Organization of the Urinary System

•Two kidneys •Two ureters •One urinary bladder •One urethra

Kidney Location

Functions of the Urinary System •Regulates blood volume and blood pressure •Regulate plasma concentrations of ions such as sodium, potassium, chloride •Helps to stabilize blood pH •Conserves valuable nutrients •Assists the liver in detoxification

Supporting Connective Tissues of Kidneys

•Paired organs that are located on each side of the vertebral column •between the levels of the 12th thoracic and 3rd lumbar vertebrae •retroperitoneal Parietal peritoneum

Liver

Renal Fascia Renal Capsule Adipose Capsule

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Blood Supply of Kidney

Kidney Anatomy

•Hilus •Renal cortex •Renal medulla •Renal pyramids •Renal columns •Renal lobe •Renal papilla •Minor Calyx •Major Calyx •Renal Pelvis

Segmental Arteries •The renal artery, which arises from the abdominal aorta, brings blood to the kidney. •the renal artery branches many Renal Artery times into smaller arteries and eventually supplies blood to the nephrons via the afferent arteriole.

lobar Artery

Interlobar Artery Arcuate Artery Interlobular Artery

Blood Supply of Kidney

Nephron

•Blood leaves the kidney through a series of veins that progressively enlarge and finally form the renal vein, which exits the kidney at the hilus.

•The functional unit (urine making unit) of the kidney. •Each kidney contains approximately 1 million nephrons.

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Nephron

Nephron

•Cortical nephrons •85% •Majority of nephron located in cortex •Juxtamedullary Nephron •15% •Renal corpuscle next to corticomeullary junction •Loops are long and go deep into medulla •Play important role in kidney’s ability to produce concentrated urine

Renal Tubule

Renal Tubule: Glomerular Capsule

•Renal tubule: The tubule that carries fluid away from the glomerular capsule and is composed of three segments •Proximal convoluted tubule •Loop of Henle •descending limb •ascending limb •Distal convoluted tubule

•A “C”-shaped structure that surrounds the glomerulus •parietal epithelium: the epithelium that lines the outer wall of the capsule •visceral epithelium: the epithelium that lines the glomerular capillaries •Podocytes:Cells that comprise the visceral epithelium and have foot-like processes •capsular space: interior region of the glomerular capsule that connects with the renal tubule

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Renal Tubule: Glomerular Capsule Foot Processes

Cell Body

Renal Tubule •Proximal convoluted tubule •Loop of Henle •descending limb •ascending limb •Distal convoluted tubule

Cell 1

Cell 2

Renal Tubule

Collecting Ducts, Papillary Ducts •Collecting ducts •Receives urine from many nephrons via the DCT •Each collecting duct begins in the cortex and descends into the medulla, carrying fluid to a papillary duct •Papillary ducts: collecting ducts fuse to form papillary ducts that dump urine into the minor calyces

Thin Segments

Loop!

Collecting tubules

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Microvasculature •Afferent arteriole: the arteriole that brings blood to the glomerulus •Glomerulus •Capillary bed that is surrounded by the glomerular capsule •The endothelium of the glomerulus is fenestrated to allow the passage of water and solutes. •Efferent arteriole •The arteriole that drains the glomerulus •The efferent tubule is smaller in diameter than the afferent arteriole.

Renal Corpuscle, JG Apparatus •Renal corpuscle: the glomerulus and the glomerular (Bowman’s) capsule •Juxtaglomerular apparatus: a specialized region in each nephron where the DCT lies against the afferent arteriole •Juxtaglomerular cells: enlarged smooth muscle cells in the walls of the afferent arterioles that secrete renin •Macula densa: a group of tall, closely packed cells in the distal tubule that monitors the solute content of the filtrate in the tubule lumen

Microvasculature •Peritubular capillaries •Network of capillaries that surround the tubular portion of the nephron •Capillaries that are specialized for absorption •Peritubular capillaries eventually reunite to form venules and the venules come together to form veins. Efferent Arteriole

Ureter •Connective tissue: continuous with renal capsule •Muscular layer: inner longitudinal & outer circular •Mucosa: transitional epithelium & underlying lamina propria

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Urinary Bladder •A hollow muscular organ that functions as a temporary storage site for urine. •trigone: Triangular area formed by ureters and urethra •internal urethral sphincter: Smooth muscle that surrounds the opening of the urethra that provides involuntary control over the release of urine from the bladder. •Mucosa: transitional epithelium •Submucosa •Muscularis: three layers = detrusor muscle

Urine Formation: Terminology •Filtrate: fluid that contains all of the same components as blood plasma except it does not contain plasma proteins •Urine: The fluid that reaches the collecting ducts that has lost most of its water, nutrients, and essential ions but does contain metabolic wastes and unneeded substances.

