Lec 40
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Lec 40 Lec 40 Presentation Transcript

  • Regulation of Glomerular Filtration
    Three mechanisms control the GFR
    Renal autoregulation
    Sympathetic control
    Hormonal control
  • Renal Autoregulation
    The ability of nephrons to adjust their own blood flow and GFR without external control
    Autoregulation entails two types of control
    Myogenic Mechanism – responds to changes in pressure in the renal blood vessels
    Tubuloglomerular Feedback Flow-dependent , senses changes in the juxtaglomerular apparatus
  • Myogenic Mechanism
    When arterial blood pressure rises, it stretches the afferent arteriole and the arteriole contracts and thus prevents blood flow into the glomerulus from changing very much
    Glomerular blood flow and filtration remain stable
  • Tubuloglomerular Feedback
    The juxtaglomerular apparatus monitors the fluid entering the distal tubule and adjust the GFR
    High GFR
    Reduced GFR
    Rapid flow of filtrate in renal tubules (NaCl)
    Constriction of afferent arteriole
    Sensed by macula densa
    Paracrine secretion
  • The Renin-Angiotensin Mechanism
    In response to low flow of filtrate and therefore low NaCl as a result of low GFR caused by low blood pressure, the macula densa cells signal a JG cells to release renin into the blood stream
    Renin triggers production of angiotensin II
    In the kidney increase Angiotensin II causes constriction of the efferent arteriole
    Glomerular hydrostatic pressure to increase and increase GFR
    Angiotensin II stimulate release of aldosterone
  • Sympathetic Nervous System Activation Decreases GFR
    All the blood vessels of the kidneys, are richly innervated by sympathetic nerve fibers
    Strong activation of the renal sympathetic nerves can constrict the renal arterioles and decrease renal blood flow and GFR
    The renal sympathetic nerves seem to be most important in reducing GFR during severe, acute disturbances, such as those elicited by severe hemorrhage
  • Hormonal Control
    Norepinephrine, Epinephrine, and Endothelin constrict renal blood vessels and decrease GFR
    Angiotensin II Constricts Efferent Arterioles
    Prostaglandins and Bradykinin cause vasodilation and increased renal blood flow and increase GFR
  • Other Factors That Increase Renal Blood Flow and GFR
    High protein intake is known to increase both renal blood flow and GFR (increased amino acid reabsorption also stimulates sodium reabsorption in the proximal tubules)
    This decreases sodium delivery to the macula densa
    increases in renal blood flow and GFR that occur with large increases in blood glucose levels
  • The Urinary Bladder
    The body, Detrusor muscle, responsible for storage and elimination of urine
    The neck, funnel shaped extension of the body and connecting with the urethra
    • The Trigone, region where ureters enter the bladder
  • The bladder neck (posterior urethra) is 2 to 3 centimeters long, and its wall is composed of detrusor muscle interlaced with a large amount of elastic tissue. The muscle in this area is called the internal sphincter
    The urethra passes through the urogenital diaphragm, which contains a layer of muscle called the external sphincter of the bladder. This muscle is a voluntary skeletal muscle
  • Pelvic nerves is the principal nerve supply of the bladder, which connect with the spinal cord through sacral plexus
    Coursing through the pelvic nerves both sensory nerve fibers and motor nerve fibers
    Sensory fibers detect the degree of stretch in the bladder wall
    The motor nerves transmitted in the pelvic nerves are parasympathetic fibers
    Innervation of the Bladder
  • Innervation of the bladder
    The pudendal nerve to the external bladder sphincter. These are somatic nerve fibers
    Sympathetic innervation through the hypogastric nerves
  • Micturition
    The process by which the urinary bladder empties when it becomes filled.
    the bladder fills progressively until the tension in its walls rises above a threshold level
    nervous reflex called the micturition reflex that empties the bladder or at least causes a conscious desire to urinate.
    Micturition can also be inhibited or facilitated by centers in the cerebral cortex or brain stem
  • Micturition Reflex
    Stretch of bladder wall when the bladder fills with urine
    Increase activity in sensory stretch receptors in the bladder wall
    Sensory signals from the bladder stretch receptors are conducted to the scaral segments of the cord through pelvic nerves and reflexively back again to the bladder through parasympathetic nerve fibers by pelvic nerve
    As the bladder becomes more filled, micturition reflexes occur more and more often and more powerfully. (increase detrusor muscle contraction)
    Once micturition reflex become powerful, it causes another reflex which passes through pudendal nerves to the external sphincter to inhibit it
    If this inhibition is more potent in the brain than the voluntary constrictor signals to the external sphincter urination will occur
    Urine voided
  • Pelvic nerves
    Excitation of stretch receptors when ~300ml of urine
    Relayed to parasympathetic NS
    Pelvic nerves
    Contraction of bladder
    Bladder outlet pulled open, increase in pressure
    micturition
    Pudendal impulses nerve inhibited
  • Facilitation or Inhibition of Micturition by the Brain
    The micturition reflex is the basic cause of micturition, but the higher centers normally exert final control of micturition as follows:
    The higher centers keep the micturition reflex partially inhibited, except when micturition is desired.
    The higher centers can prevent micturition, even if the micturition reflex occurs, by continual tonic contraction of the external bladder sphincter until a convenient time presents itself.
    When it is time to urinate, the cortical centers can facilitate the sacral micturition centers to help initiate a micturition reflex and at the same time inhibit the external urinary sphincter so that urination can occur
  • Filling of the bladder and the bladder wall tone; the cystometrogram
    A cystometrogram is a test allows to assess how the bladder and sphincter behave while storing and passing urine
    Normal cystometrogram, showing also acute pressure waves (dashed spikes) caused by micturition reflexes
  • Transport of urine from the kidney through the ureters and into the bladder
    There are no significant changes in the composition of urine as it flows through the renal calyces and ureters to the bladder
    Urine flowing from the collecting ducts into the renal calyces stretches the calyces and initiates peristaltic contractions along the length of the ureter then toward the bladder
    Peristaltic contractions in the ureter are enhanced by parasympathetic stimulation and inhibited by sympathetic stimulation