Kidney

Anatomy of the Kidney

  • The twin bean-shaped kidneys reside in the retroperitoneal space between the levels of T12 and L3.

  • The right kidney is slightly more caudal than the left kidney to accommodate the liver.

  • The hilum is an indentation that provides the points of entry and exit for:
    • Renal artery
    • Renal vein
    • Nerves
    • Lymphatics
    • Ureters

Parts of the Kidney

1. Renal Cortex

  • Includes:
    • Glomerulus
    • Bowman's capsule
    • Proximal tubules
    • Distal tubules

2. Renal Medulla

  • Includes:
    • Loops of Henle
    • Collecting ducts

Nephron - Functional Unit of the Kidney

  • Comprised of:
    • Glomerulus
    • Afferent arteriole
    • Efferent arteriole
    • Proximal tubule
    • Loop of Henle
    • Distal tubule
    • Collecting duct

  • Functional significance: • All structures reside in the renal cortex EXCEPT:

    • Distal tubule

    • Collecting duct

Gross Anatomy of the Kidney

  • Major structures include:
    • Major calyx
    • Minor calyx
    • Interlobar artery and vein
    • Arcuate artery and vein
    • Renal artery
    • Renal vein
    • Renal pelvis
    • Ureter

Medulla (Pyramids)

  • Divided into pyramids with the apex directed towards the renal pelvis (papillae).

  • The papillae drain urine into the minor calyces, converging to form major calyces which converge into the renal pelvis for urine to enter the ureter.

  • Calyces, pelvis, and ureters can contract and facilitate the movement of urine towards the bladder.

Six Functions of the Kidney

  • The kidney has six major functions:

1. Maintenance of Extracellular Volume and Composition

• Aldosterone controls extracellular fluid volume (Na+ and water are reabsorbed together).
• ADH (Antidiuretic Hormone) controls plasma osmolarity (water reabsorbed, Na+ not reabsorbed).
• Regulates: K+, Cl-, phosphate, Mg2+, H+, bicarbonate, glucose, and urea.

2. Blood Pressure Regulation

- Long-term control via thirst mechanism and sodium/water excretion. Intermediate through RAAS (Renin-Angiotensin-Aldosterone System) and short-term via baroreceptor reflex.

3. Excretion of Toxins and Metabolites

• Glomerular filtration and tubular secretion clear metabolic byproducts, drugs, and toxins.

4. Maintenance of Acid-Base Balance

• Lungs excrete volatile acids (CO2) while kidneys manage non-volatile acids.
• Kidneys titrate hydrogen in tubular fluid creating acidic/basic urine.

5. Hormone Production

• Erythropoietin - Released due to low oxygen delivery, influencing red blood cell production in bone marrow.
• Calcitriol (active Vitamin D), regulates calcium absorption in intestine, decreases calcium and phosphate excretion by kidneys, impacts bone deposition/reabsorption.
• Prostaglandins - Influence renal blood flow.

6. Blood Glucose Homeostasis

• The kidneys can synthesize glucose to maintain levels during fasting, rivaling liver capabilities.

Renal Blood Flow

  • The kidneys receive 20-25% of cardiac output, typically 1,000-1,250 mL/min.

  • 20% is filtered at glomeruli; 99% of ultrafiltrate is reabsorbed, leaving 1-1.5 L/day of urine.

Factors Affecting Renal Blood Flow

  • Directly proportional to MAP - Renal ext{ } Venous ext{ } Pressure and inversely proportional to renal vascular resistance.

Autoregulation

  • Maintains renal blood flow across varying systemic blood pressures (MAP 50 - 180 mmHg); patient-specific. Functions include:
    • Myogenic mechanism
    • Tubuloglomerular feedback
    • RAAS
    • Atrial natriuretic peptide
    • Prostaglandins

Control of Renal Blood Flow: Autoregulation

Overview of Renal Autoregulation

  • Autoregulation adjusts renal blood flow in response to perfusion pressure variations to maintain consistent filtration. Utilizes various mechanisms:

  • Myogenic Mechanism

  • Tubuloglomerular Feedback

Conditions Affecting Renal Blood Flow

  • Surgical stress response induces transient vasoconstriction and sodium retention leading to oliguria and predisposes to renal ischemic injury.