Approximately 1-1.2 liters of blood passes through the glomeruli each minute. 650 ml of the blood is plasma, and 120-125 ml of plasma is forced into the renal tubules each minute. This is equivalent to filtering your entire plasma volume more than 60 times each day.

Urine Formation

Urine Formation: Processes •Three major processes •Glomerular filtration •Tubular reabsorption •Secretion

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Foot Processes

Glomerular Filtration •A passive nonselective process that forces fluid and solids through the filtration membrane •Filtration membrane •Fenestrated endothelium of the glomerular capillaries •The visceral epithelium of the glomerular capsule (bowman’s capsule) containing podocytes •Fused basement membrane of the capillaries and visceral epithelium

Urinary Space

Filtration Membrane

Endothelial Cell

Capillary Lumen Fenestrations

Basal Lamina Podocyte Processes

Podocyte Filtration slits Polyanionic Surface coat Processes

Foot Processes Filtration slits

Basal lamina

Fenestrae Basal lamina

Capillary lumen

Capillary lumen

Pore

Plasma passes through fenestrae at high hydrostatic pressure

Glomerular Filtration

Glomerular Filtration •The glomerulus is a more efficient filter than other capillaries •The filtration membrane of the glomerulus is thousands of times more permeable than other capillary membranes •Glomerular blood pressure is much higher than that of other capillary beds •Approximately 180 L of filtrate is produced each day

•Substances must be small enough to pass through the membrane •Substances that will pass through: water, ions, glucose, amino acids, and nitrogenous wastes •Substances that do not pass through: plasma proteins. Plasma proteins that remain in the glomerular capillaries maintain colloidal osmotic pressure, which stops all of the water from passing into the renal tubules.

Urinary Space Urinary Space Podocyte Processes

Filtration slits

Fenestrae

Basal lamina

Polyanionic Surface coat

Podocyte Processes

Filtration slits

Fenestrae

Basal lamina

Polyanionic Surface coat

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Glomerular Filtration Rate •Factors affecting GFR: •glomerular hydrostatic pressure (forces fluid into the capsular space) •blood osmotic pressure (force that opposes GHP and is exerted by the plasma proteins that keeps some fluid in the glomerular capillaries) •capsular hydrostatic pressure (force that opposes GHP and tries to force fluid back into the glomerulus) •net filtration pressure: the amount of pressure that forces fluid out of the glomerulus

Regulation of Filtration •The filtration rate must be kept constant so the body can excrete waste products, maintain pH, and regulate blood volume •Proper filtration depends on adequate blood flow to the glomerulus and the maintenance of normal filtration pressures

Fig 21.5

Autoregulation •Autoregulation uncouples renal function from arterial blood pressure, meaning GFR and RBF remain constant despite fluctuations in systemic arterial blood pressure.

•A rate that is too fast will cause a loss of nutrients because flow of the filtrate is too fast to allow for reabsorption. •A rate that is too slow will cause wastes to be reabsorbed.

Autoregulation: Myogenic Mechanism •Systemic BP declines = Decrease in the glomerular filtration rate •Reduction in the filtration rate = dilation of the afferent arteriole and a constriction of the efferent arteriole = Increased GFR

•Systemic BP increases = Increase in glomerular filtration rate •High pressure is exerted against the wall of the afferent arterioles = The smooth muscle responds by vasoconstriction = decreased GFR

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Autoregulation: Tubuloglomerular Feedback •The macula densa of the juxtaglomerular apparatus is sensitive to flow rate and filtrate osmolarity. •Slow flow and low osmolarity = vasodilation of the afferent arteriole by acting on the smooth muscle cells = Increases GFR. •Fast flow and high osmolarity = vasoconstriction of the smooth muscle in the walls of afferent arterioles = Decreases GFR

Sympathetic Nervous System •Stimulation of the sympathetic nervous system causes vasoconstriction of the afferent arterioles = decreased blood flow to the glomerulus and shunts blood to the heart, muscles, and brain.

Sympathetic Nervous System Stimulation

Hormonal Mechanism •Slow flow and Low osmolarity also causes the JG cells to set the renin-angiotensin mechanism into motion. •Decreased stretch of the wall of the afferent arteriole also plays a part •Renin converts angiotensinogen to angiotensin I. •Angiotensin I is converted to angiotensin II by angiotensin converting enzyme (ACE). •Angiotensin II causes vasoconstriction of the efferent arteriole, stimulates secretion of ADH, stimulates the secretion of aldosterone, and causes vasoconstriction of peripheral precapillary sphincters.

Tubular Reabsorption •About 99% of the glomerular filtrate is reabsorbed into the blood as it passes through the renal tubules and ducts •Approximately 1% of the filtrate becomes urine and will leave the body •Reabsorption allows for the body to retain most of its nutrients. •In healthy kidneys, 100 % of filtered organic solutes such as glucose and amino acids are reabsorbed. •Epithelial cells lining the renal tubule and collecting duct carry out reabsorption, but those of the PCT reabsorb the bulk of the fluid and solutes.

