ALFATH: The Kidneys in Detail

The Kidneys in Detail

The kidneys are among the greatest blessings of the Creator upon humans, working silently and masterfully around the clock to filter blood, regulate the body's balance, and maintain the continuity of life with amazing efficiency.

First: Shape and Location (in very detail)

1. General Shape of the Kidney

• The kidney resembles a bean or kidney shape (bean-shaped), a shape that facilitates the curvature of the ureter and its connection to the kidney.
• The kidney has:
  • A convex lateral border.
  • A concave medial border called the Renal Hilum or Renal Hilus where the renal artery enters and the renal vein and ureter exit.
• Kidney surface:
  • Smooth in a normal person.
  • Dark red in color due to the high blood content.

Interestingly: Despite this size, each kidney contains over a million nephrons responsible for delicate vital processes.

2. Dimensions and Weight

• Kidney length: 10–12 cm.
• Width: 5–7 cm.
• Thickness: 2.5–3 cm.
• Weight of one kidney:
  • Men: 125–170 grams.
  • Women: 115–155 grams.

3. Location in the Body

• The kidneys are located in the posterior region of the abdominal cavity, specifically in the upper abdomen.
• They are considered retroperitoneal organs, meaning they lie behind the peritoneal membrane and not within it.
• Vertebral level:
  • The left kidney is located approximately opposite vertebrae T12 – L3.
  • The right kidney sits about 1–2 cm lower than the left due to pressure from the liver, approximately at level L1 – L3.

4. Difference Between the Right and Left Kidney

• Right kidney:
  • Lower in position.
  • Sits directly under the liver.
  • Internally related to the duodenum.
  • Anterior relations: Right lobe of the liver, ascending colon.
• Left kidney:
  • Slightly higher.
  • Directly related to:
    • Stomach
    • Pancreas
    • Spleen
    • Descending colon

5. Layers Surrounding the Kidney (Multi-level Protection)

The kidney is a highly protected organ due to its importance, and thus it is surrounded by several layers:

1. Renal Capsule
   A thin membrane adherent to the kidney that maintains its shape and protects it from infection.
2. Perirenal Fat
   Acts as a cushion that absorbs shocks.
3. Gerota's Fascia
   A fibrous layer that anchors the kidney in place and prevents excessive movement.
4. Pararenal Fat
   Protects the organ and maintains its position.

These layers make the kidney stable and protected despite its proximity to many organs and its sensitivity.

6. Relationship with Adjacent Organs

• Posteriorly:
  • Diaphragm.
  • Psoas major muscle.
  • Quadratus lumborum muscle.
  • Ribs:
    • The left kidney is often covered by ribs 11 and 12.
    • The right kidney may be covered only by rib 12.
• Anteriorly:
  • Liver (right), Spleen (left).
  • Stomach (anterior left).
  • Pancreas.
  • Parts of the colon.

7. Renal Hilum – The Passage of Life

At the concave medial border lies the "Renal Hilum," which is:

• An opening through which enters:
  • Renal Artery
• And from which exits:
  • Renal Vein
  • Ureter
  • Nerves and lymphatic vessels

Their order from front to back:
1. Renal Vein
2. Renal Artery
3. Ureter

Second: Internal Structure of the Kidney (in very precise detail)

The kidney's interior is one of the most complex anatomical structures in the human body, containing delicate systems for filtering blood and regulating fluids, with each part performing a specific and essential function.

1. Renal Cortex

• This is the outer layer of the kidney, slightly lighter in color than the medulla.
• It contains:
  • Glomeruli in high density.
  • The initial parts of the renal tubules: proximal tubule, distal tubule.
• Its primary function:
  • Initiating the filtration and purification of blood.
  • Primary reabsorption of vital substances.
• Contains what is known as Renal Columns, which are extensions of the cortex descending between the pyramids.

2. Renal Medulla

• This is the inner region with a darker color.
• It consists of:
  • Renal Pyramids: Typically 8 to 18 pyramids.
  • Each pyramid ends in a Renal Papilla that drains urine into the calyx.
• The tubules here are long and are responsible for:
  • Concentrating urine.
  • Maintaining salt balance.

Organization within the Medulla:

• Contains:
  • The long Loop of Henle
  • The straight part of the tubules
  • Collecting ducts
• The blood vessels in the medulla (vasa recta) create a countercurrent system that helps concentrate urine precisely.

3. Renal Papilla

• These are the ends of the renal pyramids.
• They open into:
  • Minor Calyces.
• They allow urine to exit from the collecting duct unit into the collection system.

4. Renal Calyces

1. Minor Calyces:
   Receive urine from the papillae.
2. Major Calyces:
   Formed by the union of several minor calyces.
3. Renal Pelvis:
   Collects urine from the major calyces.
   Connects directly to the Ureter.

