What Are The Functional Units Of The Kidneys

The kidneys, vital organs responsible for filtering blood, regulating blood pressure, and maintaining electrolyte balance, perform these complex functions through specialized microscopic structures known as nephrons. These nephrons are the functional units of the kidneys, each capable of independently producing urine. Understanding their structure and function is key to understanding overall kidney health.

Nephron: The Functional Unit of the Kidney

Each kidney contains approximately one million nephrons, working tirelessly to filter waste products and excess substances from the blood. They selectively reabsorb essential nutrients and water, returning them to the bloodstream, while excreting waste as urine. The nephron's structure is intricately designed to facilitate these processes, consisting of two main components: the renal corpuscle and the renal tubule.

Renal Corpuscle: The Filtration Hub

The renal corpuscle is the initial filtration unit of the nephron, responsible for filtering blood. It consists of two structures: the glomerulus and the Bowman's capsule.

• Glomerulus: This is a network of tiny blood capillaries responsible for the filtration of blood.
• Bowman's Capsule: A cup-shaped structure that surrounds the glomerulus, collecting the filtered fluid.

Blood enters the glomerulus through the afferent arteriole and exits through the efferent arteriole. The pressure of blood within the glomerulus forces water, ions, glucose, amino acids, urea, and other small molecules across the filtration membrane and into Bowman's capsule. This filtrate, now called glomerular filtrate, is essentially blood plasma without the large proteins and blood cells.

The filtration membrane consists of three layers:

1. The endothelium of the glomerular capillaries: This layer has small pores called fenestrations, which allow most substances to pass through but prevent blood cells and large proteins from filtering out.
2. The basement membrane: This layer is a gel-like matrix composed of glycoproteins and proteoglycans. It further restricts the passage of large proteins.
3. The podocytes of the Bowman's capsule: These specialized cells have foot-like processes called pedicels that interdigitate to form filtration slits. These slits are covered by a thin diaphragm that acts as the final barrier to protein filtration.

Renal Tubule: Fine-Tuning the Filtrate

The renal tubule is a long, convoluted tube that extends from Bowman's capsule. It is responsible for reabsorbing essential substances from the glomerular filtrate and secreting additional waste products into the filtrate. The renal tubule is divided into three main sections:

1. Proximal Convoluted Tubule (PCT): The PCT is the first and longest section of the renal tubule. It is highly coiled and lined with simple cuboidal epithelial cells with numerous microvilli on their apical (luminal) surface. These microvilli increase the surface area for reabsorption.

   • Reabsorption in the PCT: The PCT is responsible for reabsorbing approximately 65% of the glomerular filtrate. This includes:

     • All of the glucose and amino acids
     • Most of the sodium, potassium, chloride, bicarbonate, and water
     • Urea and other solutes

   • Secretion in the PCT: The PCT also secretes certain substances into the filtrate, including:

     • Hydrogen ions
     • Ammonium ions
     • Creatinine
     • Some drugs and toxins

2. Loop of Henle: The loop of Henle is a U-shaped section of the renal tubule that extends into the medulla of the kidney. It consists of two limbs:

   • Descending limb: This limb is permeable to water but impermeable to sodium chloride. As the filtrate flows down the descending limb, water moves out of the tubule and into the hypertonic interstitial fluid of the medulla, concentrating the filtrate.
   • Ascending limb: This limb is impermeable to water but permeable to sodium chloride. As the filtrate flows up the ascending limb, sodium chloride moves out of the tubule and into the interstitial fluid of the medulla, diluting the filtrate. The ascending limb also has an active transport mechanism that pumps sodium chloride into the interstitial fluid, further contributing to the hypertonicity of the medulla.

   The loop of Henle plays a crucial role in establishing a concentration gradient in the medulla, which is essential for the kidney's ability to produce concentrated urine.

3. Distal Convoluted Tubule (DCT): The DCT is the final section of the renal tubule. It is shorter and less coiled than the PCT and is lined with simple cuboidal epithelial cells with fewer microvilli.

