Renal System PDF

THE URINARY SYSTEM Hamza Abu Hilail M.Sc. Physiology  The Urinary System Consists of the 1. kidneys 2. ureters 3. bladder 4. urethra Maintains homeostasis by managing the volume and composition of fluid reservoirs, primarily blood  Functions of the Components of the Ur...

THE URINARY SYSTEM Hamza Abu Hilail M.Sc. Physiology  The Urinary System Consists of the 1. kidneys 2. ureters 3. bladder 4. urethra Maintains homeostasis by managing the volume and composition of fluid reservoirs, primarily blood  Functions of the Components of the Urinary System Ureters: transport urine from kidneys to bladder Bladder: store urine until voided from body Urethra: carry urine from bladder to the outside of the body  Homeostatic Kidney Functions Regulation of blood ionic composition (Na, K and Cl ) Regulation of blood pH (H and HCO3) Regulation of blood volume (H2O) Regulation of blood pressure  Homeostatic Kidney Functions Maintenance of blood osmolarity Production of hormones:  Calcitriol  Erythropoietin Regulation of blood glucose level Excretion of metabolic wastes and foreign substances (drugs or toxins)  Renal Anatomy The kidneys are retroperitoneal, partly protected by the lower ribs.  Renal Anatomy The indented area is called the Hilum. This is the entrance for: 1.Renal Artery 2.Renal Vein 3.Ureter 4.Nerves  External Layers Connective Tissue, Superficial to Deep: Renal Fascia: Anchors to other structures Adipose Capsule: Protects and anchors Renal Capsule: Continuous with Ureter  Internal Renal Anatomy 1.Renal Cortex: Outer layer Renal 2.Medulla: Inner region 3.Renal Pyramids: Secreting, Apparatus and Tubules 4.Renal Columns: Anchor the Cortex External Layers Internal Renal Anatomy Reham Khalaf-Nazzal, MD PhD, Faculty of Medicine, AAUP  Internal Renal Anatomy Papillary Ducts empty urine into calyces Calyces pass urine to the Ureter Blood and Nerve supply of the Kidneys Blood supply Kidneys constitute less than 0.5% of total body mass, they receive 20–25% of resting cardiac output Nerve Supply  Renal Nerves primarily carry sympathetic outflow  They regulate blood flow through the kidneys  The Nephron Nephrons are the structural and functional units of the kidney.  Each kidney has over 1 million nephrons. Each nephron consists of: 1. Glomerulus: a tuft of capillaries and a renal tubule. 2. Glomerular capsule (Bowman’s capsule): The tubule forms a cup shape around the glomerulus 3. Renal corpuscle filters the blood plasma 4. Renal tubule modifies the filtrate 5. Mesangial cells (smooth muscle-like cells) are contractile and help regulate glomerular filtration.  The Renal Corpuscle The Renal Corpuscle consists of two parts: 1. The Glomerular (Bowman’s) Capsule  Has a visceral layer of podocytes that wrap around the capillaries.  The filtrate is collected between the visceral and parietal layers.  The Renal Corpuscle 2. The Glomerulus  It is a mass of capillaries.  The Afferent Arteriole feeds it and drains into the Efferent Arteriole  The glomerular endothelial cells have large pores (fenestrations) and are leaky.  Basal lamina lies between endothelium and podocytes.  Podocytes form pedicels which are filtration slits  The Renal Tubule The filtrate passes from the glomerular capsule to the renal tubule Proximal Convoluted Tubule Nephron Loops Descending Loop Ascending Loop Distal Convoluted Tubule  The Juxtaglomerular Apparatus The ascending loop contacts the afferent arteriole at the macula densa. The wall of the arteriole contains smooth muscle cells; juxtaglomerular cells.  