This system works to transfer urine gradually without backflow to prevent infection.

5. Nephron – The Basic Functional Unit

Each kidney contains approximately one million nephrons, which are the units directly responsible for urine formation.

Components of the Nephron in detail:

Firstly: The Filtration Part

1. Glomerulus
   A dense network of capillaries.
   Blood is filtered here under pressure.
2. Bowman's Capsule
   A double-walled sac surrounding the glomerulus.
   Receives the initial filtrate (Plasma Filtrate).

Secondly: The Tubular Part

1. Proximal Convoluted Tubule (PCT)
   Where most of the following occurs:
   • Glucose reabsorption (100%).
   • Amino acid reabsorption.
   • High percentage of water reabsorption.
   • Sodium reabsorption.
2. Loop of Henle
   Consists of:
   • Descending limb: Allows water to leave.
   • Ascending limb: Allows salts (Na+, Cl−) to leave, does not allow water to pass.
   It is responsible for creating the concentration gradient within the medulla, thus concentrating urine.
3. Distal Convoluted Tubule (DCT)
   Here, salt concentration is regulated.
   Responds to hormones such as:
   • Aldosterone (for adjusting sodium and potassium).
4. Collecting Duct
   Extends through the cortex and then through the medulla.
   Plays a major role in:
   • Controlling the amount of water retained via the ADH hormone.
   • Producing final urine.

6. Juxtaglomerular Apparatus

Consists of:

• Juxtaglomerular (JG) cells: Secrete the hormone renin.
• Macula densa: Senses sodium concentration.
• Extraglomerular mesangial cells.

Its importance:

• Regulates blood pressure.
• Regulates Glomerular Filtration Rate (GFR).
• Balances fluids and salts.

7. Renal Blood Vessels

• Renal artery → Arcuate arteries → Interlobar arteries → Cortical radiate arteries → Glomerular capillaries.
• After the glomerulus:
  • Efferent arteriole then
  • Peritubular capillary network or vasa recta.

This unique system makes the kidney one of the highest blood-consuming organs (20–25% of cardiac output).

Third: Basic Kidney Functions (in very precise detail)

The kidneys are not just an organ for urine production; they are an integrated regulatory center that maintains the body's internal balance (Homeostasis). The kidney performs many vital functions that ensure the continuation of life, which can be summarized in seven main functions with precise processes:

1. Blood Purification and Waste Removal (Excretion)

This is the most famous function of the kidney, involving the removal of toxic substances resulting from metabolic processes:

Main wastes excreted by the kidney:

• Urea: Produced from protein breakdown.
• Creatinine: Result of muscle metabolism.
• Uric acid: Produced from nucleic acid breakdown.
• Drug residues and toxins.

Mechanism of action:

• Approximately 180 liters of plasma reach the kidneys daily.
• Harmful substances are filtered, beneficial ones are reabsorbed, and harmful ones are excreted in urine.

Without this function, blood poisoning could occur within hours.

2. Regulation of Body Water Balance (Water Balance)

The kidneys control the amount of water the body retains or excretes.

How is this done?

• When there is water deficiency:
  • The body secretes ADH hormone, which makes the collecting ducts reabsorb water, making urine concentrated.
• When there is excess water:
  • The amount of ADH decreases and urine becomes dilute.

Importance:

• Preventing dehydration.
• Maintaining blood volume.
• Regulating arterial pressure.

3. Regulation of Salt and Mineral Concentration (Electrolyte Balance)

The kidneys are the main system responsible for ion balance.

Main regulated ions:

• Sodium (Na+): For regulating pressure and fluid volume.
• Potassium (K+): Essential for heart rhythm and muscle functions.
• Chloride (Cl−), Calcium (Ca2+), Magnesium.

How does the kidney regulate salts?

• Reabsorption or secretion of ions according to the body's needs.
• Response to hormones such as:
  • Aldosterone: Retains sodium and excretes potassium.
  • Parathyroid hormone (PTH): Regulates calcium.

4. Maintaining Acid-Base Balance (Acid-Base Balance)

The kidneys work with the lungs to regulate blood pH.

How does the kidney do this?

• Excreting excess hydrogen ions (H+).
• Reabsorbing bicarbonate (HCO3−).
• Producing new bicarbonate to compensate for lost amounts.

Importance:
Maintaining blood pH between 7.35–7.45, which is essential for enzyme and cell function.

5. Regulation of Blood Pressure (Blood Pressure Regulation)

The kidneys control pressure through a very sensitive system called:

The Renin–Angiotensin–Aldosterone System (RAAS)

When there is a decrease in:

• Blood pressure
• Blood volume
• Sodium concentration

The kidney cells secrete the hormone renin, which initiates a chain leading to:

• Vasoconstriction (raising pressure).
• Sodium and water retention.