   • Reabsorption in the DCT: The DCT is responsible for reabsorbing sodium, chloride, and water under the influence of hormones:

     • Aldosterone: Increases sodium and chloride reabsorption.
     • Antidiuretic hormone (ADH): Increases water reabsorption.

   • Secretion in the DCT: The DCT also secretes potassium and hydrogen ions into the filtrate, helping to regulate blood pH and electrolyte balance.

Collecting Duct: Final Adjustments and Urine Collection

The collecting duct is not technically part of the nephron, but it receives filtrate from multiple nephrons and plays a crucial role in determining the final urine concentration. As the filtrate flows through the collecting duct, it passes through the hypertonic medulla. In the presence of ADH, the collecting duct becomes permeable to water, allowing water to move out of the filtrate and into the interstitial fluid, concentrating the urine. Without ADH, the collecting duct remains impermeable to water, resulting in dilute urine.

The collecting ducts eventually merge and empty into the renal pelvis, which collects the urine and directs it to the ureter.

Types of Nephrons

Not all nephrons are created equal. There are two main types of nephrons, classified based on their location within the kidney and the length of their loops of Henle:

1. Cortical Nephrons: These nephrons are located primarily in the cortex of the kidney, with short loops of Henle that barely penetrate into the medulla. They account for approximately 85% of the nephrons in the kidney. Cortical nephrons are primarily responsible for removing waste products and reabsorbing nutrients.
2. Juxtamedullary Nephrons: These nephrons are located near the medulla of the kidney, with long loops of Henle that extend deep into the medulla. They account for approximately 15% of the nephrons in the kidney. Juxtamedullary nephrons play a critical role in concentrating urine. Their long loops of Henle create a steeper concentration gradient in the medulla, allowing the kidney to produce more concentrated urine when needed.

The vasa recta are specialized peritubular capillaries that surround the loops of Henle of juxtamedullary nephrons. They run parallel to the loops of Henle and play a vital role in maintaining the concentration gradient in the medulla by preventing the washout of solutes.

Juxtaglomerular Apparatus: Regulating Blood Pressure and Filtration Rate

The juxtaglomerular apparatus (JGA) is a specialized structure located near the glomerulus. It plays a crucial role in regulating blood pressure and the glomerular filtration rate (GFR). The JGA consists of three main components:

1. Macula densa: These are specialized cells in the wall of the distal convoluted tubule (DCT). They monitor the sodium chloride concentration in the filtrate.
2. Juxtaglomerular (JG) cells: These are modified smooth muscle cells in the wall of the afferent arteriole. They secrete renin in response to low blood pressure or low sodium chloride concentration in the filtrate.
3. Extraglomerular mesangial cells: These cells are located between the afferent and efferent arterioles and may play a role in communication between the macula densa and the JG cells.

Renin-Angiotensin-Aldosterone System (RAAS)

When blood pressure or sodium chloride concentration in the filtrate decreases, the macula densa stimulates the JG cells to release renin. Renin is an enzyme that converts angiotensinogen (produced by the liver) into angiotensin I. Angiotensin I is then converted into angiotensin II by angiotensin-converting enzyme (ACE), which is primarily found in the lungs. Angiotensin II has several effects that help to raise blood pressure and increase sodium chloride reabsorption:

• Vasoconstriction: Angiotensin II is a potent vasoconstrictor, narrowing blood vessels and increasing blood pressure.
• Aldosterone secretion: Angiotensin II stimulates the adrenal cortex to release aldosterone. Aldosterone increases sodium chloride reabsorption in the DCT and collecting duct, which in turn increases water reabsorption and blood volume.
• ADH secretion: Angiotensin II stimulates the pituitary gland to release ADH. ADH increases water reabsorption in the collecting duct, which also increases blood volume.
• Thirst stimulation: Angiotensin II stimulates the thirst center in the brain, increasing fluid intake.

By increasing blood pressure and blood volume, the RAAS helps to maintain adequate blood flow to the kidneys and ensure proper glomerular filtration.

