The Renal System: Detailed Anatomy and Physiology

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Anatomy of the Kidney

The kidneys are a pair of bean-shaped organs located in the posterior abdominal cavity, responsible for filtering blood, excreting waste, and maintaining the body’s electrolyte balance. Each kidney plays a critical role in the urinary system and overall homeostasis. the kidneys are 2 in number.

1. Location and Position:-

  • Location:- The kidneys are retroperitoneal organs, meaning they are located behind the peritoneum, the lining of the abdominal cavity. They are positioned on either side of the spine, roughly between the T12 and L3 vertebrae.
  • Position:- The right kidney is slightly lower than the left due to the presence of the liver. Each kidney lies at an oblique angle, with its medial margin closer to the spine than its lateral margin.

2. Size and Shape:-

  • Shape:- The kidneys are bean-shaped organs, with a convex lateral surface and a concave medial surface.
  • Size:- On average, an adult kidney is about 10-12 cm long, 5-7 cm wide, and 3-5 cm thick.
  • Weight:- Each kidney typically weighs between 120-150 grams.

3. External Structure:-

  • Renal Capsule:- A tough, fibrous capsule surrounds each kidney, providing protection and maintaining its shape.
  • Renal Fascia:- Surrounding the renal capsule is the renal fascia, a layer of connective tissue that anchors the kidney to surrounding structures.
  • Perirenal Fat:- A layer of adipose tissue, called perirenal fat, cushions the kidney and protects it from trauma.
  • Renal Hilum:- The medial border of the kidney features a central indentation called the renal hilum, through which the renal artery, renal vein, and ureter enter and exit.

4. Blood Supply:-

  • Renal Arteries:- Each kidney receives blood from a renal artery, which branches off from the abdominal aorta. The renal artery enters the kidney through the hilum and divides into segmental arteries.
  • Segmental Arteries:- These further divide into interlobar arteries, which travel through the renal columns between the pyramids.
  • Arcuate Arteries:- At the junction of the cortex and medulla, the interlobar arteries arch into arcuate arteries, giving rise to cortical radiate arteries.
  • Cortical Radiate Arteries:- These supply blood to the afferent arterioles, which lead into the glomeruli.
  • Glomerular Capillaries:- Blood is filtered in the glomerular capillaries, and the remaining blood exits via efferent arterioles.
  • Peritubular Capillaries:- The efferent arterioles form peritubular capillaries, which surround the PCT and DCT, aiding in reabsorption and secretion.
  • Vasa Recta:- In the medulla, the efferent arterioles also give rise to the vasa recta, a network of capillaries surrounding the loop of Henle, important for concentrating urine.
  • Venous Drainage:- Blood from the kidney is collected by cortical radiate veins, arcuate veins, interlobar veins, and eventually drains into the renal vein, which exits the kidney through the hilum and empties into the inferior vena cava.

5. Nerve Supply:-

  • Renal Plexus:- The kidneys are innervated by the renal plexus, a network of nerves derived from the sympathetic nervous system. It regulates blood flow, filtration, and urine production through vasoconstriction and vasodilation of renal blood vessels.

6. Lymphatic Drainage:-

  • Lymphatic Vessels:- Lymphatic vessels from the kidney drain into the lumbar lymph nodes, which are located along the abdominal aorta. These vessels play a role in immune surveillance and fluid balance within the kidney.

The function of the Kidneys

The kidneys are essential organs responsible for several critical physiological functions:

  • Filtration of Blood:-

    • The kidneys filter approximately 120-150 liters of blood daily, removing waste products, excess substances, and toxins to form urine.
    • They play an important role in clearing nitrogenous wastes like urea, creatinine, and uric acid.

  • Regulation of Electrolyte Balance:-

    • The kidneys maintain the balance of electrolytes, such as sodium, potassium, calcium, and phosphate, by selectively reabsorbing or excreting these ions.
    • This regulation is crucial for nerve function, muscle contraction, and overall cellular homeostasis.

  • Acid-Base Balance:-

    • The kidneys regulate blood pH by reabsorbing bicarbonate and excreting hydrogen ions and ammonium, thus preventing acidosis or alkalosis.

  • Blood Pressure Regulation:-

    • Through the renin-angiotensin-aldosterone system (RAAS), the kidneys help regulate blood pressure by controlling blood volume and systemic vascular resistance.
    • They also secrete prostaglandins that modulate vascular tone.