This system is responsible for stabilizing pressure at a very precise level.

6. Production of Hormones and Vital Substances (Endocrine Function)

The kidney is also an endocrine gland, secreting several important hormones:

1. Erythropoietin (EPO)
   Stimulates bone marrow to produce red blood cells.
   Decreases in renal failure → leads to anemia.
2. Renin
   A key component in blood pressure regulation.
3. Prostaglandins
   Help dilate blood vessels and regulate blood flow.

7. Activation of Vitamin D (Vitamin D Activation)

The kidneys convert vitamin D from its inactive form to the active form, Calcitriol.

Importance of Calcitriol:

• Increases calcium absorption from the intestines.
• Maintains bone and dental health.
• Prevents osteoporosis.

Therefore, patients with kidney dysfunction suffer from calcium deficiency and bone disorders.

8. Removal of Pharmacological Substances and Regulation of Their Concentrations (Drug metabolism)

• The kidneys subject many drugs to processes:
  • Filtration
  • Secretion
  • Reabsorption
• Which determines the drug's efficacy and duration in the body.

9. Maintenance of Blood Volume and Plasma Composition

Through:

• Controlling protein ratios.
• Regulating dissolved molecules in plasma.

Summary of Functions

The kidneys are responsible for:

• Blood purification
• Water regulation
• Salt balance
• Acid balance
• Blood pressure
• Hormone production
• Vitamin D activation

Thus, the kidney forms the center of the body's internal regulation, and any disturbance in it affects all other body systems.

Fourth: Mechanism of Urine Formation (in very precise detail)

Urine formation is a delicate biological process that takes place within the nephrons of the kidney, aiming to purify the blood and regulate the balance of fluids and salts. This process goes through three main stages:

1. Glomerular Filtration

Where does it occur?
In the glomerulus and Bowman's capsule.

How filtration occurs:

• Blood enters the glomerulus via the Afferent arteriole.
• Pressure inside the glomerulus is high (~55 mmHg), higher than most capillaries in the body.
• This pressure pushes plasma (without cells or large proteins) to pass through:
  1. The capillary endothelium
  2. The basement membrane
  3. Podocyte cells in Bowman's capsule

Components of the initial filtrate (Filtrate):

• Water
• Ions (Na+, K+, Cl−)
• Glucose
• Amino acids
• Metabolic byproducts

Glomerular Filtration Rate (GFR):

• Approximately 125 ml/minute
• That is 180 liters of filtrate daily.

From this large volume, most fluids are reabsorbed, and only 1–2 liters of urine are excreted daily.

2. Tubular Reabsorption

Where does it occur?
In:

• Proximal Convoluted Tubule (PCT)
• Loop of Henle
• Distal Tubule
• Collecting Duct

Most important substances reabsorbed:

• Glucose: 100%
• Amino acids: 100%
• Water: 65–80%
• Sodium: 65–70%
• Bicarbonate (for acidity regulation)

Most important site for reabsorption:
The Proximal Convoluted Tubule (PCT) performs the largest amount of reabsorption (more than 70%).

Details according to reabsorption site:

1. Proximal Convoluted Tubule (PCT)
   • Relies on active and secondary transport.
   • Reabsorbs:
     • Glucose
     • Amino acids
     • Na+, K+, Cl−
     • Bicarbonate
   • Also reabsorbs water via osmosis.
2. Loop of Henle

   Plays an important role in concentrating urine via what is called "countercurrent multiplication."

   • Descending limb:
     • Allows only water to pass.
     • Concentrates the filtrate.
   • Ascending limb:
     • Allows salts (Na+, Cl−) to pass.
     • Does not allow water to pass.
     • Dilutes the filtrate.
3. Distal Convoluted Tubule (DCT)
   • Additional sodium reabsorption under the influence of:
     • Aldosterone
   • Final adjustment of calcium under the influence of:
     • Parathyroid hormone (PTH)
4. Collecting Duct
   • Responsible for controlling the final amount of water.
   • Depends on ADH hormone:
     • When water is deficient: Increases tubule permeability to water → concentrated urine.
     • When water is abundant: Decreases ADH effect → dilute urine.

3. Tubular Secretion

Where does it occur?
In:

• Proximal tubule
• Distal tubule

What is secreted?

• Hydrogen H+ (acidity regulation)
• Ammonia
• Drug residues (like penicillin)
• Potassium K+ under the influence of aldosterone
• Toxins that were not filtered in the glomerulus

Importance of tubular secretion:

• Disposal of toxic substances.
• Maintaining acid-base balance.
• Controlling potassium concentration, essential for regulating heart rhythm.

