Renal System Overview

Renal System Overview

• Kidneys are responsible for various functions, including regulating plasma volume, removing waste, balancing electrolytes, and regulating acid-base balance.
• These functions result from selective reabsorption and secretion of various molecules within the kidney.

Kidney Structure

• The urinary system comprises macroscopic and microscopic components.
• The microscopic structure includes glomerular filtration, including glomerular filtration rate and regulation, and mechanisms of salt and water reabsorption for water balance, Na/K balance, and acid-base balance.

Mammalian Kidney

• The mammalian kidney features a nephron, renal pelvis, renal pyramid, cortex, and medulla.

Nephron Types

• Two main types of nephrons exist: cortical nephrons (most common) with short loops of Henle, and juxtamedullary nephrons with long loops of Henle.
• The length of the loop of Henle affects the kidney's ability to concentrate urine.

Kidney Tubules

• Kidney tubules are made of epithelial cells, which have a brush border, tight junctions, and apical and basolateral surfaces.

Urine Formation

• Urine formation involves three key processes: filtration, reabsorption, and secretion.

Glomerular Filtration Properties

• Filtration in the Bowman's capsule includes water, salts, glucose, urea, and amino acids.
• Blood cells and most proteins are not filtered.
• The filtrate is driven by glomerular pressure.

Glomerular Filtration Dependence

• Glomerular filtration relies on net filtration pressure and the structure of the glomerular endothelium, Bowman's capsule, and the basement membrane.
• Fenestrated capillaries, filtration slits in the Bowman's capsule, and negative charges on the basement membrane are important factors.

Juxtaglomerular Apparatus

• The juxtaglomerular apparatus consists of the distal tubule, Bowman's capsule, the glomerular capillaries, afferent and efferent arterioles as well as the granular cells and macula densa.

Glomerular Filtration Rate (GFR)

• GFR is measured in mL/min or L/day.
• It is typically 120 mL/min (180 L/day) in healthy adults, and is equivalent to filtering about 5.5 liters every 40 minutes

GFR Regulation

• Renal blood flow (RBF) is a key factor in determining GFR.
• RBF is closely regulated to maintain a relatively constant GFR in the face of changes in arterial blood pressure (between 80 to 200 mm Hg).
• Changes in RBF or GFR are detected and adjusted within the kidney itself through myogenic and tubuloglomerular feedback mechanisms.

Intrinsic GFR Regulation

• Myogenic regulation involves smooth muscle contraction in the afferent arteriole in response to stretch in the glomerulus to regulate GFR.
• Tubuloglomerular feedback involves changes in glomerular blood flow affecting the distal tubule's macula densa to alter the release of chemical messengers for regulating GFR.

Extrinsic GFR Regulation

• The sympathetic nervous system can increase or decrease renal blood flow, altering GFR.
• Hormones like angiotensin II and aldosterone can alter GFR by affecting blood volume or blood pressure.

Mechanisms of Salt & Water Reabsorption

• Different nephron segments possess varying ion transport mechanisms and permeabilities to water.
• This sets up different conditions within the nephron for reabsorbing essential materials and eliminating harmful waste products.

Proximal Convoluted Tubule

• Reabsorption in the proximal tubule involves the active transport of sodium, potassium, and chloride ions.
• Glucose, amino acids, and phosphates are reabsorbed via secondary active transport mechanisms.

Loop of Henle

• The juxtamedullary nephron has more of an impact than cortical nephrons in the salt & water reabsorption process.
• About 20% of salt and water is reabsorbed, not regulated.
• The countercurrent multiplier system is involved in reabsorption.

Thin Descending Limb

• The thin descending limb is highly permeable to water but has low permeability to ions.
• Water reabsorption occurs in response to the osmotic gradient produced by the loop of Henle.

Thick Ascending Limb

• The thick ascending limb actively transports sodium, potassium, and chloride ions from the filtrate into the interstitial fluid surrounding the nephron.

Distal Convoluted Tubule

• The distal convoluted tubule performs essential functions: maintaining electrolyte balance (sodium, potassium) and acid-base balance.

Counter Current Multiplier Significance

• Loops of Henle play a critical role in establishing an osmotic gradient in the renal medulla.
• Longer loops create a steeper gradient, enabling higher urine concentration.

Vasa Recta Significance

• The vasa recta helps maintain the osmotic gradient by recirculating fluids and salts, trapping these substances within the interstitial tissues of the kidney.

Collecting Duct

• The collecting duct plays a role in fine-tuning salt and water reabsorption, and this process is regulated by hormones such as antidiuretic hormone (ADH).
• In the collecting duct, the last 10-15% of filtrate is reabsorbed.

Renal Control of Electrolyte Balance

• Approximately 85-90% of sodium and potassium is reabsorbed in the proximal tubule and loop of Henle independently of hormones.
• The remaining 10-15% is regulated by aldosterone and occurs mainly in the distal convoluted tubule (DCT).
  • Aldosterone affects sodium reabsorption and potassium secretion.
  • Several hypotheses explain sodium absorption mechanisms.
  • Increased potassium secretion is thought to involve increased synthesis of potassium channels in the apical membrane

Acid-Base Balance

• The kidney plays a role in mammals, working alongside other regulatory mechanisms for the regulation of plasma pH. It regulates H+ and HCO3- concentrations both in the plasma and in the filtrate
• pH regulation occurs inside the DCT and collecting ducts by A and B cells
• When urine pH is below 4.5, the secretion of H+ is inhibited

ADH Mechanism

• ADH is secreted in responses to low blood pressure or high osmolarity.
• ADH binds to a GPCR on the collecting duct cells.
• This leads to the exocytosis of aquaporins, resulting in increased water permeability and increased water reabsorption.

Aquaporins

• Aquaporins form channels allowing water to move across the membranes of the collecting ducts.

Explore the intricate functions and structures of the renal system, including the kidney's roles in regulating plasma volume, waste removal, and electrolyte balance. Dive into the details of nephron types, kidney tubules, and their impact on urine concentration. Perfect for students of biology or medicine.