The Urinary System: Filtration and Fluid Balance

CHAPTER

ESSENTIALS OF A&PFOR EMERGENCY CARE

Copyright ©2011 by Pearson Education, Inc.All rights reserved.

Essentials of A&P for Emergency CareBruce J. Colbert • Jeff Ankney • Karen T. Lee • Bryan E. Bledsoe

The Urinary System: Filtration and Fluid Balance

17



Multimedia Asset Directory

Slide 13 Urinary System AnimationSlide 24 Renal Blood Flow AnimationSlide 40 Hypovolemic Shock AnimationSlide 50 Blood Loss and Blood Pressure Regulation AnimationSlide 51 Renin-Angiotension System AnimationSlide 54 Urinalysis VideoSlide 111 Renal Failure VideoSlide 112 Kidney Stones VideoSlide 113 Ultrasound Video



Introduction

• The urinary system acts as a purification plant, cleaning the blood of waste materials.

• The liver does some purification, but the urinary system controls electrolyte and fluid balances for your body.

• The kidneys filter blood, reabsorb and secrete ions, and produce urine.

• Without this important function you would die in a few days.



Learning Objectives

• Present an overview of the organs and functions of the urinary system.

• Describe the internal and external anatomy and physiology of the kidneys.

• Discuss the importance of renal blood flow.

• Describe the process of urine formation.



Learning Objectives

• Trace the pathways of reabsorption or secretion of electrolytes and other chemicals.

• List and discuss the importance of hormones for proper kidney function.

• Describe the anatomy and physiology of the bladder and urine removal from the body.

• Discuss several common disorders of the urinary system.



Pronunciation GuideClick on the megaphone icon before each item to hear the pronunciation.

afferent arterioles (AFF er ent ahr TEE ree ohlz)aldosterone (al DOSS ter own)antidiuretic hormone (ADH) (AN tih dye yoo RET

ick)atrial natriuretic peptide (AY tree al NAY tree your

ET ick PEP tide)calyx, calyces (KAY licks, KAY leh seez)cortical nephron (CORE tih cull NEFF rahn)efferent arterioles (EFF er ent ahr TEE ree ohlz)




external urethral sphincter (yoo REE thral SFINK ter)

glomerular capsule (gloh MAIR you ler)glomerulus (gloh MAIR yoo lus)juxtaglomedullary nephron (JUX tuh glow med

DULL lair ee NEFF rahn)juxtaglomerular cells (JUX tuh glow MARE you lair)renal hilium (REE nal HIGH lum)renal medulla (REE nal meh DULL lah)




renin-angiotensin-aldosterone (RIN en-an gee oh TEN sen-al DOSS ter own)

ureter (yoo REE ter)urethra (you REE thrah)



System Overview

• The urinary system consists of two kidneys; bean-shaped organs located in the superior dorsal abdominal cavity that filter blood and make urine, and accessory structures.

• A ureter is a tube that carries urine from each kidney to the single urinary bladder, located in the inferior ventral pelvic cavity.



System Overview

• The urinary bladder is basically an expandable sac that holds urine.

• The urethra is the tube that transports urine from the bladder to the outside of the body.

• The job of the urinary system is to make urine, thereby controlling the body’s fluid and electrolyte balance, and eliminating waste products.



System Overview

• To make urine, three processes are necessary:– Filtration – filtering the blood. What passes

through the filter is called a filtrate. – Reabsorption – substances stay in the body

after being removed from urine– Secretion – substances move from the blood

stream and leave the body in the urine



Figure 17-1 Anatomy of the urinary system.



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External Anatomy of the Kidney

• The kidney is covered by a fibrous layer of connective tissue called the renal capsule.

• The indentation that gives the kidney it’s bean-shape is called the renal hilum.

• At the hilum, renal arteries bring blood to the kidneys to be filtered and renal veins take the filtered blood away from the kidney. The ureter is also attached at the hilum to transport urine from the kidney to the bladder.