Enlarged, smooth muscle cells  Have secretory granules containing renin  Act as mechanoreceptors The apparatus regulates blood pressure in the kidney in conjunction with the ANS. Juxtaglomerular Apparatus  Two Kinds of Nephrons 1. Cortical Nephrons Represents 80-85% of nephrons The renal corpuscle found in the outer portion of cortex Loops are short, loop of Henle extend only into outer region of medulla Create urine with osmolarity similar to blood Cortical Nephrons 2. Juxtamedullary Nephrons Renal corpuscle deep in cortex with long nephron loops Receive blood from peritubular capillaries and form the vasa recta that arise from efferent arterioles. Ascending limb has thick and thin regions Enable the kidney to secrete very concentrated urine Juxtamedullary  Renal Tubule 1. Proximal convoluted tubule (PCT):  Composed of cuboidal cells with numerous microvilli and mitochondria  Convoluted means the tubule is tightly coiled rather than straight  Function: Reabsorbs water and solutes from the filtrate and secretes substances into it 2. Loop of Henle:  Hairpin-shaped loop of the renal tubule  The proximal part is similar to the proximal convoluted tubule  The proximal part is followed by the thin segment (Descending limb) and thick segment (Ascending limb)  Renal Tubule 3. Distal convoluted tubule (DCT):  cuboidal cells without microvilli that function more in secretion than reabsorption Two different types of cells are present In the last part of the DCT: 1. Principal cells: which have receptors for both antidiuretic hormone (ADH) and aldosterone 2. Intercalated cells: Play a role in the homeostasis of blood pH. Renal Tubule  Collecting duct (CD) The distal convoluted tubules of several nephrons empty into a single collecting duct (CD). Collecting ducts unite and converge into large papillary ducts, which drain into the minor calyces. The collecting ducts and papillary ducts extend from the renal cortex through the renal medulla to the renal pelvis. Capillaries Associated with Nephrons Nephrons are associated with 2 sets of capillaries: glomerular and peritubular. The glomerular capillaries are specialized for filtration. glomerular capillaries are fed and drained by afferent and efferent Glomerular capillaries allow the blood pressure in the capillary bed to be very high and force fluid and solute out of the blood into the glomerular capsule. Most of the filtrate is reabsorbed in the renal tubule cells and returns to the blood through the peritubular capillaries  Capillary Beds Peritubular beds are low-pressure adapted for absorption that:  Arise from efferent arterioles  Empty into the renal venous system Vasa recta – long, straight efferent arterioles of juxtamedullary nephrons Capillary Beds Renal Physiology - Urine Formation Excretion of a solute = glomerular filtration + secretion - reabsorption  Urine Formation 1.Glomerular filtration 2.Tubular reabsorption 3. Tubular secretion Excretion of a solute = glomerular filtration + secretion - reabsorption Urine Formation  Glomerular Filtration Driven by blood pressure Opposed by capsular hydrostatic pressure and blood colloid osmotic pressure Water and small molecules move out of the glomerulus. Each day, 150–180 liters of water pass out into the glomerular capsule Filtration is a nonselective, passive process  Glomerular Filtration Both proteins and blood cells are normally too large to pass through the filtration membrane Depends on the systemic blood pressure If arterial blood pressure drops too low, the glomerular pressure becomes inadequate to force substances out of the blood into the tubules, and filtrate formation stops. Glomerular Filtration Reham Khalaf-Nazzal, MD PhD, Faculty of Medicine, AAUP  Glomerular Filtration Regulation of the GFR is critical to maintaining homeostasis and is regulated by: 1. Renal autoregulation 2. Neural regulation 3. Hormonal regulation  Glomerular Filtration 1. Renal autoregulation of GFR occurs by two means:  Stretching in the glomerular capillaries triggers the contraction of smooth muscle cells in afferent arterioles (reduces GFR).  Pressure and flow are monitored in the macula densa, causing the afferent arterioles to constrict (decreasing blood flow and (GFR) or dilate (increasing blood flow and (GFR) appropriately.  