Renal Physiology (PYS 221) PDF

RENAL PHYSIOLOGY (PYS 221) DR. J. E TORYILA GROSSANATOMY OF THE KIDNEY There are normally two separate kidneys each with its own fibrous capsule. They are located in a retroperitoneal location in the upper abdomen, one in each paravertebral gutter adjacent to T12 to L3. They are approxi...

RENAL PHYSIOLOGY (PYS 221) DR. J. E TORYILA GROSSANATOMY OF THE KIDNEY There are normally two separate kidneys each with its own fibrous capsule. They are located in a retroperitoneal location in the upper abdomen, one in each paravertebral gutter adjacent to T12 to L3. They are approximately 12cm long and weigh150 g each. The right kidney is slightly lower than the left due to the presence of the liver in the right upper abdomen. The upper part (upper pole) of each kidney is protected posteriorly by the 11th and 12th ribs  The kidney has two distinct regions a cortex around the outer edge, and an inner medulla. The medulla is composed of numerous renal pyramids. At the innermost ends of the pyramids are calyces which receive urine , which then drain to the renal pelvis and the ureter. Both kidneys account for 0.5% of body weight In humans  kidney functions Regulation of the volume and composition of the body fluids, Body fluid osmolality and volume Electrolyte balance Acid-base balance Excretion of metabolic products and xenobiotics citrate, succinate, urea, uric acid, creatinine, end-products of metabolisms of hemoglobin and hormones, antibiotics, drugs, Secretion of hormones renin, prostaglandins, kinins, 1-25 di- hydroxyvitamin D 3, erythopoietin  Nephron The nephron is the functional unit of the kidney. There are two types of nephrons, cortical and juxtaglomerular nephrons. Approximately 25 % of the nephrons are juxtaglomerular and the majority are cortical.  Juxtaglomerular nephrons extends deep into the medulla of the kidney, towards the tip of the renal pyramid. Cortical nephrons most of the tubular component resides in the cortex of the kidney.  Nephron, functional unit of the kidney, the structure that actually produces urine in the process of removing waste and excess substances from the blood. There are about 1,000,000 nephrons in each human kidney  Each nephron in the mammalian kidney is a long tubule, or extremely fine tube, about 30–55 mm (1.2–2.2 inches) long. At one end this tube is closed, expanded, and folded into a double- walled cuplike structure. This structure, called the renal corpuscular capsule, or Bowman’s capsule, encloses a cluster of microscopic blood vessels— capillaries—called the glomerulus. The capsule and glomerulus constitute the renal corpuscle.  The renal tubule is a long, convoluted structure that emerges from the glomerulus. It can be divided into three parts based on function. The first part is called the proximal convoluted tubule (PCT), due to its proximity to the glomerulus. The second part is called the loop of Henle, or nephritic loop, because it forms a loop (with descending and ascending limbs) that goes through the renal medulla. The third part of the renal tubule is called the distal convoluted tubule (DCT); this part is also restricted to the renal cortex  PROXIMAL CONVOLUTED TUBULES The proximal convoluted tubule is attached to the glomerular capsule and is the first structure through which fluid passes. The largest amount of solute and water reabsorption from filtered fluid occurs in the proximal convoluted tubules. Reabsorbed substances include glucose, amino acids, lactic acid, water-soluble vitamins, and ions such as Na+, K+, Cl-, Ca2+, Mg2+, HCO3- and HPO42-. Waste products such as NH4+, urea and small amounts of creatinine are secreted into urine  LOOP OF HENLE The loop of Henle connects the proximal and distal convoluted tubules. The descending part of this structure dips into the renal medulla. It next makes a hairpin turn and ascends to the renal cortex. Additional filtered ions and water are reabsorbed from the loop of Henle  The distal convoluted tubule (DCT) The distal convoluted tubule (DCT) is a short nephron segment, interposed between the macula densa and collecting duct. It plays a key role in regulating extracellular fluid volume and electrolyte homeostasis. DCT cells are rich in mitochondria, and possess the highest density of Na+/K+-ATPase along the nephron, where it is expressed on the highly amplified basolateral membranes. DCT cells are largely water impermeable, and reabsorb sodium and chloride across the apical membrane via electroneurtral pathways.  DUCTS, CALYCES AND THE RENAL PELVIS Distal convoluted tubules from several nephrons empty into a single collecting duct. The collecting ducts unite and later converge into large papillary ducts, which then drain into cuplike structures called calyces. Urine then moves into a single large cavity called the renal pelvis, and subsequently through the ureter into the urinary bladder.  Urine Formation Urine formation involves 3processes: Glomerular Filtration–small molecules are filtered from glomerulus's to bowman's capsule. Selective Rebasorption–nutrient molecules are transported from PCT and DCT to peritubular capillaries. Concentration–water is reabsorbed from descending limb of loop of handle and from collecting duct into peritubular capillaries. Secretion –waste or harmful substances are transported from peritubular capillaries to PCT and DCT  Glomerular Filtration The formation of urine begins with a passive ultrafiltration process, in which the movement of water and associated dissolved small molecules is determined by hydrostatic and oncotic pressures. Glomerular capillaries are about 100 times more permeable to water and crystalloids than are muscle capillaries. The plasma oncotic pressure rises as fluid moves along the capillary (due to net loss of water into Bowman's space)  Reabsorption Reabsorption = Recovery of substances from the lumen of the tubule Requires both favorable electrochemical gradients and transport capacities. Most molecules are reabsorbed through a combination of facilitated diffusion and active transport.  