Internal Anatomy of the Kidney

• The kidney can be divided into three layers:– Renal cortex – outer layer – grainy in

appearance and has little obvious structure to the naked eye; this is where blood filtration occurs

– Renal medulla – middle layer – contains a number of triangle-shaped, striped areas called renal pyramids Composed of collecting tubules for the urine that is

formed in the kidney Adjacent pyramids are separated by narrow renal

columns – extensions of cortical tissue



Internal Anatomy of the Kidney

• The kidney can be divided into three layers:– Renal pelvis – inner layer – a funnel, divided

into two or three large collecting cups called major calyces. Each major calyx is divided into several minor

calyces, forming cup-shaped areas around the tips of the pyramids.



Internal Anatomy of Kidney

• The blood is filtered by millions of tiny filters in the cortex, and the filtrate flows through tiny tubules in the medulla and collects in the renal pelvis.

• The renal pelvis, the enlarged proximal portion of the ureter, empties into the ureter where urine is carried to the bladder.



Figure 17-2 The internal and external anatomy of the kidney.




• Blood vessels– Good blood supply to the kidney is essential

to allow it to function properly – network of blood vessels throughout kidney tissue.

– A single renal artery enters each kidney at the hilum, branching into five segmental arteries.

– The segmental arteries branch into lobar arteries in the renal sinus.




• Blood vessels– The lobar arteries branch into interlobar

arteries which pass through the renal columns.

– Arcuate arteries originate from the interlobar arteries and arch around the pyramids in the renal medulla.

– Arcuate arteries give rise to cortical radiate arteries which give rise to afferent arterioles.




• Blood vessels– Each afferent arteriole leads to a ball of

capillaries called a glomerulus.– Efferent arterioles leave from the glomerulus.

Travel to a specialized series of capillaries called the peritubular capillaries and vasa recta (straight collecting tubes) that are part of the renal nephron, the functional unit of the kidney.

The peritubular capillaries wrap around the collecting tubules of the nephron, allowing efficient movement of ions between blood and the fluid in the nephron.




• Blood vessels– From each set of peritubular capillaries, blood

flows out the interlobular veins.– From there, the blood flows out a series of

veins that are the direct reverse of the arteries with the exception that there are no segmental veins.

– The blood finally leaves the kidney via the renal vein.



Figure 17-3 Renal blood vessels and the pathway of blood through the renal system.



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The Nephron

• The functional unit of the kidney is the nephron, consisting of millions of microscopic funnels and tubules.

• The nephron can be divided into two distinct parts:– The renal corpuscle – a filter– The renal tubule – where reabsorption and

secretion take place



Renal Corpuscle

• Blood enters the renal corpuscle via the glomerulus, a ball of capillaries.

• Surrounding the glomerulus is a double-layered membrane called the glomerular capsule, or Bowman’s capsule.

• The layers of the glomerular capsule are similar to the layers of a serous membrane.



Renal Corpuscle

• The inner layer of the glomerular capsule, the visceral layer, surrounds the glomerular capillaries and is made of specialized squamous epithelial cells called podocytes. This makes for a very efficient filter.

• The outer layer, the parietal layer, of the glomerular capsule is simple squamous epithelium and completes the filter.



Renal Corpuscle

• Blood flows into the glomerulus and everything BUT blood cells and a few large molecules, mainly protein, are pushed from the capillaries across the filter and into the glomerular capsule.

• The material filtered from the blood into the glomerular capsule is called glomerular filtrate.

• If blood or protein leaks into urine it can indicate a kidney filtration problem.



Figure 17-4 The nephron.



Renal Tubule

• The rest of the nephron is a series of tubes known as renal tubules.

• Glomerular filtrate travels from the glomerular capsule into the first part of the renal tubule, the proximal tubule.– The wall of the proximal tubule is made of

cuboidal epithelium with microvilli.



Renal Tubule

• From the proximal tubule, glomerular filtrate flows into the nephron loop (or the Loop of Henle).– The nephron loop consists of the descending

loop (similar in structure to the proximal tubule) and the ascending loop (simple cuboidal epithelium).