  • Erythropoiesis:-

    • The kidneys produce erythropoietin (EPO), a hormone that stimulates the production of red blood cells in the bone marrow in response to hypoxia.

  • Calcium and Phosphate Homeostasis:-

    • The kidneys activate vitamin D to calcitriol, which enhances calcium and phosphate absorption in the intestines, playing a crucial role in bone health.

  • Detoxification:-

    • The kidneys detoxify blood by excreting water-soluble toxins and metabolites of drugs, making them a critical component of the body’s defense against harmful substances.

2. Microscopic Structure of the Kidney

The kidney’s structure is organized into two main regions:-

  • Cortex:-

    • The outer region of the kidney contains the renal corpuscles (glomeruli and Bowman’s capsule) and the proximal and distal convoluted tubules of the nephrons.
    • It is rich in blood vessels, providing the necessary blood supply for filtration, and it appears granular due to the presence of numerous glomeruli.

  • Medulla:-

    • The inner region consists of 8-18 renal pyramids, which are cone-shaped structures containing the loops of Henle and collecting ducts.
    • The apex of each pyramid, known as the renal papilla, points towards the renal pelvis. which drains urine into the minor calyx.
    • The medulla is organized into a series of columns (renal columns) that extend from the cortex and separate the pyramids.
  • Renal Columns:-
    • The renal columns are extensions of the cortex that dip into the medulla, separating the renal pyramids.
    • These columns contain blood vessels and urinary tubules, supporting the kidney’s functional units.
  • Urinary Drainage System:-
    • Minor Calyces:- The renal papillae of the pyramids open into small chambers called minor calyces. Each kidney has 8-18 minor calyces.
    • Major Calyces:- Several minor calyces merge to form 2-3 major calyces, which funnel urine into the renal pelvis.
    • Renal Pelvis:- The renal pelvis is a large, funnel-shaped chamber that collects urine from the major calyces and channels it into the ureter.

3. Structure of the Nephron

The nephron is the functional unit of the kidney, responsible for filtering blood and forming urine. Each kidney contains approximately 1 to 1.5 million nephrons.

3.1. Renal Corpuscle

  • Glomerulus:-
    • A tuft of capillaries with fenestrated (porous) endothelium, allows the passage of water, ions, and small molecules from the blood into the nephron while retaining larger molecules like proteins and blood cells.
    • The glomerulus is the primary site of blood filtration.
  • Bowman’s Capsule:-
    • A double-walled capsule surrounds the glomerulus, which collects the filtrate produced by the glomerulus.
    • The inner layer, known as the visceral layer, is made of specialized cells called podocytes, which play a role in the filtration process.

3.2. Renal Tubule

  • Proximal Convoluted Tubule (PCT):-
    • The first segment of the renal tubule is located in the cortex.
    • The PCT reabsorbs approximately 65% of the filtered water, glucose, amino acids, and electrolytes back into the bloodstream.
    • It also secretes hydrogen ions, creatinine, and certain drugs into the tubular fluid.
  • Loop of Henle:-
    • A U-shaped loop extending from the cortex into the medulla and back.
    • Descending Limb:- Permeable to water but not solutes, allowing water to be reabsorbed and concentrating the tubular fluid.
    • Ascending Limb:- Impermeable to water but actively reabsorbs sodium, potassium, and chloride, diluting the tubular fluid.
  • Distal Convoluted Tubule (DCT):-
    • The segment of the nephron following the loop of Henle, primarily in the cortex.
    • The DCT reabsorbs sodium and calcium under the influence of hormones like aldosterone and PTH and secretes potassium and hydrogen ions.
  • Collecting Duct:-
    • The final segment of the nephron extends from the cortex through the medulla.
    • Collecting ducts from multiple nephrons converge to form larger ducts that eventually drain into the renal pelvis.
    • The permeability of the collecting duct to water is regulated by antidiuretic hormone (ADH), controlling the concentration of urine.

4. Urine Formation

Urine formation is a complex process that involves three main steps: filtration, reabsorption, and secretion.