Secretion Secretion = transferring solutes from the blood into the tubule lumen. Uses transporters found in the cells that line the lumen. Most important secretory products are K+, NH4+, and H+.  MICTURATION PHYSIOLOGY Micturition is a process by which urinary bladder empties when it becomes filled It is a reflex process. However, in grown up children and adults, it can be controlled voluntarily to some extent. The functional anatomy and nerve supply of urinary bladder are essential for the process of micturition.  Begins in 5th month of intrauterine life Remain as reflex up to 2-3years of age. Filling of Bladder - progressively till the tension of the wall rise above threshold Emptying of Bladder - Nervous reflex,micturition reflex that emptied the bladder or at least conscious desire of urination  URINARY BLADDER AND URETHRA Gross Anatomy External Features – hollow muscular viscous for urine Pyramidal , having apex, base & 3 surfaces Neck – lowest part, continues as urethra Interior Of Bladder – mucosa shows irregular folds, & smooth trigone.  STRUCTURE OF THE BLADDER. Wall – 3 layers, outer serous, middle smooth muscle & inner mucous membrane. Mucous membrane – transitional epithelium. Stretches when distends Complete barrier for fluid & electrolytes. Glycosaminoglycan barrier that prevents bacterial adherence.  MUSCULAR LAYER. Smooth muscle fibre of Detrusor Muscle Arranged in interlacing longitudinal, circular, spiral bundles Responsible for emptying of Bladder.  URETHRA & ITS SPHINCTERS. Male urethra – 20 cm 3 parts – prostatic, membranous, penile Membranous surrounded by external sphincters. Female urethra-3.8 cm Traverses external sphincters Lies in front of vagina.  SPHINCTERS OF THE URETHRA Internal sphincters – circular smooth muscle fiber in area of neck. Prevents emptying up to pressure of threshold level. External sphincters--at the level of Urogenital Diaphargm encircled by ring of voluntary muscles Provides voluntary control.  INNERVATION OF THE URINARY BLADDER. Urinary bladder and the internal sphincter are supplied by sympathetic and parasympathetic divisions of autonomic nervous system where as, the external sphincter is supplied by the somatic nerve fibers Preganglionic fibers of sympathetic nerve arise from first two lumbar segments (L1 and L2) of spinal cord. The postganglionic fibers arising from this ganglion form the hypogastric nerve, which supplies the detrusor muscle and internal sphincter The stimulation of sympathetic (hypogastric) nerve causes relaxation of detrusor muscle and constriction of the internal sphincter. It results in filling of urinary bladder and so, the sympathetic nerve is called nerve of filling.  Preganglionic fibers of parasympathetic nerve form the pelvic nerve Pelvic nerve fibers arise from second, third and fourth sacral segments (S1, S2 and S3) of spinal cord. Stimulation of parasympathetic (pelvic) nerve causes contraction of detrusor muscle and relaxation of the internal sphincter leading to emptying of urinary bladder. So, parasympathetic nerve is called the nerve of emptying or nerve of micturition.  External sphincter is innervated by the somatic nerve called pudendal nerve. It arises from second, third and fourth sacral segments of the spinal cord.Pudendal nerve maintains the tonic contraction of the skeletal muscle fibers of the external sphincter and keeps the external sphincter constricted always. During micturition, this nerve is inhibited. It causes relaxation of external sphincter leading to voiding of urine. Thus, the pudendal nerve is responsible for voluntary control of micturition.  MICTURITION REFLEX Micturition reflex is the reflex by which micturition occurs. This reflex is elicited by the stimulation of stretch receptors situated on the wall of urinary bladder and urethra. When about 300 to 400 mL of urine is collected in the bladder, intravesical pressure increases. This stretches the wall of bladder resulting in stimulation of stretch receptors and generation of sensory impulses.  APPLIED PHYSIOLOGY The abnormalities of micturition  Atonic bladder  Automatic bladder  The uninhibited neurogenic bladder  Urinary incontinence  ATONIC BLADDER Due to the destruction of sensory nerve fibers from urinary bladder - Syphilis, tabes dorsalis Due to the absence of sensory impulses, the detrusor muscle loses the tone and becomes flaccid. No contraction of muscle It is also called overflow dribbling / overflow incontinences / Tabetic bladder  AUTOMATIC BLADDER After complete Transection of spinal cord above sacral segments The urinary bladder loses the tone and fails to give response to the micturition reflex. The voluntary control of micturition is lost. Whenever, the bladder is filled with some amount of urine, there is automatic evacuation of bladder.  THE UNINHIBITED NEUROGENIC BLADDER Du to lesion in brainstem, there is continuous excitation of spinal micturition centers The Frequency of Micturition is increased with a small quantity of urine collected in bladder will elicit the micturition reflex.  NOCTURNAL MICTURITION Enuresis or bed wetting. Common in infants and children below 3 years. It is because of the underdevelopment of voluntary control of micturition which is due to incomplete myelination of motor nerve fibers of the bladder. When myelination is complete, voluntary control of micturition develops and enuresis stops in these children.  Diseases of the kidney are numerous. Commonly occurring clinical conditions include acute kidney injury, chronic kidney disease, diabetic kidney disease, nephritic and nephrotic syndromes, polycystic kidney disease, urinary tract obstruction, urinary tract infection, and renal cancer. When renal function decreases such that the kidneys are no longer functioning to maintain health, patients undergo dialysis and eventually kidney transplantation. The prevalence of kidney disease is increasing across the world as is the cost of treating the diseases that cause kidney damage. The two diseases that cause the largest prevalence of kidney dysfunction are diabetes and hypertension