Lecture 18 The Urinary System
5 Functions of the Urinary System 1.
Regulate blood volume and blood pressure:
2.
Regulate plasma ion concentrations:
3.
sodium, potassium, and chloride ions (by controlling quantities lost in urine) calcium ion levels (through synthesis of calcitriol)
Help stabilize blood pH:
4.
by controlling loss of hydrogen ions and bicarbonate ions in urine
Conserve valuable nutrients:
5.
by adjusting volume of water lost in urine releasing erythropoietin and renin
by preventing excretion while excreting organic waste products
Assist liver to detoxify poisons
Urinary System Organs Kidney – produces urine Urinary bladder – provides a temporary storage reservoir for urine Paired ureters – transport urine from the kidneys to the bladder Urethra – transports urine from the bladder out of the body
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Quick Facts on the Kidneys
The kidneys filter 200 liters of blood daily, allowing toxins, metabolic wastes, and excess ions to leave the body in urine Approximately one-fourth (1200 ml) of systemic cardiac output flows through the kidneys each minute All the blood in the body is filtered 60 times everyday (2½ times every hour)
Internal Anatomy of the Kidney
A frontal section shows three distinct regions
Cortex – the light colored, granular outer region Medulla – the inner region that exhibits cone-shaped medullary (renal) pyramids 6-18 Pyramids are made up of parallel bundles of urine-collecting tubules Renal columns are inward extensions of cortical tissue that separate the pyramids A Lobe is a medullary pyramid and its surrounding capsule Papillae – drain urine from a lobe into a minor calyx
Draining passages Minor Calyces – small branches between the lobes and the major calyces Major calyces – large branches of the renal pelvis Renal pelvis – flat, funnel-shaped tube lateral to the hilus within the renal sinus
Urine flows through the pelvis and ureters to the bladder
Renal Pyramids Contain Nephrons A nephron is composed of three regions 1. A Filter (the Renal Corpuscle): Glomerular capsule encloses a Glomerulus a fine network of capillaries
2. A Tubule (3 sections): Proximal Convoluted Tubule (PCT) Loop of Henle is bent back on itself in the center Distal Convoluted Tubule (DCT)
Site of reabsorption & secretion 3. A Collecting Duct Water conservation device
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Filtration Membrane
Figure 25.7a
The glomerulus is a filter that lies between the blood and the interior of the glomerular capsule It is composed of three layers Fenestrated endothelium of the glomerular capillaries Visceral membrane of the glomerular capsule (podocytes) Basement membrane composed of fused basal laminae of the other layers
2 Types of Nephrons
1. Cortical nephrons – 85% of nephrons; located in the cortex 2. Juxtamedullary nephrons: Are located at the cortex-medulla junction Have loops of Henle that deeply invade the medulla Have extensive thin segments Are involved in the production of concentrated urine
Capillary Beds
Every nephron has two capillary beds Glomerular capillaries Peritubular capillaries
Each glomerulus is: Fed by an afferent arteriole Drained by an efferent arteriole
Blood pressure in the glomerulus is high because: Arterioles are high-resistance vessels Afferent arterioles have larger diameters than efferent arterioles Fluids and solutes are forced out of the blood throughout the entire length of the glomerulus Peritubular beds are low-pressure, porous capillaries adapted for absorption that: Arise from efferent arterioles Cling to adjacent renal tubules Empty into the renal venous system Vasa recta – long, straight efferent arterioles of juxtamedullary nephrons
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Juxtaglomerular Apparatus (JGA)
The distal tubule lies against the afferent (sometimes efferent) arteriole
Arteriole walls have juxtaglomerular (JG) cells
Enlarged, smooth muscle cells Have secretory granules containing renin Act as mechanoreceptors
Macula densa cells
Tall, closely packed distal tubule cells Lie adjacent to JG cells Function as chemoreceptors or osmoreceptors
Mechanisms of Urine Formation
The kidneys filter the body’s entire plasma volume 60 times each day The filtrate: Contains all plasma components except protein Loses water, nutrients, and essential ions to become urine The urine contains metabolic wastes and unneeded substances