4.1. Glomerular Filtration

  • Process:-
    • Blood enters the glomerulus under pressure, and water, along with solutes like glucose, electrolytes, and waste products, is filtered through the glomerular capillaries into Bowman’s capsule.
    • The filtration membrane consists of three layers: the fenestrated endothelium of the glomerulus, the basement membrane, and the podocyte layer of Bowman’s capsule.
    • This filtration process results in the formation of a filtrate, which is similar to plasma but not have proteins and blood cells.
  •  Factors Affecting GFR:-
    • Hydrostatic Pressure:- The pressure exerted by the blood within the glomerular capillaries drives filtration.
    • Oncotic Pressure:- The osmotic pressure due to plasma proteins in the glomerular capillaries opposes filtration.
    • Capsular Hydrostatic Pressure:- The pressure in Bowman’s capsule opposes filtration.
  •  Regulation of GFR:-
    • Autoregulation:- The kidneys maintain a relatively constant GFR despite fluctuations in systemic blood pressure, primarily through the myogenic response and tubuloglomerular feedback.
    • Neural Regulation:- Sympathetic nervous system activity can reduce GFR during stress by constricting the afferent arterioles.
    • Hormonal Regulation:- RAAS, ANP (atrial natriuretic peptide), and prostaglandins can modulate GFR.

4.2.Tubular Reabsorption and Secretion

  • Proximal Convoluted Tubule (PCT):-
    • Reabsorption:- Reabsorbs approximately 99% of the filtered water and solutes, including glucose, amino acids, bicarbonate, sodium, and chloride.
    • Secretion:- Actively secretes substances like hydrogen ions, organic acids, and bases into the tubular fluid.
  • Loop of Henle:-
    • Descending Limb:- Permeable to water but not to solutes, allowing water reabsorption and concentrating the tubular fluid.
    • Ascending Limb:- Impermeable to water but actively reabsorbs sodium, potassium, and chloride, diluting the tubular fluid.
  • Distal Convoluted Tubule (DCT):-
    • Selective Reabsorption:- Sodium reabsorption is regulated by aldosterone; PTH influences calcium reabsorption.
    • Secretion:- Continues the secretion of potassium, hydrogen ions, and ammonium to maintain electrolyte and acid-base balance.
  • Collecting Duct:-
    • Final Concentration of Urine:- Under the influence of ADH, water permeability increases, leading to concentrated urine.
    • Regulation by Hormones:- Aldosterone increases sodium reabsorption, while ADH controls water reabsorption.

4.3.Acid-Base Balance

  • Bicarbonate Reabsorption:-
    • The kidneys reabsorb nearly all filtered bicarbonate to maintain blood pH.
    • Carbonic Anhydrase:- Catalyzes the conversion of bicarbonate and hydrogen ions into carbon dioxide and water, facilitating reabsorption.
  • Ammonium Production and Excretion:-
    • Ammonium is produced in the proximal tubule from glutamine metabolism and is excreted in urine, providing a mechanism to excrete excess hydrogen ions.
  • Phosphate Buffer System:-
    • The kidneys excrete hydrogen ions in the form of dihydrogen phosphate, a key buffer in urine.
  • Role of Distal Tubules:-
    • The distal tubules and collecting ducts fine-tune acid-base balance by secreting hydrogen ions and reabsorbing bicarbonate.

5. Composition of Urine

Urine is the final product of the filtration, reabsorption, and secretion processes in the kidneys. It is composed of the following:-
  • Water:-
    • Approximately 95% of urine is water, which serves as a solvent for the excreted substances.
    • The amount of water in urine varies depending on hydration status and the body’s needs.
  • Nitrogenous Wastes:-
    • Urea:- A byproduct of protein metabolism, urea is the most abundant nitrogenous waste in urine.
    • Creatinine:- A waste product from muscle metabolism, creatinine levels in urine can be used to assess kidney function.
    • Uric Acid:- Formed from the breakdown of purines, uric acid is less soluble and can form crystals if its concentration is too high.
  • Electrolytes:-
    • Sodium, Potassium, and Chloride:- These ions are excreted in varying amounts depending on dietary intake and the body’s electrolyte balance.
    • Calcium, Phosphate,  and Magnesium:- These electrolytes are excreted in smaller amounts and are regulated by hormones like PTH and calcitriol.
  • Other Substances:-
    • Hormones:- Small amounts of hormones, such as aldosterone and ADH, may be present in urine.
    • Metabolites:- Waste products from drugs, toxins, and other metabolites are excreted in urine.
  • pH and Specific Gravity:-
    • pH:- The pH of urine typically ranges from 4.5 to 8.0, depending on diet, hydration, and metabolic activity.
    • Specific Gravity:- A measure of urine concentration, specific gravity typically ranges from 1.005 to 1.030.

