Renal Physiology Quiz

Renal Physiology Overview

• Kidneys are the primary organs for homeostasis, regulating volume and composition of body fluids, and excreting metabolic waste products.
• Bean-shaped, located on the dorsal side of the visceral cavity, and protected by a tough fibrous coat—the renal capsule.
• Adipose tissue surrounds the renal capsule, cushioning the kidney.

Renal System Components

• Kidneys: Form urine.
• Ureters: Transport urine from kidneys to the bladder.
• Urinary bladder: Temporarily stores urine.
• Urethra: Transports urine from the bladder out of the body.

Kidney Structure & Location

• Bean-shaped, retroperitoneal, in the lumbar region.
• Right kidney is slightly lower than the left, and crowded by the liver.
• Renal hilus: Inlet/outlet for ureters, arteries, veins, lymphatics & nerves.
• Adrenal gland (suprarenal gland) sits atop each kidney.

Kidney Anatomy

• Two distinct regions: Cortex & Medulla.
• Cortex surrounds darker triangular structures called pyramids, which form the medulla.
• Renal pelvis collects urine from calyces and drains into the ureter.

Nephrons

• Basic functional unit of the kidneys.
• Each kidney has approximately 1 million nephrons, comprising:
  • Renal corpuscle: Glomerulus & Bowman's capsule.
  • Renal tubules: Proximal convoluted tubule, Loop of Henle & Distal convoluted tubule.

Types of Nephrons

• Cortical nephrons: 85% of nephrons, located in the cortex, have short Loops of Henle, supplied with peritubular capillaries, involved in formation of diluted urine.
• Juxtamedullary nephrons: Fewer in number, located in the medullary region, have long Loops of Henle, supplied with vasa recta, involved in concentrated urine production.

Functions of the Urinary System

• Regulates blood pressure (ABP) through controlling blood volume, electrolyte concentration, Acid-base balance, and Osmolitity, RBC formation
• Endocrine function: Erythropoietin (EPO), calcitriol & prostaglandins.
• Filter 200 liters of blood daily to remove metabolic wastes & excess ions.
• Drug metabolism & detoxification.
• Excretion: Elimination of metabolic wastes (ammonia, urea, bilirubin, creatinine & uric acid) & foreign substances (drugs & toxins).
• Secretion of excess materials.
• Activation of Vitamin D to increase calcium absorption and reabsorption.
• Release of erythropoietin for RBC production for appropriate oxygen & carbon dioxide transport
• Release of renin which causes the powerful vasoconstrictor angiotensin II. This elevates blood pressure to maintain proper homeostasis
• Release of prostaglandins to maintain proper blood flow in the kidneys

Kidney Structures

• Capsule: Outer membrane protecting the kidney.
• Cortex: Outer layer, containing most of the nephron, involved in filtration, reabsorption, & secretion.
• Medulla: Inner core, containing pyramids, columns, papillae, calyces, pelvis & nephron parts.
• Renal Pyramids: Triangular units, site of counter-current system for concentrated salt & water/urea conservation.
• Renal Columns: Passageway between the renal pyramids, space for blood vessels.
• Nephron: The physiological unit of the kidney, involved in filtration, reabsorption and secretion of materials.
• Renal Papilla: Tip of the renal pyramid releasing urine into a calyx.
• Calyx: Collecting sac surrounding the renal papilla, transferring urine to the renal pelvis.
• Renal Pelvis: Collects urine from the calyces of kidney.
• Ureter: Transports urine from the renal pelvis to the bladder.

Blood Supply to Kidneys

• Receive 20% of cardiac output (approximately 4 ml/min/g).
• High capillary pressure supports the kidneys’ filtration capacity.
• Renal artery: Transports oxygenated blood from heart & aorta to kidneys for filtration.
• Renal vein: Transports filtered, deoxygenated blood from kidney to posterior vena cava & then the heart.

Capillary Beds of the Nephron

• Each nephron has two capillary beds:
  • Glomerulus
  • Peritubular capillaries or vasa recta
• Glomerulus: Fed by an afferent arteriole, drained by an efferent arteriole.

Glomerular Filtration Rate (GFR)

• Amount of fluid filtered per minute in all nephrons.
• GFR = 125 ml/min or 180 L/day.

Glomerular Filtration (GFR) Regulation

• GFR influenced by factors such as filtration pressure, glomerular capillary membrane permeability, diameters of afferent & efferent arterioles, plasma protein concentration, and renal blood flow.
• Autoregulation, neural, and hormonal mechanisms adjust renal blood pressure & resulting blood flow.

Juxtaglomerular Apparatus (JGA)

• Initial portion of the distal tubule passes between afferent & efferent arterioles
• Epithelial cells (macula densa) and smooth muscle cells (JG cells) are involved in regulating renin & erythropoietin secretion.

Tubular Reabsorption & Secretion

• Proximal Convoluted Tubule (PCT): Reabsorbs water & solutes; secretes substances.
• Loop of Henle: Has descending and ascending limbs with thin & thick segments for water/electrolyte reabsorption/secretion.
• Distal Convoluted Tubule (DCT): Primarily functions in secretion, though reabsorption also occurs.

Tubular Transport Maximum (Tm)

• Maximum amount of a substance a nephron can reabsorb or secrete per minute.
• Renal threshold for substances is the plasma concentration where substances appear in the urine.

Tubular Load (T_load) of Substances

• The rate of a substance filtered through glomeruli into tubules per minute, calculated by (substance concentration in filtrate) x GFR.

Renal Plasma Clearance

• Volume of plasma completely cleared of a substance per minute, determined by the formula.

Importance of Renal Plasma Clearance

• Measurement of GFR.
• Indicating renal handling of substances through reabsorption and/or secretion.
• Providing quantitative information about renal diseases.

Function of the Loop of Henle

• Creates and maintains an osmotic gradient in the renal medullary interstitium.
• Facilitates the formation of concentrated urine (up to 1200 mOsm/L).
• Acts as a counter-current multiplier system, creating osmotic gradients in both the renal medullary interstitium and the tubule.

Water Reabsorption

• Facultative water reabsorption: Primarily in collecting ducts, regulated by ADH—selective water reabsorption based on body's needs.
• Obligatory water reabsorption: Occurs throughout the nephron, primarily driven by sodium reabsorption.

The Role of ADH

• Osmoreceptors in the hypothalamus detect low water levels (high osmolarity) and stimulate ADH secretion.
• ADH makes the distal tubules and collecting ducts more permeable to water, increasing water reabsorption.

Formation of Water Pores

• Vasopressin, also known as ADH, binds to membrane receptors triggering the activation of a secondary messenger system.
• Water is absorbed by osmosis into the blood through inserted water pores (aquaporins).

Acid-Base Balance Regulation

• Normal blood pH range: 7.35-7.45.
• Chemical buffer system (bicarbonate, phosphate, and hemoglobin/protein), respiratory buffer system (regulating CO2), and the renal buffer system regulate pH through excretion of H+ and conservation of bicarbonate.

Pathology of Kidneys

• Renal failure—a decrease or cessation of glomerular filtration.

• Renal stones—insoluble materials accumulating in the kidneys.

• Glomerulonephritis—inflammation of the glomeruli, frequently caused by an allergic reaction to toxins produced by streptococcal bacteria.

• Nocturnal enuresis—discharge of urine during sleep

• Urinary tract infections—caused by bacteria migrating from the anal area.

• Other renal pathologies detailed in the provided text such as acute & chronic renal failure, their causes, and classifications.

Diuretics

• Drugs increasing urine output through various mechanisms, either by enhancing solute excretion, increasing GFR, or inhibiting ADH release.