Mechanisms of Urine Formation
Urine formation and adjustment of blood composition involves three major processes Glomerular filtration Tubular reabsorption Tubular secretion
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Glomerular Filtration Rate (GFR) The total amount of filtrate formed per minute by the kidneys Factors governing filtration rate at the capillary bed are: Total surface area available for filtration Filtration membrane permeability Net filtration pressure (NFP) Changes in GFR normally result from changes in glomerular blood pressure GFR is directly proportional to the NFP
Glomerular Filtration The glomerulus is more efficient than other capillary beds because: Its filtration membrane is significantly more permeable Glomerular blood pressure is higher It has a higher net filtration pressure
Plasma proteins are not filtered and are used to maintain osmotic pressure of the blood If the GFR is too high: Needed substances cannot be reabsorbed quickly enough and are lost in the urine
If the GFR is too low: Everything is reabsorbed, including wastes that are normally disposed of
Three mechanisms control the GFR Renal autoregulation (intrinsic system) Neural controls Hormonal mechanism (the renin-angiotensin system)
Absorption in Renal Tubules and Collecting Ducts PCT reabsorbs substances including: Sodium, all nutrients, cations, anions, and water Urea and lipid-soluble solutes Small proteins
Loop of Henle reabsorbs:
H2O, Na+, Cl−, K+ in the descending limb Ca2+, Mg2+, and Na+ in the ascending limb
DCT absorbs:
Ca2+, Na+, H+, K+, and water HCO3− and Cl−
Collecting duct absorbs: Water and urea
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How Kidney Tubules Create an Osmotic Gradient and produce the initial dilute urine
~ 300 mOsm
(Filtrate ~ 100 mOsm)
Proximal Convoluted Tubule Active reabsorption Na+ Permeable to Water Not to Solutes
Na+/K+-Clsymporter
Additional 10% of original water passively reabsorbed
Na+-Hantiporter
Permeable to Solutes Not to Water Additional 40% of original NaCl reabsorbed
200 mOsm
~ 1200 mOsm
Passive reabsorption Na+
Loop of Henle
The Countercurrent Multiplier
Vasa Recta: Countercurrent Exchange
The vasa recta is a countercurrent exchanger that: Delivers blood to the cells in the area While maintaining the osmotic gradient
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How Kidney Tubules Produce Concentrated Urine Common Process
Variable Process
Producing initial dilute urine
Producing concentrated urine
Water Reabsorption IF plasma > 300 mOsm Pituitary releases ADH
~ 300 mOsm (Filtrate ~ 100 mOsm)
Distal Convoluted Tubule
Proximal Convoluted Tubule
Permeable to Solutes Not to Water Additional 40% of original NaCl reabsorbed
Additional 10% of original water passively reabsorbed 200 mOsm
Medullary Collecting Duct
Permeable to Water Not to Solutes
Insertion of aquaporins into luminal membrane Water (& urea) Reabsorbed
~ 1200 mOsm
Loop of Henle
Summary of Urine Concentration
Two things are required to concentrate urine A high osmotic gradient in the kidney’s medulla The presence of ADH
The loop of Henle is the primary structure responsible for maintaining the hypertonic medullary interstitium The presence of ADH is mediated by blood plasma osmolality as sensed by the hypothalamus The final concentration of urine cannot exceed the mOsm level of the most concentrated part of the hypertonic medullary interstitium.
Summary Quiz on Concentration of Urine 6.
~ 300 mOsm
When the DCT in this region is permeable to water, will water flow into or out of the tubule?
Distal Convoluted Tubule
Proximal Convoluted Tubule
(Filtrate ~ 100 mOsm)
What flows out of the filtrate in the ascending limb?
1. What flows out of the filtrate in the descending limb?
3. What hormone mediates the process in the previous question? 4. Does the filtrate have higher osmolality in the Descending or Ascending limb?
~ 1200 mOsm
Loop of Henle
5. This difference in osmolality is called the?
Medullary Collecting Duct
2.
7. What hormone mediates water permeability in the tubules? 8. What condition stimulates its release? 9. In addition to water, what solute flows out of the duct in this region? 10. What does this solute help maintain?
11. As water is removed from the filtrate what happens to the filtrate volume?
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