Renal Tubule

• Glomerular filtrate travels from the nephron loop to the distal tubule.– The wall of the distal tubule is like that of the

ascending branch of the nephron loop.– From the distal tubule, glomerular filtrate flows

into one of several collecting ducts, made of cuboidal epithelium.



Renal Tubule

• The collecting ducts lead to minor calyces, then to major calyces, the renal pelvis, and the ureter.

• At this point, the glomerular filtrate is urine.



Figure 17-5 A functional renal unit.



Renal Tubule

• Blood vessels are in close proximity to the nephrons because substances move between the tubules and the bloodstream.– Blood approaches the nephron via the

afferent arteriole. – Blood flows from the afferent arteriole into the

glomerulus.– Blood flows from the glomerulus via the

efferent arteriole into the peritubular capillaries and vasa recta.



Renal Tubule

• Blood vessels are in close proximity to the nephrons because substances move between the tubules and the bloodstream.– These surrounding blood vessels allow for

reabsorption and secretion.– Blood leaves the nephron via the cortical

radiate veins.



Clinical Application: Trauma, Ischemia, and Kidney Damage• The kidney is well vascularized, with each

nephron surrounded by blood vessels. • The flow of blood is controlled by the

afferent arteriole. When blood flow decreases for a period of time, oxygen delivery to the nephron decreases and ischemia results.



Clinical Application: Trauma, Ischemia, and Kidney Damage• Blood flow can be decreased by any

number of hormonal mechanisms causing prolonged vasoconstriction, such as severe blood loss.

• If the situation continues long enough, the tissues will become ischemic and eventually die, causing kidney failure that can be temporary or permanent.



From the Streets:Renal Trauma

• The kidneys are well protected in the retroperitoneal space.

• Trauma to the kidneys usually is the result of penetrating trauma.

• Signs and symptoms



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Urine Formation

• The kidney controls fluid and electrolyte balance by controlling urine volume and composition.

• In order to form urine, the nephron must perform three processes:– Glomerular filtration– Tubular reabsorption– Tubular secretion



Filtration

• During glomerular filtration, fluid and molecules pass from the glomerular capillaries into the glomerular capsule.– Across a filter composed of the walls of the

capillaries and the podocytes of the glomerular capsule

– The filtrate flows into the renal tubule where the chemistry is controlled by reabsorption and secretion



Filtration

• During glomerular filtration, fluid and molecules pass from the glomerular capillaries into the glomerular capsule.– Filtration moves fluid and chemicals into the

nephron from blood– Glomerular filtrate is chemically similar to

blood



From the Streets:Kidney Stones

• Kidney stones, or renal calculi, are formed by crystals in the kidney’s collection system.



Figure 17-10 Cross section through a kidney showing multi- ple stones, including one “staghorn” stone in the renal pelvis.



From the Streets:Kidney Stones

• Causes• Risk factors• Signs and symptoms• Diagnostic tests• Treatment



From the Streets:Hemodialysis

• Renal failure (RF) can be either acute (ARF) or chronic (CRF).

• ARF is a sudden drop in urine output to less than half a liter per day.

• Causes• Signs and symptoms• Treatment



Reabsorption and Secretion

• Urine is chemically different from plasma because reabsorption and secretion control the concentration of chemicals and volume of urine.– Substances that are reabsorbed pass from

the renal tubule into the peritubular capillaries and return to the blood stream.

– Substances that are secreted pass through the peritubular capillaries into the renal tubule and eventually leave the body as urine.



Reabsorption and Secretion

• Urine is chemically different from plasma because reabsorption and secretion control the concentration of chemicals and volume of urine.– Some substances, like glucose, are

completely reabsorbed while substances like metabolic waste products (urea and creatinine) are almost completely secreted as urine.



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Figure 17-7 The processes involved in urine formation.



Table 17-1 Kidney Fluid Chemistry.



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Control of Filtration

• Filtration is controlled by several factors.– Filter size – determines what gets through the

filter Podocytes and capillary walls of the renal

corpuscle create a filter with fixed openings. Plasma, and many of the substances dissolved in

plasma, pass through the filter, but blood cells, platelets, and large molecules, can’t get into a healthy kidney.