Renin-Angiotensin-Aldosterone System (RAAS)

Introduction

  • Regulates renal perfusion and systemic vascular resistance through monitoring renal performance.

Juxtaglomerular Apparatus

  • Monitors renal perfusion and solute concentration, linked to sodium delivery, and manages resistance changes.

Aldosterone

  • Enhances sodium reabsorption while promoting potassium and hydrogen excretion; no direct effect on osmolarity.

Renin Release Factors

  • Decreased renal perfusion; SNS stimulation; Tubuloglomerular feedback influences.

Nephron Function

Glomerular Filtration Rate (GFR)

  • Normal rate: 125 mL/min; key determinants include:
    • Arterial blood pressure
    • Afferent and efferent arterioles resistance.

Reabsorption, Secretion, and Excretion

  • Reabsorption: Substance transfer from tubules to capillaries

  • Secretion: Substance transfer from capillaries to tubules

  • Excretion: Removal from body via urine.

Maximum Transport

  • There is a maximum reabsorption capacity; once exceeded, excess excreted, exemplified by renal glycosuria in diabetes.

Nephrons - Segment Functionality

  • Proximal Tubule:
    • 65% Na+ and water reabsorbed here, including bicarbonate, chloride, and potassium.

  • Loop of Henle:
    • Descending limb - permeable to water, concentrates urine.
    • Ascending limb - impermeable to water, pumps sodium out, diluting urine.

  • Distal Tubule:
    • Further regulates sodium and water reabsorption based on aldosterone and ADH.

  • Collecting Duct:
    • Final adjustments; ADH and aldosterone act here for sodium and water handling.

Diuretics and Their Mechanisms

Types of Diuretics

  • Carbonic Anhydrase Inhibitors: Inhibit bicarbonate reabsorption, leading to metabolic acidosis. Use in altitude sickness.

  • Loop Diuretics: Inhibit sodium-reabsorption mechanism in the loop of Henle. Common side effects include hypokalemia, metabolic alkalosis.

  • Thiazide Diuretics: Inhibit sodium reabsorption in distal tubules; can cause hypercalcemia.

  • Potassium-Sparing Diuretics: Aldosterone antagonists that prevent potassium loss, with attention to risks of hyperkalemia.

Renal Function Tests and Assessment

  • Tests assess glomerular vs tubular function; includes serum creatinine, BUN, fractional excretion of sodium, etc.

AKI Classification

  • Based on prerenal, intrinsic, or postrenal injury; distinguish treatment based on underlying causes.

Management of AKI

  • Focus on restoring renal perfusion, avoiding nephrotoxins, and addressing underlying conditions.

Conditions Causing CKD

  • Most common are diabetes and hypertension; complications include uremic syndrome, anemia, cardiovascular issues, metabolic acidosis, disorders related to mineral and bone.

Dialysis Indications

  • Volume overload, hyperkalemia, severe metabolic acidosis, symptomatic uremia, drug overdose management.

Impact of Anesthetic Agents on Renal Function

  • Awareness of how common anesthetics interact with renal metabolism and pharmacokinetics is crucial during surgical procedures.

TURP Specifics

  • Neuraxial anesthesia preferred; irrigating fluids pose risks, monitor for TURP syndrome, managing potential complications like bleeding, bladder perforation, etc. Determining choices of irrigation fluids based on osmolality considerations is essential.

Summary of Urinary Stone Management

  • Treatments like ESWL and PCNL and insights into their contraindications, complications, and considerations.

Conclusion

  • Understanding kidney physiology, pathology, and management strategies is integral for health practitioners involved in renal care, surgeries, or patients with kidney vulnerabilities.