Renal System

• Renal physiology


  The study of the functions of the kidneys
• Mechanism of urine formation

  1. Glomerular filtration

  2. Tubular reabsorption

  3. Tubular secretion
• Nephron


  The functional unit of the kidney
• The kidney is a powerful regulatory organ which maintains water balance, electrolyte balance, and acid-base balance in the extracellular fluid
• The kidney achieves this regulation through the formation of urine
• Nervous and endocrine systems


  • They regulate homeostasis by sensing system deviations and sending nerve impulses or hormones to appropriate organs
• An individual must maintain internal homeostasis to maintain health
• Intracellular fluid (ICF)


  Fluid inside the cells, constitutes about one half to two thirds of the total body fluid in an adult

  
• Extracellular fluid (ECF)


  Fluid outside the cells, constitutes about one third of the total body fluid in an adult
• Interstitial fluid (IF)


  Fluid found between cells

  
• Transcellular fluids


  Specialized ECFs within epithelial-lined spaces, including synovial fluid, CSF, aqueous fluid, GI tract fluids, and bladder urine

  
• Major ions in ECF

  • Sodium, chlorine, and carbonate
• Thirst center


  Part of the hypothalamus that stimulates or inhibits the desire to drink
• Vasopressin (ADH)


  Hormone released by the pituitary gland that tells the kidneys how much water to conserve
• RAA system

  Renin-angiotensin-aldosterone system that controls fluid volume
• Atrial natriuretic peptide (ANP)


  Hormone released by the heart that helps regulate fluid balance
• Edema


  Excess accumulation of fluid in interstitial (tissue) spaces
• Dehydration


  Deficiency of body water or excessive loss of water, where water output is greater than input
• Polydipsia


  Excessive fluid intake, particularly of plain water

  
• Polyuria


  Production of abnormally large volumes of dilute urine
• Diuresis


  Increased or excessive production of urine
• Nephron


  Functional unit of the kidney consisting of a glomerulus and a single long tubule

  
• Nephrons


  • Similar in basic structure and function across mammals
  • Number of nephrons differs among mammals (e.g. 4 million for cattle, 500,000 for dogs)
• Bowman's capsule


  Spherical structure that contains a capillary tuft (glomerulus) and is connected to the single long tubule
• Segments of the single tubule

  • Proximal (convoluted) tubule
  • Loop of Henle
  • Distal (convoluted) tubule
• Collecting duct


  Tubular structure in the kidney that connects the distal tubules of numerous nephrons

  
• Renal microcirculation

  • Glomerular capillaries are between two arteriolar vessels rather than between an arteriole and a venule
  • Afferent arterioles lead into glomeruli, and efferent arterioles leave glomeruli
  • Efferent arterioles from glomeruli lead into capillary networks that surround tubules in the cortex (peritubular capillaries)
  • Efferent from glomeruli deep in the cortex next to the medulla contribute blood to vessels that extend into the medulla (vasa rectae)

  
• Juxtaglomerular (JG) cells


  Specialized cells in the walls of afferent arterioles near glomeruli that contain the enzyme renin
• Macula densa


  Specific region of the wall of the distal tubule where the cellular nuclei appear to be bunched closely together
• Juxtaglomerular apparatus

  Consists of the JG cells, the macula densa, and extraglomerular mesangial cells
• Glomerular filtration


  Movement of water and solutes from the plasma in the glomerulus, across the glomerular-capsule membrane, and into the capsular space of the Bowman's capsule
• Glomerular filtrate


  Fluid constituents that pass from the blood plasma in the glomerulus into the urinary space of Bowman's capsule
• Glomerular filtration barrier


  • Acts like a sieve, allowing substances up to a molecular weight of about 65,000 to pass through
  • Blood cells are too large to pass, and only a small percentage of plasma proteins pass through
• Plasma constituents (e.g. glucose, amino acids, urea, creatinine, sodium, potassium, chlorine, and bicarbonate ions) readily cross the glomerular filtration barrier, and their concentrations in the initial filtrate are about the same as in plasma
• Proteinuria


  Presence of abnormal amounts of protein in voided urine
• Hematuria


  Blood in voided urine
• Factors determining rate of fluid movement across glomerular filtration barrier

  1. Blood (hydrostatic) pressure in the capillary

  2. Osmotic pressure generated by the plasma proteins of the blood in the capillaries

  3. Hydrostatic pressure in the urinary space of Bowman's capsule

  4. Osmotic pressure generated by proteins in the fluid in the urinary space
• In mammals, GFR and renal blood flow (RBF) remain relatively stable in normally hydrated animals in spite of minor short-term fluctuations in arterial blood pressure (20–30 mm Hg)

  
• Renal autoregulation


  Mechanism intrinsic to the kidney that maintains stability of GFR and RBF

  
• Mechanism of renal autoregulation

  Vasoconstriction produced by increases in sympathetic nerve activity to the kidneys and increases in vasoconstrictors such as angiotensin II