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Sodium Reabsorption •Most of the energy used by the kidneys is for the reabsorption of sodium ions. •Sodium reabsorption occurs across cell membranes (transcellular) and is an active process. •Primary active transport: Sodium is pumped out of the cell on the basolateral membrane and then diffuses into the peritubular capillaries. •Na+ is “pulled” into the cell from the tubule lumen by facilitated diffusion (passive transport) using a symporter.

Passive Transport •The electrical gradient established by the positively charged sodium ions moving into the peritubular capillaries “pulls” (passive transport) negative ions into peritubular capillaries. •To equal the difference in charges, anions such as Cl, and HCO3- are transported into the peritubular capillaries.

Passive Transport •Reabsorption of water is by the process of osmosis •Reabsorption of molecules and ions creates a concentration gradient and as a result water follows •Obligatory water reabsorption

Nonreabsorbed Substances ƒMost important substances not reabsorbed ƒUrea ƒUric acid ƒcreatinine (used for measuring GFR) ƒReasons not reabsorbed ƒLack carriers ƒAre not lipid soluble ƒDon’t fit through plasma membrane pores in tubular cells

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Secondary Active Transport •As a carrier (symporter) transports Na+ into a tubule cell by facilitated diffusion, another molecule attaches to the transporter and is also transported into the cell. •Glucose, amino acids, lactate, vitamins, and most cations are reabsorbed in this way.

Tubular Secretion •The transport of molecules and ions from the blood in the peritubular capillaries or from tubule cells into the filtrate. •to dispose of substances that were not filtered, such as drugs (penicillin) •to eliminate unwanted substances that were reabsorbed, such as urea and uric acid •to eliminate excess K+ ions •to control blood pH •decreased blood pH causes the renal tubule to secrete more H+ and reabsorb more HCO3•increased blood pH causes more Cl- to be reabsorbed instead of HCO3-

HCO3-

H+

pH

Urine Concentration & Volume

H+

Cl-

pH

Regulation of Urine Concentration & Volume

•The kidneys play an important role in regulating blood volume, blood pressure, and blood concentration •If the concentration of solutes in the blood increases above normal, the kidneys excrete a small amount of concentrated urine. This conserves water and gets rid of solutes. •If the concentration of solutes decreases below normal, the kidneys conserve solutes and gets rid of water.

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Regulation of Urine Concentration & Volume

Hormonal Regulation: ADH

H2O

Concentrated Urine

Hormonal Regulation: Aldosterone

Renin Decreased BP

Aldosterone causes increased reabsorption of Na+and as a result water follows by osmosis.

Angiotensinogen

Angiotensin I

•A hormone released from the posterior lobe of the pituitary gland •ADH makes the distal convoluted tubule and collecting duct more permeable to water. •The result is more concentrated urine with less water present and increased water in the blood causing an increased blood volume and BP.

Hormonal Regulation: ANP •a hormone produced by the stretching of the atria (increased blood volume) •ANP acts on the kidney tubules to inhibit Na+ reabsorption and inhibits ADH, renin, and aldosterone. •The result of ANP is more Na+ and water in the urine and less water in the blood causing a decrease in blood volume and blood pressure.

ACE Angiotensin II

Vasoconstriction

Inhibit Na+, ADH, Aldosterone = Less H2O Reabsorption

More H20 in urine Decrease BP and Volume

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Micturition

Acid-Base Balance •Blood has a normal pH of 7.35-7.45. •Deviations below this range are called acidosis. •Deviations above this range are called alkalosis. •Cellular metabolism produces substances that alter the pH balance. •Glycolysis produces lactic acid. •CO2, which is produced from aerobic respiration combines with H2O to produce carbonic acid. •Metabolism of fatty acids produces acidic ketone bodies.

Acid-Base Balance •Buffers •substances that prevent significant changes in pH and are important for many small adjustments in H+ ion concentration •If H+ ion concentration is too high the buffer combines with H+ ions to bring the pH back to normal. •If H+ ion concentration is too low the buffer releases H+ ions to lower the pH. •Important blood buffer system •Rise in pH: H2CO3 Æ HCO3- + H+ •Low pH: H+ HCO3- Æ H2CO3

Acid-Base Balance •The removal of CO2 from the blood as it flows through the lungs •The release of CO2 will allow the pH of the blood to elevate •H+ + HCO3- ÅÆ H2CO3 ÅÆ H2O + CO2 CO2

CO2 + H2O

7% blood Plasma

CO2-Hb

H2CO3 H+ + HCO3-

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Acid-Base Balance •Removal of H+ ions by the kidney •If the blood is too acidic, the tubule cells of the kidney secrete H+ ions into the lumen of the tubule. •If the blood is too alkaline, the tubule cells of the kidney conserve H+ ions.

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