Myogenic Mechanism: Smooth muscle cells in afferent arterioles contract in response to elevated blood pressure  Glomerular Filtration 2. Neural regulation of GFR is possible because: The renal blood vessels are supplied by sympathetic ANS fibers that release norepinephrine causing vasoconstriction. Sympathetic input to the kidneys is most important with extreme drops of B.P. (as occurs with hemorrhage)  Glomerular Filtration 3. Hormonal Regulation Renin-Angiotensin-Aldosterone System Is triggered when the JG cells release renin→ Renin acts on angiotensinogen to release angiotensin I → converted to angiotensin II→ causes the arterial pressure to rise → stimulates the adrenal cortex to release aldosterone→ both systemic and glomerular hydrostatic pressure rise  Hormonal Regulation Renin-Angiotensin-Aldosterone System  Glomerular Filtration Two hormones contribute to the regulation of GFR 1. Angiotensin II is a vasoconstrictor of both afferent and efferent arterioles (reduces GFR) 2. A sudden large increase in BP stretches the cardiac atria and releases atrial natriuretic peptide (ANP) ANP causes the glomerulus to relax, increasing the surface area for filtration.  Tubular Reabsorption 99%of the glomerular filtrate is reabsorbed Tubular reabsorption Is the process of returning important substances from the filtrate back into the renal interstitium, then into the renal blood vessels, and ultimately back into the body. Important substances are: Glucose, electrolytes, vitamins, water, amino acids, and any small proteins  Tubular Reabsorption Amount Total returned to Amount in 180 blood/d Amount in Amount L of filtrate Urine in Plasma (Reabsorbe (/day) d) (/day) Water 3L 180 L 178-179 L 1-2 L (passive) Protein 200 g 2g 1.9 g 0.1 g (active) Glucose 3g 162 g 162 g 0g (active) 24 g 30 g Urea (passive) 1g 54 g (about 1/2) (about 1/2) 0g 1.6 g  Tubular Reabsorption Reabsorption into the interstitial has two routes: Paracellular reabsorption is a passive process that occurs between adjacent tubule cells Transcellular reabsorption is movement through an individual cell.  Tubular Reabsorption Reabsorption of fluids, ions, and other substances occurs by active & passive means. A variety of symporters & antiporters actively transport Na , Cl, Ca2, H, HCO3, glucose, HPO4,SO4, NH4 , urea, all amino acids, and lactic acid. Reabsorption of water can be obligatory or facultative Water always moves by osmosis down its concentration gradient depending on the permeability of the tubule cells  Tubular Reabsorption Obligatory reabsorption of water Occurs when the water is obliged to follow the solutes as they are reabsorbed (to maintain the osmotic gradient). Facultative reabsorption Describes variable water reabsorption, adapted to specific needs. It is regulated by the effects of ADH and aldosterone on the principal cells of the renal tubules and collecting ducts. Transport Mechanisms Symport Antiporter Passive Reabsorption in the PCT  Tubular Reabsorption Countercurrent multiplication is the process by which a progressively increasing osmotic gradient is formed in the interstitial fluid of the renal medulla as a result of countercurrent flow Countercurrent exchange is the process by which solutes and water are passively exchanged between the blood of the vasa recta and interstitial fluid of the renal medulla due to countercurrent flow.  This provides oxygen and nutrients to the renal medulla without washing out or diminishing the gradient dilute urine through obligatory reabsorption of water. Urine can be up to 4 times more concentrated than blood plasma  Tubular Secretion If higher than normal amounts of a substance are present in the filtrate, then the renal threshold for reabsorption of that substance may be surpassed. When that happens, the substance cannot be reabsorbed fast enough, and it will be excreted in the urine. For example, the renal [reabsorption] threshold of glucose is 180-200mg/dl. When this level is exceeded (as in diabetes mellitus), the glucose is said to “spill” into the urine  Meaning a substance which is not normally present in urine begins to appear).  Tubular Secretion Hydrogen and ammonium ions are secreted to decrease the acidity in the body, and bicarbonate is conserved. Secreted substances include H, K, NH4, and some Drugs; the amount often depends on body needs.  