• Filtration is controlled by several factors.– Pressure

Higher pressure on one side of the filter allows chemicals to be pushed though the filter more quickly.

Higher blood pressure in the glomerular capillaries increases filtration, while lower pressure decreases filtration.



Figure 17-8 Filter selectivity.



Figure 17-9 Comparison of damaged and healthy kidneys.




• Changes in blood pressure change filtration rate.– Minor changes in systolic blood pressure do

not change glomerular pressure because it is protected by a mechanism called autoregulation. As systemic BP increases, the afferent arterioles

leading into the glomerulus constrict, decreasing the amount of blood getting into the glomerulus.




• Changes in blood pressure change filtration rate.– Autoregulation can be overridden.

Since the kidney regulates fluid volume, the kidney can work with the cardiovascular system to regulate blood pressure.

Glomerular filtration can decrease to conserve fluid when blood pressure falls, or increase filtration if blood pressure rises.

The sympathetic nervous system can control urine production through the adrenal medulla.



Clinical Application:Diabetic Nephropathy

• Diabetes Mellitus (DM) is characterized by high glucose levels from inadequate, or lack of, insulin. The high glucose levels wreak havoc on the osmotic balance of blood. The kidneys work hard to remove excess glucose, and urine output is high.




• Over time the kidney is damaged, (diabetic nephropathy), beginning with a thickening of the filter surface of the glomerular capsule, eventually leading to breakdown of kidney tissue, destroying the filtering ability. Substances that would normally not pass through, like protein and blood, begin to appear in urine.




• Kidney function deteriorates. Diabetics can prevent the onset of kidney damage by controlling their blood sugar levels, preventing high blood pressure, and reducing blood cholesterol levels. This is the leading cause of kidney disease in America.



Control of Tubular Reabsorption and Secretion

• Tubular reabsorption and secretion control the chemistry and volume of urine.– Substances that are reabsorbed move from

the tubule back to the blood stream via the peritubular capillaries and stay in the body.

– Substances that are secreted stay in the tubule and eventually leave the body via the urine.

• Anything that affects reabsorption and secretion affects urine chemistry.




• The first thing that affects tubular reabsorption and secretion is tubule permeability.– Each section of the tubule can reabsorb and

secrete different substances.– Molecules move across membranes through

several different methods including diffusion and active transport.

– Differences in tubular permeability result in dramatic differences in what molecules are reabsorbed or secreted in each part of the tubule.



Table 17-2 Individual Tubule Functions.



Clinical Application: Kidney Stones

• Kidney stones result when substances in the urine crystallize in the renal tubule, often because the concentration of the molecule is higher than normal. The cause of stones is frequently a mystery.




• Stones can be made of calcium, uric acid, or caused by kidney infections. Some people are more susceptible than others. Some stones pass unnoticed, while others that are larger or irregularly shaped may lodge in the tubule, obstructing flow and irritating nearby tissues.




• Symptoms include blood in the urine and severe flank pain. Stones may move on their own or may be treated with increased fluid intake, lithotripsy (shock waves to break the stone), or surgery if these don’t work. Patients are asked to filter their urine to look for the passage of these stones, some as small as sand.




• Special type of circulation around the nephron loop, called countercurrent circulation– Ions move across cell membranes from

higher to lower areas of concentration.




• Special type of circulation around the nephron loop, called countercurrent circulation– Water and ions tend to move in opposite

directions, making it impossible to move both without the special environment around the nephron loop. Concentration gradient, with low ion concentration

at the beginning of the descending loop and high concentration at the tip of the loop




• Special type of circulation around the nephron loop, called countercurrent circulation– Water and ions tend to move in opposite

directions, making it impossible to move both without the special environment around the nephron loop. Differences in permeability between the

descending loop (water) and ascending loop (ions)




• Countercurrent circulation (cont’d)– Filtrate flows into the descending loop,

reabsorbing water and increasing the concentration of ions. As the filtrate enters the ascending loop, fluid is concentrated because of water loss, the membrane is permeable to only ions, and ions move across the membrane.



Figure 17-12 Sites of tubular reabsorption and secretion.



