Renal Physiology Review

Kidney Structure and Function

• Location: Kidneys are retroperitoneal organs situated on either side of the vertebral column, directly adjacent to the abdominal aorta and inferior vena cava, and they play a crucial role in maintaining homeostasis.

• Anatomical Layers:

  • Renal Fascia: The outermost connective tissue layer that anchors the kidneys to surrounding structures and provides additional support.

  • Peritoneum: A delicate membrane lining the abdominal cavity that surrounds the kidneys and other abdominal organs, contributing to the compartmentalization of internal organs.

  • Adipose Capsule: A layer of fat that cushions the kidneys against physical shocks and serves as insulation.

  • Renal Capsule: A tough outer fibrous layer that provides a protective barrier against external trauma and infection.

• Nephron: The functional unit of the kidney, with approximately 1.5 million nephrons in each kidney, each responsible for filtering blood and forming urine.

  • Components:

    • Glomerulus: A tuft of capillaries where blood filtration occurs.

    • Bowman's Capsule: Encases the glomerulus and collects the filtrate.

    • Proximal Convoluted Tubule: The first segment of the renal tubule where significant reabsorption of nutrients, ions, and water occurs.

    • Loop of Henle: A U-shaped segment that plays a critical role in concentrating urine by creating a countercurrent multiplier system.

    • Distal Convoluted Tubule: Further adjusts electrolyte balance and volume via reabsorption and secretion.

    • Collecting Duct: Final segment responsible for water reabsorption regulated by hormones, leading to urine concentration.

Functions of the Kidneys

• Filtration and Excretion:

  • Glomerular Filtration: Plasma and dissolved substances are filtered from the blood into the Bowman's capsule, producing an ultrafiltrate.

    • Tubular Reabsorption: Process where water, ions, and nutrients are selectively reabsorbed back into the bloodstream from the renal tubules, with about 99% of the filtered water reclaimed.

    • Tubular Secretion: Additional substances from the blood, including waste products, drugs, and excess ions (such as K+ and H+), are secreted into the renal tubules, aiding in maintaining acid-base balance and electrolyte homeostasis.

• Waste Removal: Major waste products include nitrogenous wastes (urea, creatinine) and toxins, which are eliminated through urine.

Urine Production Mechanism

1. Glomerular Filtration:

   • Roughly 180 liters of blood are filtered daily, a process driven by glomerular hydrostatic pressure that enables the passage of fluids and small solutes such as water, electrolytes, vitamins, glucose, and amino acids.

2. Tubular Reabsorption:

   • Proximal Convoluted Tubule: Almost 65% of the filtered water and solutes (e.g., Na+, glucose, amino acids) are reabsorbed here, essential for conserving vital nutrients.

   • Loop of Henle: The descending limb is permeable to water but not ions, creating a hyperosmotic environment in the medulla; the ascending limb is impermeable to water and actively transports Na+ and Cl− ions out to dilute urine.

   • Distal Convoluted Tubule and Collecting Duct: Reabsorption and secretion are fine-tuned here, influenced by hormones such as aldosterone (increases Na+ reabsorption) and ADH (increases water reabsorption).

3. Tubular Secretion:

   • Involves the active and passive transport of non-filtered substances, including creatinine, drugs, and ions, from the bloodstream into the renal tubules.

Regulation of Filtration Pressure

• Intrinsic Mechanisms:

  • Myogenic Feedback: Smooth muscle cells in arterioles react to changes in pressure; increased pressure causes constriction of the afferent arteriole to prevent damage and maintain stable GFR.

  • Tubuloglomerular Feedback: Specialized cells called macula densa detect variations in NaCl concentration in the filtrate, adjusting the afferent arteriole diameter to regulate glomerular filtration rate (GFR).

• Extrinsic Mechanisms:

  • Neural Regulation: The baroreceptor reflex responds to low blood pressure by activating sympathetic nerves that constrict afferent arterioles, reducing GFR to maintain systemic blood pressure.

  • Hormonal Regulation: The renin-angiotensin-aldosterone system responds to low blood volume or pressure, aiming to enhance GFR and promote sodium and water reabsorption to restore blood volume.

Hormonal Regulation

• Aldosterone: A steroid hormone released from the adrenal cortex that promotes Na+ reabsorption in the distal tubules and collecting duct, leading to water retention and increased blood volume. Increased potassium excretion is also a crucial effect of aldosterone. The juxtaglomerular cells respond to decreased blood pressure, sympathetic nervous system innervation, and macula densa cells send signals to release renin.

• ADH (Antidiuretic Hormone): Secreted by the posterior pituitary, ADH increases water permeability in the collecting duct, facilitating water reabsorption depending on hydration state, thus concentrating urine and reducing urine volume.

Urine Concentration

• Countercurrent Multiplier System: The unique structure of the nephron loops creates osmotic gradients which are vital for enhancing the kidney's ability to concentrate urine, preventing dehydration and maintaining fluid balance.

• Osmolality Changes:

  • Normal osmolality is approximately 300 mOsm/kg;

  • In an overhydrated state, urine can be as dilute as 100 mOsm/kg;

  • In a dehydrated state, urine may reach concentrations as high as 1200 mOsm/kg, reflecting kidney adaptability to hydration levels.

Micturition Process

• Reflex Actions: When the bladder fills, stretch receptors signal the brain, stimulating the pontine micturition center. This initiates smooth contraction of the detrusor muscle and relaxation of the internal and external sphincters, allowing urination to occur voluntarily once appropriate conditions are met.

Common Clinical Issues

• Chronic Kidney Disease (CKD): A progressive loss of kidney function over time, leading to imbalances in electrolytes, accumulation of wastes, and complications such as hypertension, anemia, and increased cardiovascular risks.

• Dialysis: A life-sustaining treatment that artificially removes waste products and excess fluid from the blood in cases of kidney failure, mimicking normal kidney functions through hemodialysis or peritoneal dialysis.

• Key Learning Objectives:

  1. Identify the anatomy and functional components of the nephron.

  2. Describe the process of glomerular filtration and its significance in waste removal.

  3. Outline the significance of tubular reabsorption and secretion processes in kidney function.

  4. Explain the renal system’s vital role in maintaining fluid, electrolyte, and acid-base balance within the body.