6. Ureter

The ureters are muscular tubes that transport urine from the kidneys to the bladder. Each ureter is about 25-30 cm long and has a diameter of 3-4 mm.
  • Structure:-
    • Mucosa:- The inner lining of the ureter is composed of transitional epithelium, which allows the ureter to stretch as the urine passes through.
    • Muscularis:- The middle layer consists of smooth muscle fibers arranged in longitudinal and circular layers, which contract rhythmically (peristalsis) to propel urine toward the bladder.
    • Adventitia:- The outer layer of connective tissue anchors the ureter to surrounding structures.
  • Physiology:-
    • The ureters are equipped with valves at their junction with the bladder (ureterovesical junction) that prevent the backflow of urine.
    • Peristaltic waves, occurring every 10-15 seconds, ensure the continuous flow of urine from the kidneys to the bladder.

7. Bladder

The bladder is a hollow, muscular organ that stores urine until it is excreted. It is located in the pelvic cavity, posterior to the pubic symphysis.
  • Structure:-
    • Mucosa:- The inner lining of the bladder is made of transitional epithelium, which allows the bladder to expand as it fills with urine.
    • Detrusor Muscle:- The middle layer of smooth muscle, known as the detrusor muscle, contracts to expel urine during micturition (urination).
    • Serosa/Adventitia:- The outer layer of the bladder is composed of connective tissue (adventitia) or serous membrane (serosa) in the superior surface.
  • Capacity:-
    • The bladder can hold approximately 400-600 ml of urine, although the urge to urinate typically occurs when it contains about 200-300 ml.
  • Function:-
    • The bladder serves as a temporary storage reservoir for urine, and its distension activates stretch receptors that signal the brain when it is time to urinate.
    • The process of micturition is coordinated by the central nervous system, involving voluntary control over the external urethral sphincter and the involuntary contraction of the detrusor muscle.

8. Urethra

The urethra is a tube that conveys urine from the bladder to the outside of the body. Its length and structure differ between males and females.
  • Male Urethra:-
    • Approximately 20 cm long, the male urethra serves a dual function in excreting urine and transporting semen during ejaculation.
    • It is divided into three regions: the prostatic urethra, membranous urethra, and spongy (penile) urethra.
  • Female Urethra:-
    • About 4 cm long, the female urethra is shorter and only serves the function of excreting urine.
    • It opens to the outside at the external urethral orifice, located anterior to the vaginal opening.
  • Sphincters:-
    • Internal Urethral Sphincter:- Located at the junction of the bladder and urethra, this involuntary sphincter is controlled by the autonomic nervous system.
    • External Urethral Sphincter:- A voluntary sphincter composed of skeletal muscle, located in the pelvic floor.

9. Hormone Release by the Kidneys

The kidneys are not only involved in waste elimination but also play a crucial role in the endocrine system by secreting several important hormones:
  • Erythropoietin (EPO):-
    • Stimulates the production of red blood cells in the bone marrow in response to low oxygen levels (hypoxia) in the blood.
    • Essential for maintaining adequate oxygen-carrying capacity of the blood, especially in conditions like chronic kidney disease.
  • Renin:-
    • An enzyme that is part of the renin-angiotensin-aldosterone system (RAAS), crucial for blood pressure regulation.
    • Renin is released by the juxtaglomerular cells of the kidneys in response to low blood pressure or low sodium levels, leading to the production of angiotensin II, which constricts blood vessels and stimulates aldosterone release.
  • Calcitriol (Active Vitamin D):-
    • The kidneys convert inactive vitamin D into its active form, calcitriol, which is essential for calcium and phosphate absorption in the intestines.
    • Calcitriol plays a vital role in maintaining bone health by regulating calcium and phosphate levels in the blood.
  • Prostaglandins:-
    • Local hormones produced by the kidneys help regulate blood flow within the kidneys, influence the glomerular filtration rate, and modulate the effects of other hormones like ADH and aldosterone.
  • Kallikrein:-
    • An enzyme that plays a role in blood pressure regulation by converting kininogen into bradykinin, a peptide that dilates blood vessels.

