Renal Physiology Part 2 PDF

Reabsorption by peritubular capillaries - The efferent arteriole upon leaving the glomerulus will form another capillary networks, these are: § The peritubular capillaries: these are the blood capillaries that surround the PCT and DCT § The vasa recta: these are the blood capillarie...

Reabsorption by peritubular capillaries - The efferent arteriole upon leaving the glomerulus will form another capillary networks, these are: § The peritubular capillaries: these are the blood capillaries that surround the PCT and DCT § The vasa recta: these are the blood capillaries that surround the loop of Henle. - The two types of blood capillaries are involved in reabsorption and secretion. 1  Reabsorption by peritubular capillaries - Fluid and electrolytes are reabsorbed from the renal tubules into the peritubular capillaries by a net reabsorption pressure. § Hydrostatic pressure of peritubular capillaries pushes fluid out of the capillary. It equals to 12 mmHg (most of the water has been already filtered) § Oncotic pressure of peritubular capillaries pulls fluid into the capillary. It equals to 32 mmHg. Net reabsorption pressure = hydrostatic pressure - H.P. Hydrosta-c pressure colloid pressure πb : Onco-c pressure = 12 - 32 = - 20 mmHg (the –ve sign means reabsorption) 2  Tubular reabsorption - The glomerular filtrate flows sequentially through the successive parts of the tubule (by a specific co-transporter in each part of tubules) : The proximal tubule → the loop of Henle → the distal tubule → the collecting tubule → finally, the collecting duct, before it is excreted as urine. - Along this course, the useful substances such as glucose, amino acids, electrolytes (Na+, K+, Cl−, HCO3−) and vitamins are reabsorbed. Useless substances such as creatinine, uric acid are not reabsorbed. 3  Tubular secretion Is the transport of useless or excess substances (like H+ ions and toxins) from the peritubular capillaries into the tubular lumen, i.e. it is the addition of a substance to the glomerular filtrate 4  How kidneys concentrate or dilute urine 1.Osmolarity Regulation: ü The function of Loop of Henle is to create and maintain osmolarity gradient in medullary that help in dilute or concentrate the urine. As well as the recycling of urea in medulla that reabsorbed from collecting duct (Countercurrent Mechanism) under control of ADH. Ascending loop Not premable to H2O BUT prem. For Urea verses to Desending loop 5 2. Hormonal Regulation: a. ADH: controls the permeability of the collecting ducts, allowing for the reabsorption of water back into the bloodstream, thus concentrating the urine in (fasting and sweating and vomiting). b. Aldosterone: Regulates the reabsorption of sodium and water in the distal tubules and collecting ducts, affecting the concentration of urine. c. PTH: Circulating parathyroid hormone targets the distal convoluted tubule and collecting duct, directly increasing Ca reabsorption  Urine Transportation, Storage, & Elimination Ureters Slender tubes that transport urine from the kidney to the urinary bladder Urine stretches the calyces and increases their inherent pacemaker activity, which in turn initiates peristaltic contractions that spread downward along the ureter forcing urine toward the bladder. 7 Autorhythmicity in the renal pelvis may be susceptible to influences such as: 1. Muscle wall stretch 2. The localized endogenous release of prostaglandins or sensory neuropeptides (like tachykinins) The smooth muscle of ureteric walls are innervated by both sympathetic and parasympathetic nerves Peristaltic contractions in the ureter are enhanced by parasympathetic stimulation and inhibited by sympathetic stimulation  Anatomy & Physiology of Lower Urinary tract Bladder: a reservoir for holding urine - bladder body - Trigone consist of 3 layers serosa, sm.m (detrusor m.) & endothelium. Bladder outlet: release urine - bladder neck (internal urethral sphincter ), - urethra - external urethral sphincter Soma+c Urinary bladder and its innerva-on.  Micturition (Voiding) The act of urination Both sphincter muscles must open to allow voiding When urine volume exceeds 200-400 ml, the internal urethral sphincter is relaxed as stretch receptors send signal to spinal cord (spinal reflex). The external urethral sphincter (composed of skeletal muscle) must be voluntarily relaxed. 12  Detrusor muscle ̶ ve + ve M3 β3 + ve ̶ ve Internal sphincter a1 + ve External sphincter Nm  Neurophysiology of the LUT On–off switching Storage phase switched to the Voiding phase either involuntarily (children) or Voluntarily. Storage (filling): the bladder wall stretch sufficiently to accommodate a reasonable amount of urine. Voiding : the individual has sensory awareness of a full bladder and the guarding reflex is intensified until voluntary elimination is possible.  During storage distention of the bladder produces (low level) afferent firing, which in turn stimulates: (1) sympathetic outflow to the bladder outlet & inhibits detrusor m. (guarding reflexes). (2) somatic outflow contract external urethral sphincter. Relax Noradrenergic dominance Contract contraction  During , (intense) bladder afferent firing activates spinobulbospinal reflex pathways passing thru the pontine micturition center to periaqueductal gray (PAG), which stimulate the parasympathetic outflow to the bladder and internal sphincter and inhibit the sympathetic and somatic INHIBITION outflow to the urethral outlet. contraction Cholinergic dominance relaxation