Glomerular Filtration Rate (GFR)

The glomerular filtration rate (GFR) is the volume of filtrate formed per minute by all the nephrons in both kidneys. It is a key indicator of kidney function. A normal GFR is typically around 125 mL/min, which means that the kidneys filter about 180 liters of fluid per day. However, most of this fluid is reabsorbed, and only about 1-2 liters is excreted as urine.

The GFR is influenced by several factors, including:

• Blood pressure: Higher blood pressure increases the GFR, while lower blood pressure decreases it.
• Afferent and efferent arteriolar resistance: Constriction of the afferent arteriole decreases the GFR, while constriction of the efferent arteriole increases it (up to a point).
• Plasma protein concentration: Higher plasma protein concentration decreases the GFR, while lower plasma protein concentration increases it.
• Bowman's capsule pressure: Higher Bowman's capsule pressure decreases the GFR, while lower Bowman's capsule pressure increases it.

The kidneys have several mechanisms to maintain a relatively constant GFR, even when blood pressure fluctuates. These mechanisms include:

• Myogenic mechanism: This is an intrinsic ability of the afferent arteriole to constrict or dilate in response to changes in blood pressure.
• Tubuloglomerular feedback: This is a mechanism by which the macula densa senses changes in sodium chloride concentration in the filtrate and signals the afferent arteriole to constrict or dilate accordingly.

Common Kidney Diseases Affecting Nephron Function

Several diseases and conditions can damage the nephrons and impair their ability to function properly. Some common examples include:

• Glomerulonephritis: Inflammation of the glomeruli, which can be caused by infection, autoimmune disease, or other factors. Glomerulonephritis can damage the filtration membrane, leading to protein and blood in the urine.
• Diabetic nephropathy: Damage to the nephrons caused by diabetes. High blood sugar levels can damage the glomeruli and other parts of the nephron, leading to kidney failure.
• Hypertensive nephrosclerosis: Damage to the nephrons caused by high blood pressure. High blood pressure can damage the blood vessels in the kidneys, leading to decreased blood flow and nephron damage.
• Polycystic kidney disease (PKD): A genetic disorder characterized by the growth of numerous cysts in the kidneys. These cysts can damage the nephrons and lead to kidney failure.
• Acute kidney injury (AKI): A sudden loss of kidney function, which can be caused by various factors, including dehydration, infection, and medications.
• Chronic kidney disease (CKD): A progressive loss of kidney function over time. CKD can be caused by various factors, including diabetes, high blood pressure, and glomerulonephritis.

Maintaining Healthy Nephrons

Taking care of your kidneys is essential for overall health. Here are some tips to help maintain healthy nephrons:

• Control blood pressure: High blood pressure can damage the nephrons, so it is important to keep blood pressure under control through diet, exercise, and medication if needed.
• Manage blood sugar: High blood sugar levels can also damage the nephrons, so it is important to manage blood sugar levels through diet, exercise, and medication if needed.
• Maintain a healthy weight: Being overweight or obese can increase the risk of developing kidney disease.
• Eat a healthy diet: A diet that is low in sodium, saturated fat, and processed foods can help protect the kidneys.
• Drink plenty of fluids: Staying hydrated helps the kidneys flush out waste products.
• Avoid smoking: Smoking can damage the blood vessels in the kidneys.
• Limit alcohol consumption: Excessive alcohol consumption can damage the kidneys.
• Avoid overuse of pain medications: Nonsteroidal anti-inflammatory drugs (NSAIDs) can damage the kidneys if taken in large doses or for long periods of time.
• Get regular checkups: Regular checkups with a doctor can help detect kidney problems early when they are most treatable.

Conclusion

The nephron is the fundamental functional unit of the kidney, orchestrating the complex processes of filtration, reabsorption, and secretion to maintain fluid and electrolyte balance, regulate blood pressure, and eliminate waste. Understanding the nephron's structure and function is crucial for appreciating the remarkable capabilities of the kidneys and for maintaining overall health. By adopting healthy lifestyle habits and seeking medical attention when needed, individuals can protect their nephrons and ensure optimal kidney function for years to come.