) that surrounds them like a protective padding. Learn more about the kidney structure and function from this short article.

Kidney Structure

The bean-shaped kidneys have an outer convex side and an inner concave side called the renal hilus, where the renal artery, vein, and ureter are found.

A thin connective tissue called the renal capsule surrounds each kidney. This capsule maintains the kidneys’ shape and protects the inner tissues.

Inside the renal capsule is the outer layer called the renal cortex, asoft, dense, and vascular tissue. Deep to this layer is the renal medulla, which consists of several renal pyramids, thecone-shaped structures with apices pointing toward the kidney’s center.

Each apex of the renal pyramid is connected to a minor calyx, a hollow collecting tube for urine. These minor calyces merge and form three major calyces that also merge into the renal pelvis at the hilus of the kidney. From here, urine drains into the larger ureter.

Here’s a summary of kidney structure and function based on different parts of the kidney:

Parts of the Kidney

Description

Renal hilus

An indentation near the center of the concavity of the kidney where the renal vein and ureter leave the kidney and the renal artery enters the kidney.

Renal capsule

A smooth, transparent membrane surrounding the kidney. It protects and helps maintain the kidney’s shape. It is also surrounded by fatty tissue which helps protect the kidney from damage.

Renal cortex

The outer reddish part of the kidney that has a smooth texture. It is where the Bowman’s Capsules, glomeruli, proximal and distal convoluted tubules and blood vessels are found.

Renal medulla

The inner striated red-brown part of the kidney.

Renalpyramids

Striped, triangular structures within the medulla, which are made of straight tubules and corresponding blood vessels.

Renal pelvis

The funnel-shaped cavity that receives urine drained from the nephrons through the collecting ducts and papillary ducts.

Renal artery

The blood vessel that delivers oxygen-rich blood to the kidney. It enters the kidney through the hilus and divides into smaller arteries, which separate into afferent arterioles that serve each of the nephrons.

Renal vein

The blood vessel that receives deoxygenated blood from the kidney and returns it to the systemic circulation.

Interlobular artery

The blood vessel that delivers oxygen-rich blood to the glomerular capillariesunder high pressure.

Interlobular vein

The blood vessel that receives low pressure deoxygenated blood that drains from the glomeruli and the loops of Henle.

Kidney nephrons

The functional units where the kidney’s main functions are performed. There are about a million nephrons in each kidney.

Collecting duct

The part of the kidney nephron that collects urine and drains into papillary ducts, minor calyx, and major calyx, and finally into the ureter and urinary bladder.

Ureter

The structure which conveys urine from the kidney to the urinary bladder.

The Functional Unit of Kidney: Nephron

The nephron is the kidney’s functional unit that removes waste from the body. Each kidney has more than a million nephrons in the renal cortex, which gives it a granular appearance on sagittal section.

There are 2 types of nephrons. The cortical nephrons, which make up about 85 percent, are found deep in the renal cortex, while the juxtamedullary nephrons, which make up about15 percent of total nephrons, lie close to the medulla.

The nephron consists of a renal corpuscle, a tubule, and a capillary network that originates from the small cortical arteries. Each renal corpuscle is composed of a glomerulus (a network of capillaries) and a Bowman’s capsule(the cup-shaped chamber that surrounds it.

The glomerulus connects to a long, convoluted renal tubule which is divided into three functional parts. These consist of the loop of Henle (nephritic loop), the proximal convoluted tubule, and the distal convoluted tubule, which empties into the collecting ducts. These collecting ducts fuse together and enter the papillae of the renal medulla.

Urine passes through the renal medulla as a fluid with high sodium content and leaves through the renal papillae, into the renal calyces, into the renal pelvis, and into the bladder through the ureter.

For clearer demonstration of nephron watch this: 

Blood Supply of the Kidney

Proper kidney structure and function is dependent on adequate blood supply:

  • The renal artery, which branches out from your abdominal aorta enters the kidney in the renal hilus.
  • Each renal artery divides into the smaller afferent arterioles in the kidney.
  • The afferent arterioles in the renal cortex separate into bundles of capillaries called glomeruli.
  • Each glomerulus recollects into a smaller efferent arteriole that descends into the renal medulla.
  • Each efferent arteriole separates into peritubular capillaries near the renal tubules.
  • Peritubular capillaries merge, forming veins that drain into the renal vein.
  • The renal vein exits each kidney to join the inferior vena cava, which transports blood back to your heart.

Kidney Function

The urinary system depends on proper kidney structure and function. Some of these core actions include:

  • Excretes waste: The kidneys get rid of toxins, urea, andexcess salts. Urea is a nitrogen-based waste product of cell metabolism that is produced in the liver and transported by the blood to the kidneys.
  • Maintains water balance: The kidneys help maintain water and electrolyte balance in the body. They react to changes in the water level, which may increase or decrease throughout the day.
  • Regulates blood pressure: The kidneys help regulate blood pressure by producing angiotensin, a substance that constricts blood vessels and signals the body to retain waterand sodium when blood pressure is low.
  • Regulates red blood cells: The kidneys produce erythropoietin, a hormone that stimulates your bone marrow to produce more red blood cells when the bodydoes not get enough oxygen.
  • Regulates acid levels: Acids are products of metabolism. The kidneys help maintain proper acid-base balance to keep the body healthy.

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