Structure of Kidneys | Nephrolounge

The Physical Structure of Kidneys

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Understanding the composition and structure of the kidneys is crucial for appreciating their vital role in maintaining overall health and well-being.

Shape and Size:

The Kidneys are Bean-shaped organs.

A healthy kidney is about the size of a fist. On average, adult human kidneys measure about 10-12 centimeters (4-5 inches) in length, 5-7 centimeters (2-3 inches) in width, and 2-3 centimeters (0.8-1.2 inches) in thickness. The total volume thus ranges from 125cc to 170cc.

Anatomy:

Anatomically they are located in the abdominal cavity, in the back of the Abdomen, behind the peritoneum. The 2 Kidneys are positioned symmetrically on either side of the spine, just below the rib cage. Typically, the right kidney sits slightly lower than the left kidney due to the presence of the liver on the right side.

Each kidney is positioned adjacent to the adrenal gland on its respective side.

Their anatomical location places them in close proximity to other organs such as the liver, spleen, pancreas, and intestines.

This strategic placement allows the kidneys to efficiently perform their essential functions of filtering waste products from the blood and regulating fluid and electrolyte balance in the body.

Main Components of a Kidney:

The kidneys have 7 key components that work together to perform its basic function of filtering and other physiological functions.

1. Cortex:

This is the outer layer of the kidney.

It comprises of renal corpuscles, convoluted tubules, and blood vessels. The renal cortex contains nephrons, the functional units responsible for filtering blood and producing urine.

2. Medulla:

The inner region of the kidney, arranged in a pyramid-like structure called renal pyramids. The renal medulla consists of renal tubules, collecting ducts, and blood vessels. It plays a role in transporting urine from the nephrons to the renal pelvis.

Note: A diseased kidney often is found to lack "Cortico-Meduallary differentiation", which in simple terms means lacks the visual differentiation between Cortex and Medulla.

3. Renal Pelvis:

The renal pelvis is connected to the ureter, which carries urine from the kidneys to the bladder. It is a funnel-shaped structure located at the center of the kidney, where urine collects before it is transported out to ureter.

4. Nephrons:

The microscopic functional units of the kidney responsible for filtering blood and producing urine. We will know more in details about Nephrons in the next section.

5. Renal Blood vessels:

The kidneys receive a rich blood supply through the renal arteries, which branch into smaller vessels called arterioles.

Arterioles supply blood to the glomeruli within the renal cortex, where filtration occurs. After filtration, blood exits the kidneys through the renal veins.

6. Renal Capsule

A fibrous, transparent membrane that surrounds each kidney, providing support and protection. The renal capsule helps maintain the structural integrity of the kidney and protects it from injuries or damage.

7. Renal Calcyces:

These are Cup-shaped structures within the renal pelvis that collect urine from the collecting ducts. Renal calyces merge to form the renal pelvis, which serves as a reservoir for urine before it is expelled from the kidney through the ureter.

Nephrons:

Blood is filtered in the kidneys through the Nephrons. Each kidney contains millions of these tiny structures.

The nephrons consist of 2 parts : a) A filtration unit called Renal Corpuscle, and b) A tubule where further processing of filtrate occurs.

The renal corpuscle (filtration unit) comprise of a network of capillaries called the Glomerulus, sorrounded by a capsule known as Bowman's Capsule.

How Glomerulus filter your blood

As blood enters into the Glomerulus, thin walls of the glomerulus allow smaller molecules, wastes, and fluid to pass through the walls into the tubule. The glomerulus walls thus act as a sieve that retains the larger molecules such as Proteins and Red blood cells, while allows smaller molecules and fluids to pass through.

What does the tubule do

The tubule has a blood vessel running alongside, which absorbs almost all of the fluids plus the necessary nutrients. These nutrients are absorbed into the body, while the remaining fluid and wastes turn into urine.

Its important to note that the Tubule helps in removing Acid from our body, helping maintain the pH balance.

The blood that entered the nephron from the renal artery is filtered by tiny blood vessels of the kidneys (glomerulus) and exits via the renal veins.

In case the glomeruli are not functioning properly, larger molecules - protein and red blood cells - can get leaked into the tubule and hence into the urine.

Thus one of the key signs of reduced kidney function is frothy urine, which is cause by Protein in Urine. In advanced stages of Kidney ailment, Blood cells also leak out into the urine.

Appearance of a Diseased Kidney:

Physical changes observed in a diseased kidney can offer valuable insights into the extent of damage and provide valuable data points to nephrologists (kidney doctors) for diagnosis & treatment. Early detection can help in significantly slowing down the progression of kidney disease.

Size alternations: A diseased kidney would be smaller in size compared to a healthy kidney because of death of nephrons. Initially kidneys may enlarge due to compensatory hypertrophy as a natural response of body to nephron loss. In later stages of the disease, kidneys will shrink in size (atrophy) due to loss of nephrons & their replacement with fibrous tissue.

Surface irregularities and Loss of cortico-medullary differentation: Diseased kidneys may exhibit scarring, indentations, or cysts due to conditions such as glomerulonephritis or polycystic kidney disease among others.

CKD will also lead to cortical thinning, as a portion of nephrons (say 10-20%) present in cortex region die and get replaced by fibrous tissue. Fiborsis or scarring may also occur in the Medulla thus impairing flow of urine through collecting ducts. Both these factors together cause a visible change of appearance of the kidneys, where the Cotex and Medulla lose their differentiating physical aspects. This is known as Cortico-Medullary differentiation loss, and is one of the common known characteristics of individuals with Chronic Kidney Disease.

Calcifications: In advanced stages of CKD, calcium deposits or calcifications may occur within the kidney tissue. These calcificatiosn can be seen on Ultra-sonographic scans (USG scans) or other radiological scans such as MRI or CT scans.

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