Kidney Anatomy: The Inside Story

The kidneys, those bean shaped organs tucked away inside your body, play an incredibly crucial role in maintaining your overall health. While we might not think about them often, these unassuming organs are responsible for filtering waste, regulating blood pressure, balancing electrolytes, and more. Let’s take a closer look at the intricate anatomy of these unsung heroes.

Gross structure of the kidney

The two kidneys, each weighing 150g in adults, are located retroperitoneally in the abdominal cavity on either side of the vertebral column. The vertical section of the kidneys shows the following : 

• Outer cortex – Reddish in color
• Inner medulla – Pale in color. It contains 10 – 15 pyramids which terminate medially in the renal papillae. 

Key components

Nephrons

Inside each kidney, you’ll find around a million tiny filtering units called nephrons. These nephrons are the workhorses of the kidney, responsible for filtering your blood and removing waste products and excess fluids to create urine. Each nephron consists of a renal corpuscle and a renal tubule, working together to filter and regulate your body’s fluid balance.

There are two types of nephrons in the kidney. They are cortical and juxta medullary nephrons. Cortical nephrons have smaller size glomeruli located in the renal cortex. They comprise 85% – 86% nephrons in the kidney. These nephrons play a major role in excretion of waste products in dissolved form in the urine. Juxta medullary  nephrons  have larger size glomeruli located at the junction of the cortex and the medulla of the kidney. They possess long loops of Henle which penetrate deep into medulla. Both the descending and ascending limbs of the loop of Henle contain thin segments. Juxta medullary nephrons are important in the ‘countercurrent system’ by which the kidneys concentrate urine. 

Renal arteries and veins

The kidneys receive blood through the renal arteries, which branch off from the abdominal aorta. Blood is filtered in the nephrons, and the filtered blood returns to circulation through the renal veins.

Ureters

The ureters are muscular tubes that connect the kidneys to the urinary bladder. They transport urine from the kidneys to the bladder for storage before elimination.

Renal pelvis

The renal pelvis is a funnel-shaped structure that collects urine from the renal pyramids. From the renal pelvis, urine flows into the ureters.

Functions of kidney

Excretion of metabolic waste products

The main function of the kidney is to filter blood and reject its waste products dissolved in the urine. Some major compounds that the kidney remove are : 

• Uric acid from the breakdown of nucleic acid.
• Drugs and their metabolites.
• Urea, which results from the breakdown of proteins. 

Reabsorption of nutrients

They reabsorb nutrients from the blood using tubules and transport them to where they will best support health. They also reabsorb other products to help maintain homeostasis. Reabsorbed products include:

• Water
• Phosphate
• Glucose
• Bicarbonate
• Amino Acids
• Chloride, sodium, magnesium and potassium ions. 

Maintaining pH

The kidneys control the acid-base balance of the blood. When the blood becomes too acidic (lower pH), they increase the excretion of hydrogen ions and enhance bicarbonate reabsorption. Conversely, when the blood becomes too alkaline (higher pH), the kidneys reduce the reabsorption of bicarbonate and increase hydrogen ion secretion. 

Regulation of blood pressure

They play a critical role in regulating blood pressure. They do so by controlling the volume of blood and the concentration of salts (sodium and potassium) in the body. The renin-angiotensin-aldosterone system is a key mechanism in blood pressure regulation.

Metabolism of vitamin D 

The kidneys activate vitamin D by converting it into its active form, which is essential for calcium absorption in the intestines and bone health

Glucose Regulation

While the primary function of the kidneys is not glucose regulation, they do contribute to maintaining stable blood glucose levels by reabsorbing glucose from the urine when necessary. 

Blood supply

They have an extensive blood supply. They receive approximately 20% of the blood pumped by your heart. This rich blood flow ensures that they can efficiently filter waste products and maintain a stable internal environment.

Blood supply of kidney

Renal Arteries:

The journey of blood to the kidneys begins with the renal arteries, which branch off from the abdominal aorta. The two renal arteries, one for each kidney, carry oxygenated blood rich in nutrients to fuel the organ’s demanding metabolic processes. This arterial blood supply is fundamental for it’ ability to perform their critical functions.

Renal Segmental Arteries:

As the renal arteries penetrate the kidney, they branch into segmental arteries, further dividing into inter-lobar arteries. These vessels traverse the renal columns, eventually reaching the cortex of the kidney.

Inter-lobar Arteries and Arcuate Arteries:

The inter-lobar arteries ascend between the renal pyramids, giving rise to arcuate arteries at the junction of the renal cortex and medulla. These arcuate arteries arch along the corticomedullary junction, giving off branches into the renal cortex.

Afferent Arterioles and Glomerular Filtration:

At the cortex, the arcuate arteries give rise to afferent arterioles, which play a pivotal role in the filtration process. The afferent arterioles lead to the formation of glomerular capillaries within the renal corpuscles, where blood filtration initiates. The high pressure in the glomerular capillaries facilitates the movement of water and solutes into Bowman’s capsule, initiating the formation of urine.

Efferent Arterioles and Peritubular Capillaries:

The blood that exits the glomerular capillaries through different arterioles enters a network of peritubular capillaries that surrounds the renal tubules. This intricate network provides oxygen and nutrients to the nephrons, ensuring that essential substances are reabsorbed and waste products are secreted into the urine.

Renal Veins:

After the blood has been filtered and the necessary processes have taken place within it, it is collected by the renal veins. Renal veins mirror the arterial pathway, with blood returning to the inferior vena cava and subsequently re-entering systemic circulation.

The blood supply of it is a symphony of arteries, arterioles, and capillaries that ensures the organs receive the nutrients they need while efficiently filtering and cleansing the bloodstream.

Conclusion

The anatomy of the kidney is a testament to the precision and complexity of the human body’s design. These small organs play a monumental role in maintaining homeostasis and ensuring the elimination of waste. Understanding the intricacies of it’s structure provides us with a deeper appreciation for the remarkable processes that contribute to our overall health and well-being.