Applied Science: Electrolyte and Acid Balance

• The kidney maintains electrolyte balance by selectively excreting or reabsorbing electrolytes within the tubular system.

• The relationship between hydrogen ions (H+) and bicarbonate ions (HCO3) determines the blood pH (level of acidity or alkalinity). This is called the acid/base relationship.



Applied Science: Electrolyte and Acid Balance

• If too much acid is present, H+ will be excreted to a greater level in the urine and more bicarbonate ions will be reabsorbed.

• The respiratory system also plays a role by blowing off more carbon dioxide, which is an acid.



From the Streets:Disturbances in Acid-Base Balance

• Maintenance of normal acid-base balance is one of the body’s most important homeostatic functions.

• The body compensates by three main mechanisms:– Buffer system– Respiratory system– Renal system



From the Streets:Acid-Base Abnormalities

• Respiratory acidosis• Respiratory alkalosis• Metabolic acidosis• Metabolic alkalosis




• The third factor that affects reabsorption and secretion are several hormones that regulate blood pressure.– Antidiuretic Hormone– Aldosterone– Atrial Natriuretic Peptide– Renin-Angiotensin-Aldosterone




• Hormonal control– Antidiuretic hormone (ADH) is made by

the hypothalamus and secreted from the posterior pituitary when BP decreases or ionic concentration increases. ADH increases permeability of distal

tubules and the collecting duct.





the hypothalamus and secreted from the posterior pituitary when BP decreases or ionic concentration increases. More water is reabsorbed, increasing blood

volume, increasing blood pressure, and diluting the ionic concentration.

Less urine is produced.





the hypothalamus and secreted from the posterior pituitary when BP decreases or ionic concentration increases. Alcohol or caffeine inhibit ADH production,

increasing urine production.




• Hormonal control– Aldosterone is an adrenocorticosteroid

secreted by the adrenal cortex. It is secreted when plasma sodium decreases or

plasma potassium increases. It increases the reabsorption of sodium ions and

secretion of potassium ions – increasing serum sodium levels and decreasing serum potassium levels – by the distal tubule and ascending limb of the nephron loop.




• Hormonal control– Aldosterone is an adrenocorticosteroid

secreted by the adrenal cortex. As sodium is reabsorbed, water is also reabsorbed,

decreasing urine volume.




• Hormonal control– Atrial natriuretic peptide (ANP) is secreted by

the atria of the heart when blood volume increases. ANP decreases sodium reabsorption and thus

increases urination.

– The renin-angiotensin-aldosterone system is a series of chemical reactions that regulate blood pressure.




• Hormonal control (cont’d)– Decrease in blood flow to the kidney:

Causes a special group of cells near the glomerulus, the juxtaglomerular apparatus, to secrete renin into the blood stream.

The liver secretes a chemical called angiotensinogen.

Renin converts angiotensinogen into angiotensin I.




• Hormonal control (cont’d)– Decrease in blood flow to the kidney:

Another enzyme made by the lungs, angiotensin converting enzyme (ACE), converts angiotensin I to angiotensin II, increasing thirst, increasing ADH secretion, increasing aldosterone secretion, and causing vasoconstriction. These increase blood pressure by increasing fluid volume.



Clinical Application: Polycystic Kidney Disease

• Polycystic kidney disease (PKD) is a genetic disorder. Large cysts form in the kidneys. One form of PKD is so serious that patients die in infancy. The more common form is an adult onset disorder characterized by decreasing kidney function as normal nephrons are destroyed by cysts.



Clinical Application: Polycystic Kidney Disease

• As more cysts develop the kidneys get very large – one weighed 22 pounds. There is no cure except transplantation of the kidney. It occurs in about half a million people in the United States.



The Urinary Bladder and Urination Reflex

• Glomerular filtrate flows out the collecting duct, into the minor calyces, and then into the major calyces, forming the renal pelvis.

• Once the glomerular filtrate leaves the collecting ducts, its concentration can’t be changed and it is urine.

• Urine collects in the renal pelvis and flows down the ureters to the urinary bladder, where it is stored.