4. Collection and Final Excretion of Urine

What happens after urine formation is complete?

1. Urine reaches the renal papilla.
2. Enters the minor calyces → major calyces.
3. Collects in the renal pelvis.
4. Passes through the ureter to the bladder.
5. Stored until urination via the urethra.

Summary of Urine Formation Mechanism

The process relies on three main steps:

1. Filtration
   Separating small substances from the blood.
2. Reabsorption
   Recovering the beneficial (water, glucose, salts).
3. Secretion
   Disposing of harmful substances.

Final result: Urine containing waste and fluids unnecessary for the body, while the body retains what it needs.

Fifth: Diseases that may affect the kidneys (a scientific summary)

The kidneys are subject to a number of disorders that may affect their ability to filter and regulate body fluids. Among the most important of these diseases are:

1. Renal Failure

a) Acute Kidney Injury

• Occurs suddenly within hours or days.
• Causes:
  • Reduced blood flow to the kidney (ischemia).
  • Obstruction of the ureter or bladder.
  • Intake of nephrotoxic drugs.
• Treatment possibility: High if the cause is treated quickly.

b) Chronic Kidney Disease

• Occurs gradually over years.
• Causes:
  • Diabetes.
  • High blood pressure.
  • Chronic kidney inflammation.
• Consequences:
  • Accumulation of toxins.
  • Electrolyte disturbances.
  • Anemia.
  • Osteoporosis.
• The patient may need dialysis or kidney transplantation.

2. Kidney Stones

• Form as a result of the deposition of salts and minerals in the urine.
• Most common types:
  • Calcium oxalate stones.
  • Uric acid stones.
• Symptoms:
  • Severe flank pain.
  • Blood in urine.
  • Nausea.
• Causes:
  • Insufficient water intake.
  • High salt content.
  • Metabolic disorders.

3. Kidney Infections

a) Pyelonephritis

• Bacterial infection affecting the pelvis and kidney tissue.
• Symptoms:
  • Fever.
  • Back pain.
  • Painful urination.
• May cause damage if not treated.

b) Glomerulonephritis

• Affects the glomeruli responsible for filtration.
• May occur after:
  • Bacterial infection.
  • Autoimmune diseases.
• Symptoms:
  • Blood in urine.
  • Swelling of the face and legs.
  • High blood pressure.

4. Renal Syndromes

a) Nephrotic Syndrome

• Caused by damage to the glomerular membrane leading to protein loss.
• Main signs:
  • Large amounts of protein in urine.
  • Severe swelling (Edema).
  • High blood lipids.

b) Nephritic Syndrome

• Characterized by:
  • Blood in urine.
  • High blood pressure.
  • Reduced glomerular filtration rate.

5. Polycystic Kidney Disease (PKD)

• An inherited disease characterized by the appearance of fluid-filled cysts inside the kidneys.
• Gradually leads to:
  • Kidney enlargement.
  • Decline in kidney function.
• May cause kidney failure in middle age.

6. Urinary Obstruction

• Caused by:
  • Stones.
  • Tumors.
  • Prostate enlargement.
• Leads to:
  • Backflow of urine towards the kidney.
  • Dilation of the pelvis (Hydronephrosis).
  • Kidney tissue damage if obstruction persists.

7. Renovascular Hypertension

• Caused by narrowing of the renal artery.
• Leads to:
  • Excessive renin secretion.
  • Severely high blood pressure.

8. Drug-Induced Kidney Diseases (Drug-Induced Nephropathy)

• Some drugs harm the kidneys, such as:
  • Non-steroidal anti-inflammatory drugs (NSAIDs).
  • Some antibiotics.
  • Chemotherapeutic drugs.
• Causes:
  • Acute kidney failure.
  • Tubular inflammation.

Brief Summary

Kidney diseases can be:

• Acute (occur suddenly)
• Chronic (develop gradually)
• Inflammatory
• Genetic
• Metabolic (like stones)
• Obstructive
• Drug-induced

All affect the kidney's ability to purify blood and regulate salts and fluids.

Conclusion

The kidneys are among the greatest blessings of the Creator upon humans, working silently and masterfully around the clock to filter blood, regulate the body's balance, and maintain the continuity of life with amazing efficiency. By understanding their precise structure, vital functions, and complex mechanisms of action, we realize their importance and the necessity of protecting them from factors that may threaten their integrity. Maintaining a healthy lifestyle and awareness of common diseases that may affect the kidneys are essential steps to ensure this vital organ continues to perform its ideal role. Ultimately, the kidneys remain an example of the precision of creation and the magnificence of the biological system that grants humans the ability to live in health and balance.