4. Detailed Physiology: Hormonal Regulation

4.1. Erythropoietin (EPO):-

  • Production:- EPO is produced by the peritubular fibroblasts in the renal cortex.
  • Role in Erythropoiesis:-
    • Stimulates the differentiation of hematopoietic stem cells into erythrocytes.
    • Increases the survival of erythroid progenitor cells.
  • Regulation:-
    • Hypoxia-Inducible Factor (HIF):- In response to hypoxia, HIF accumulates, leading to increased EPO gene expression.
    • Negative Feedback:- As oxygen levels increase, EPO production decreases, preventing excessive erythrocyte production.

4.2. Renin and the RAAS Pathway

  • Renin Release Mechanism:-
    • Juxtaglomerular Apparatus (JGA):- The JGA, located near the glomerulus, is crucial for renin release. It senses blood pressure via the afferent arteriole and sodium concentration in the distal tubule.
    • Baroreceptor Response:- A drop in blood pressure or volume stimulates renin release through the baroreceptors in the afferent arteriole.
    • Sympathetic Nervous System:- Activation of the beta-1 adrenergic receptors in the JGA also stimulates renin release.
  • RAAS Cascade:-
    • Angiotensinogen to Angiotensin I:- Renin converts angiotensinogen, produced by the liver, into angiotensin I.
    • Angiotensin I to Angiotensin II:- ACE, mainly in the lungs, converts angiotensin I to angiotensin II, a potent vasoconstrictor.
    • Effects of Angiotensin II:-
      • Vasoconstriction:- Increases systemic vascular resistance, raising blood pressure.
      • Aldosterone Secretion:- Promotes sodium and water reabsorption in the kidneys, increasing blood volume.
      • Antidiuretic Hormone (ADH):- Stimulates the release of ADH, enhancing water reabsorption in the kidneys.

4.3. Calcitriol and Calcium Homeostasis

  • Synthesis of Calcitriol:-
    • Liver Conversion:- Vitamin D3 is first converted to 25-hydroxyvitamin D3 in the liver.
    • Kidney Activation:- 25-hydroxyvitamin D3 is converted to calcitriol in the proximal tubules of the kidneys.
  • Function of Calcitriol:-
    • Intestinal Absorption:- Increases the absorption of calcium and phosphate from the gut.
    • Bone Resorption:- Works with PTH to release calcium from bones when blood calcium levels are low.
    • Renal Reabsorption:- Increases calcium reabsorption in the distal convoluted tubule, reducing calcium loss in urine.
  • Regulation by PTH:-
    • Low blood calcium levels trigger PTH release, which increases calcitriol synthesis in the kidneys.

4.4. Prostaglandins and Renal Function

  • Prostaglandin Synthesis:-
    • Synthesized from arachidonic acid by the action of cyclooxygenase (COX) enzymes.
    • Produced locally in the renal cortex and medulla.
  • Role in Kidney Function:-
    • Vasodilation:- Prostaglandins like PGE2 dilate afferent arterioles, increasing renal blood flow.
    • Glomerular Filtration Rate:- Help maintain GFR, especially during conditions of stress or injury.
    • Interaction with RAAS:- Prostaglandins counteract the vasoconstrictive effects of angiotensin II, balancing the RAAS to prevent excessive blood pressure increases.
  • Clinical Relevance:-
    • NSAIDs:- Non-steroidal anti-inflammatory drugs (NSAIDs) inhibit prostaglandin synthesis, which can reduce renal blood flow and impair kidney function in susceptible individuals.

4.5. Kallikrein-Kinin System

  • Kallikrein Function:-
    • Converts inactive kininogens into active kinins, particularly bradykinin.
  • Role of Bradykinin:-
    • Vasodilation:- Causes vasodilation of renal blood vessels, increasing blood flow and promoting natriuresis (excretion of sodium in urine).
    • Interaction with RAAS:- Acts as a counter-regulatory mechanism to the vasoconstrictive effects of the RAAS.
  • Regulation of Blood Pressure:-
    • Natriuresis:- Promotes sodium excretion, which lowers blood volume and blood pressure.
    • Anti-inflammatory Effects:- Kinins also have anti-inflammatory properties, protecting renal tissues from damage during inflammatory responses.

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