From the Streets:Urinary Tract Infection (UTI)

• Urinary Tract Infections (UTI) affect the urethra, bladder, ureter, kidney, and even the prostate gland.

• Risk factors• Signs and symptoms• Treatment



Urinary Bladder

• The urinary bladder is a small hollow organ posterior to the pubic symphysis and behind the peritoneum.– It is lined with transitional epithelium, the only

epithelium stretchy enough to expand as the bladder fills.

– The ability to stretch is enhanced by a series of pleats called rugae.



Urinary Bladder

• The urinary bladder is a small hollow organ posterior to the pubic symphysis and behind the peritoneum.– The bladder has a muscular wall consisting of

several layers of circular and longitudinal smooth muscle and is covered by connective tissue and parietal peritoneum.



Figure 17-13 The urinary bladder.



Urination Reflex

• As urine accumulates, the bladder fills and stretches.– This stretch triggers the urinary reflex and the

need to void to empty the bladder.– Urination had been thought to be a spinal

reflex, but new research indicates it is controlled by the brain.



Urination Reflex

• As urine accumulates, the bladder fills and stretches.– When the bladder is full, signals are sent from

the bladder to the spinal cord to the pons. The pons sends parasympathetic signals down the spinal cord, causing contraction of the muscular walls of the bladder, and the bladder empties.



Urination

• Urine leaves the bladder via the urethra, a thin muscular tube lined with several different types of epithelium along its length.– Part of your brain can inhibit urination by

controlling the internal urethral sphincter, a valve at the junction of the bladder and the urethra, and the external urethral sphincter, a valve that is part of the muscles of the pelvic floor.



Urination

• Urine leaves the bladder via the urethra, a thin muscular tube lined with several different types of epithelium along its length.– Sympathetic stimulation of these sphincters

prevents urine from leaving the body.– Although you have little control over bladder

contraction, you have good control over the sphincters starting from age 2, or slightly later in boys.



Figure 17-15 Control of urination.



Clinical Application: Urinary Tract Infections (UTIs)• Urinary tract infection is caused by the

movement of fecal bacteria into the urinary tract.

• Symptoms may include frequent, painful urination, bloody or cloudy urine with an unusual odor, and low abdominal/pelvic pain caused by bladder spasm.



Clinical Application: Urinary Tract Infections (UTIs)• Treatment should begin promptly to avoid

kidney damage from infection traveling from the bladder to the kidney.

• UTIs are more common in women because their urethra is shorter. Drinking plenty of water can help prevent infections. Once diagnosed, they are treated with antibiotics and increased fluid intake.



From the Streets:Acute Renal Failure

• Acute renal failure (ARF) is a deterioration in renal function over hours or days.

• Recovery depends upon restoration of renal blood flow and clearance of tubular toxins.



Common Disorders of the Urinary System

• Overuse or abuse of drugs can severely affect renal function because the kidney excretes most medications after the liver breaks them down. Analgesic nephropathy is caused by long term use of pain relievers, particularly non-steroidal anti-inflammatory drugs (NSAIDS) like ibuprofen or naproxen, particularly if combined with caffeine, codeine, or acetaminophen. OTCs can cause chronic kidney damage leading to kidney failure.



Chronic Renal Failure

• Chronic renal failure (CRF) is an ongoing, progressive disease of the kidney.

• The progression may be controlled by treating the underlying cause of the damage, or controlling BP and cholesterol.

• CRF can lead to end-stage renal disease (ESRD), the final stage of renal failure.

• Treatment for ESRD is dialysis or transplantation.



Diabetes Insipidus

• Diabetes insipidus (DI) is an endocrine disorder characterized by too little ADH or insensitivity of the kidney to ADH.

• The result is copious amounts of urine being produced, dehydration, and rising sodium levels as the body tries, unsuccessfully, to conserve water. If not treated it can be fatal.



Diabetes Insipidus

• The opposite of this disorder is water intoxication, in which patients have uncontrollable thirst and drink so much water they create dangerously low sodium levels because of the volume of water diluting it. This can lead to brain damage or death.