Tubular Secretion Tubular secretion is the movement of substances from the capillaries which surround the nephron into the filtrate. It occurs at a site other than the filtration membrane (in the proximal convoluted tubule, distal convoluted tubule, and collecting ducts) by active transport. The process of tubular secretion controls pH.  Tubular Secretion Maintaining the body in proper pH requires cooperation mainly between the lungs and the kidneys. According to the lung The lungs eliminate CO2. Provides a rapid response (minutes) According to the kidneys The kidneys eliminate H+ and NH+ This is a slower response(hours-days).  Hormones and Homeostasis Five hormones affect the extent of Na, Cl, Ca2, and water reabsorption as well as K secretion by the renal tubules. These hormones, play a role in maintaining homeostasis of blood flow and blood pressure (B.P) 1. Angiotensin II 2. Antidiuretic hormone (ADH) 3. Aldosterone 4. Atrial natriuretic peptide (ANP) 5. Parathyroid hormone (PTH)  Hormones and Homeostasis ADH is released by the posterior pituitary in response to low blood flow in this part of the brain. ADH affects facultative water reabsorption by increasing the water permeability of principal cells in the last part of the distal convoluted tubule and throughout the collecting duct. In the absence of ADH, the apical membranes of principal cells When blood volume and blood pressure decrease or the sympathetic NS is stimulated, the walls of the afferent arterioles are stretched less, and the cells of the JGA secrete renin.  Hormones and Homeostasis Renin clips off angiotensin I from angiotensinogen, which is synthesized by hepatocytes. angiotensin converting enzyme (ACE) converts angiotensin I to angiotensin II, which is the active form of the hormone. Angiotensin II has 3 main effects: 1. Vasoconstriction decreases GFR. 2. It increases blood volume by increasing the reabsorption of water and electrolytes in the PCT. 3. It stimulates the adrenal cortex to release aldosterone Renin-Angiotensin-Aldosterone (RAA) pathway  Hormones and Homeostasis Aldosterone stimulates the principal cells in the collecting ducts to reabsorb more Na and Cl and secrete more K. The osmotic consequence of reabsorbing more Na and Cl is that more water is reabsorbed, which increases blood volume and blood pressure.  Urine Increases in GFR usually increase urine production. The mechanisms that control electrolyte and water reabsorption in the various parts of the nephron and collecting ducts are subject to many complex controls. Normal urine output (UOP) is 1-2 L/d.  Urine A urinalysis analyzes the physical, chemical, and microscopic properties of urine. Water accounts for 95% of total urine volume. The solutes normally present in urine are filtered and secreted substances that are not reabsorbed. If the disease alters metabolism or kidney function, traces of substances normally not present or normal constituents in abnormal amounts may appear (bacteria, albumin protein, glucose, white blood cells, and red blood cells to name a few).  Urine Blood urea nitrogen (BUN) measures nitrogen wastes in blood from catabolism and deamination of amino acids (7 - 20 mg/dL). Creatinine levels appear in the blood as a result of catabolism of creatine phosphate in skeletal muscle (0.7 to 1.3 mg/dL) The serum creatinine test measures the amount of creatinine in the blood, which increases in states of renal dysfunction.  Urine Transportation and Storage Each ureter transports urine from a renal pelvis by peristaltic waves, hydrostatic pressure, and gravity. No anatomical valve at the opening of the ureter into the bladder – when the bladder fills, it compresses the opening and prevents backflow. The bladder is a hollow, distensible, muscular organ with a capacity averaging 700–800 mL Ureters, Bladder, and Urethra in a female  Micturition The discharge of urine involves voluntary and involuntary muscle contractions. Stretch receptors trigger a spinal reflex, which we learn to control in childhood. The urethra carries urine from the internal urethral orifice to the exterior of the body. In males, it discharges semen as well as urine. Male and Female Urethras