Glomerulonephritis

• Glomerulonephritis is inflammation of the glomerulus.

• Glomerulosclerosis is scarring of the glomerulus.

• Both cause damage to the delicate filter apparatus.

• When the filter is damaged, blood cells and blood proteins enter the filtrate and eventually appear in urine.



Glomerulonephritis

• Removal of waste products is decreased and electrolyte balance is usually abnormal due to the change in urine chemistry.

• There are many causes, including bacterial infection, diabetic nephropathy, systemic lupus erythematosus (SLE), and genetic disorders including Alport syndrome and Goodpasture’s syndrome.



Hemolytic Uremic Syndrome

• Hemolytic uremic syndrome is a disorder caused by an infection with the bacteria E. coli, typically from eating undercooked meat.

• The bacteria infects the digestive tract and releases toxins which destroy RBCs.



Hemolytic Uremic Syndrome

• The damaged RBCs lodge in blood vessels in the kidney, blocking them and preventing blood flow to the nephron.

• Without treatment, permanent kidney damage may result.

• A blood transfusion of the wrong type of blood can have much the same effect.



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Snapshots from the Journey

• The urinary system consists of paired kidneys and paired ureters, which carry urine to the single urinary bladder. The urethra transports urine from the bladder to outside the body. The function of the urinary system is control of fluid and electrolyte balance and elimination of nitrogen-containing waste.




• The kidney is bean shaped and covered in a capsule. It has an indentation known as the renal hilum and an interior cavity known as the renal sinus. The kidney can be divided into three layers: the renal cortex, renal medulla, and renal pelvis. The renal pelvis is a funnel that is divided into large pipes, the major calyces.




• Each major calyces is divided into several minor calyces. The renal pelvis empties into the ureter.




• The kidney is very well vascularized. Blood is supplied to each kidney by a renal artery. The blood vessels split into smaller and smaller branches until there are millions of tiny arterioles, the afferent arterioles. The afferent arterioles supply millions of nephrons, the functional unit of the kidney, with blood. Blood leaves the kidney by a series of veins and ultimately returns to circulation via the renal vein.




• The nephron is the functional unit of the kidney. There are millions of nephrons in each kidney. The nephron is divided into two parts. The renal corpuscle, consisting of the glomerulus (capillaries) and the glomerular capsule, filters blood and produces glomerular filtrate.




• The renal tubule, consisting of the proximal tubule, nephron loop, distal tubule, and collecting ducts, control the concentration and volume of urine by reabsorbing and secreting water, electrolytes, and other molecules. The walls of the nephron are made of epithelium. The type of epithelium changes depending on the specific function of each part of the nephron.




• Urine is formed by a combination of three processes: glomerular filtration, tubular reabsorption, and tubular secretion. The selectivity of the glomerular filter is determined by the size of the openings in the filter and the difference between the blood pressure of the glomerulus and the pressure in the glomerular capsule.




• The size of the filter does not change unless the glomerulus is damaged. Protein, for example, cannot pass through the filter. However, the filtration rate will change if the pressure in the glomerulus changes. Most of the time, autoregulation, control of the diameter of the afferent arteriole, keeps glomerular pressure and the glomerular filtration rate constant.




• But sympathetic stimulation can regulate (in this case decrease) glomerular filtration and urine output due to constriction of afferent arterioles.




• Tubular reabsorption and secretion is controlled by differences in tubular permeability. The proximal tubule is the most versatile, reabsorbing dozens of different molecules. The nephron loop is part of an elaborate countercurrent mechanism, with the descending loop permeable to water and ascending loop permeable to ions.




• The distal tubule and collecting ducts reabsorb water. The permeability of the renal tubule can be regulated by a number of hormones that control blood pressure. These hormones, aldosterone, ADH, atrial natriuretic peptide, and others, regulate blood pressure by regulating urine volume and ion secretion. Changes in urine volume change total body fluid volume and thereby change blood pressure.




• The urinary bladder is a collecting and storage structure for urine and is located in the pelvic cavity. It has a muscular wall. Contractions of the muscle result in voiding (urination), emptying the bladder. Urination is a reflex controlled by parasympathetic neurons in the pons. Signals from a full bladder reach the pons.




• The neurons in the pons then send signals for the bladder to contract. Sympathetic neurons control two valves, the internal and external urethral sphincters, which allow significant conscious control of the urination reflex.



Case Study

Jane has recently developed very annoying symptoms. She has to go to the restroom several times a day. Sometimes it seems she spends every waking moment in there. She hasn’t slept through the night for more than a week. She goes to the doctor, who orders a series of tests to differentiate between several disorders that cause frequent urination: diabetes mellitus, overactive bladder, and urinary tract infection.



Jane’s Test Results

• Urine bacteria – no• Blood in the urine – no• Leukocytes in urine – no• Glucose – normal• Proteins – no

• What is your diagnosis and why?



From the Streets

You are called to the scene of a 38-year-old male complaining of “severe waves of pain” that radiates from his right side to his groin. He states, “it feels like pieces of glass are moving through my side”. He is restless and claims his urine is reddish in color. You note facial grimacing, tachycardia, low grade fever, and pink, warm and moist skin. He drinks a 2-liter of caffeinated soda a day.



From the Streets Questions

• What is his most likely diagnosis? • Why is his urine reddish in color? • Why does he complain of “waves of pain”

and “it feels like pieces of glass are moving through my side”?




• Identify a key risk factor for his condition. • What is his prognosis?




• What is his most likely diagnosis? Kidney stone (renal calculus)

• Why is his urine reddish in color? He has blood in his urine (hematuria)

• Why does he complain of “waves of pain” and “it feels like pieces of glass are moving through my side”? As peristalsis moves the stone down the ureter toward the urinary bladder it feels as he describes.




• Identify a key risk factor for his condition. Caffeine consumption

• What is his prognosis? Once the stone passes he will be more comfortable. He will want to be transported for hydration and pain management. Upon discharge a urine strainer will be given so that he can save the stone and take it with him when he follows up with his physician.



End of ChapterReview Questions

1. The function of this part of the renal tubule is filtration of blood:a. Renal calyxb. Renal corpusclec. Renal cortexd. Renal columns




2. The collecting ducts are found in this part of the kidney.a. Renal Cortexb. Renal Medullac. Renal Pelvisd. Renal Pyramids




3. This tube leads from the urinary bladder to the outside.a. Collecting ductsb. Distal tubulec. Ureterd. None of these




4. The ion responsible for causing acidic blood is:a. Na+

b. H+

c. K+

d. HCO3-




5. The renal hormone secreted by the hypothalamus when blood pressure decreases to promote the reabsorption of water is:a. Aldosteroneb. Atrial natriuretic peptide c. Antidiuretic hormoned. Epinephrine




6. Why do the cells in the proximal tubule have microvilli?a. To increase surface areab. To move particles along the mucociliary

escalatorc. To differentiate it from the distal tubuled. To increase filtration rate




7. Which of the following is a possible cause of blood in the urine?a. Decreased filtrationb. UTI c. Decreased tubular reabsorptiond. Decreased blood pressure




8. If a patient experiences severe dehydration, what would you expect to happen to urine volume?a. It would increaseb. It would decreasec. It would stay the same as usuald. Not enough information




1. Most substances are reabsorbed or secreted in this part of the renal tubule: __________.

2. This part of the renal tubule has an elaborate counter-current mechanism for reabsorption of sodium and water: __________.

3. This hormone is released by the heart when fluid volume increases: __________.




4. Urination reflex is mediated by this part of the CNS __________.

5. As blood pressure decreases, this hormone is released by the adrenal cortex. __________

6. The _______ is the funnel-shaped end of the ureter.




1. List and explain the activity of three regulators of kidney function.

2. Explain the three processes necessary for urine formation. In which part of the nephron are these functions performed?

3. Describe the structure of the wall of the urinary bladder.

4. Explain the control of urination reflex.




5. Trace the flow of blood into, through, and out of the kidney.

6. Explain the symptoms of one kidney disorder. Relate the symptoms to kidney function.

The Urinary System: Filtration and Fluid Balance

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