The Urinary System

The Urinary System Chapter 26

Introduction • The kidneys are perfect examples of homeostatic organs • Maintain constancy of fluids in our internal environment • Filter 200 liters of fluid a day • Remove toxins, metabolic wastes, and excess ions to leave the body in urine • Return needed substances to the blood • A primary organ of excretion

Kidney Functions • Kidneys regulate volume and chemical makeup of the blood • Maintain the proper balance between water and salts as well as between acids and bases • Gluconeogenesis - supply glucose during fasting • Produce enzyme renin which helps regulate blood pressure and kidney function • Produce hormone erythropoietin which stimulates red blood cell production

Urinary System Organs • Structures of the urinary system include; • Kidneys • Urinary bladder • Ureters • Urethra

Kidney Location • The kidneys extend approximately from the level of the 12th thoracic vertebra to the third lumbar vertebra • Receive some protection from ribs • Right lies somewhat lower than left as it is positioned under liver

External Antomy • The adult kidney weights about 150 g (5 oz.) • Dimensions are 12 cm long, 6 cm wide, 3 cm thick • Lateral surface is convex while the medial surface is concave

External Antomy • Medial surface has a vertical cleft called the renal hilus that leads into the space within the kidney called the renal sinus • Atop each kidney is an adrenal gland which is unrelated to kidney function

External Anatomy • Structures such as the ureters, the renal blood vessels, lymphatics, and nerves enter the kidney at the hilus • These structures occupy the renal sinus

Position of the Kidneys • The kidneys are retroperitoneal, or behind the peritoneum

Position of the Kidneys • Kidneys supported by three layers of supportive tissue • The renal capsule adheres directly to the kidney surface and isolates it from surrounding region

Position of the Kidneys • The adipose capsule attaches the kidney to the posterior body wall and cushions it against trauma

Position of the Kidneys • The renal fascia is dense fibrous connective tissue which surrounds the kidney and anchors these organs to the surrounding structures

Internal Anatomy • The kidney has three distinct regions • Cortex • Medulla • Pelvis

Internal Anatomy • The most superficial region • The renal cortex is light in color and has a granular appearance

Internal Anatomy • Deep to the cortex is the renal medulla • Darker tissue which exhibits cone shaped tissue masses called medullary or renal pyramids Medullary pyramids

Internal Anatomy • Each renal pyramid has a base which is convex, and a apex which tapers toward its apex or papilla Medullary base Medullary apex

Internal Anatomy • The apex, or papilla, points internally • The pyramids appear striped because they are formed almost entirely of roughly parallel bundles of urine collecting tubules Pyramidal stripes

Internal Anatomy • Inward extensions of cortical tissue called renal columns separate the pyramids • Each medullary pyramid is surrounded by a capsule of cortical tissue to form a lobe

Internal Anatomy • Within the renal sinus is the renal pelvis • This flat, funnel shaped tube is continuous with the ureter leaving the hilus

Internal Anatomy • Branching extensions of the renal pelvis form 2-3 major calyces, each of which sub-divides to form several minor calyces • These cup shaped areas collect the urine which drain continuously from the papillae

Internal Anatomy • Urine flows through the renal pelvis into the ureter, which transports it to the bladder • The walls of the calyces, pelvis, and ureter contain smooth muscle which contract to move urine

Blood Supply • The kidney continuously cleanse the blood and adjust its composition • Kidneys possess an extensive blood supply • Under normal resting conditions, the renal arteries deliver approximately one-fourth of the total systemic cardiac output (1200 ml) to the kidneys each minute

Blood Supply • The renal arteries issue at right angles from the abdominal aorta • Each renal artery divides into five segmental arteries that enter the hilus • Each segmental artery divides into lobar and interlobar arteries

Nephrons • Each kidney contains over 1 million tiny blood processing units called nephrons, which carry out the processes that form urine • In addition, there are thousands of collecting ducts, each of which collects urine from several nephrons and conveys it to the renal pelvis

Nephron • Each nephron consists of a glomerulus, a tuft of capillaries associated with a renal tubule • The end of the renal tubule is a blind, enlarged, and cup-shaped and completely surround the glomerulus Glomerulus

Nephron • The renal corpuscle refers to the enclosed glomerulus and the capsule of the glomerulus referred to as Bowman’s capsule

Nephron • The glomerulus endothelium is fenestrated, (penetrated by many pores), which make these capillaries exceptionally porous • The capillaries allow large amounts of solute-rich, virtually protein free fluid to pass from the blood into the glomerulus capsule • This plasma-derived fluid or filtrate is the raw material that is processed by the renal tubules to form urine

Nephron

Nephron • The external parietal layer of the glomerular capsule is simple squamous epithelium • This layers contributes to the structure of the capsule and plays no part in forming filtrate • The visceral layer that clings to the glomerulus consists of highly modified, branching epithelial cells called podocytes

Nephrons • Podocytes terminate in foot processes, which intertwine and form filtration silts or slit pores • The silts allow filtrate to pass to the interior of capsule

Nephrons • The filtration membrane is the actual filter that lies between the blood and the interior of the glomerular capsule • It is a porous membrane that allows free passage of water and solutes

Nephrons • It is a porous membrane that allows free passage of water and solutes smaller that plasma proteins • The capillary pores prevent passage of blood cells, but plasma components are allowed to pass

Nephron • Once filtered out of the plasma the urine enters the collecting duct • Urine passes into larger ducts until it reaches the ureters • It leaves the kidneys and moves toward the bladder in the ureters Glomerulus

Renal Physiology • Skip to sections on Ureters located on page 1029

Ureters • The ureters are slender tubes that convey urine from the kidneys to the bladder

Ureters • Each leaves the renal pelvis, decends behind the peritoneum to the base of the bladder, turns and then runs obliquely through the medial bladder wall

Ureters • The ureters are protected from a backflow of urine because any increase within the bladder compresses and closes the ends of the ureters

Ureters • Histologically, the walls of the ureter is trilayered • An inner layer of transitional epithelium lines the inner mucosa • The middle muscularis layer is composed of a an inner longitudinal layer and an outer circular layer • The outer layer is composed of fibrous connective tissue

Ureters • The ureters play an active role in transporting urine • Distension of the ureters by incoming urine stimulates the muscularis layer to contract, which propels the urine into the bladder • The strength and frequency of peristaltic waves are adjusted to the rate of urine formation

Urinary Bladder • The urinary bladder is a smooth, collapsible, muscular sac that stores urine

Urinary Bladder • In males, the bladder lies immediately anterior to the rectum

Urinary Bladder • In females, the bladder is anterior to the vagina and uterus

Urinary Bladder • The interior of the bladder has openings for both ureters and the urethra • The triangular region of the bladder base outlined by these openings is called the trigone which is a common site of infections

Urinary Bladder • The bladder wall has three layers • A mucosa containing transitional epithelium • A thick muscular layers • A fibrous adventitia • The muscle layer consists of smooth muscle arranged inner and outer longitudinal layers • Collectively the muscle layer is called the detrusor muscle (literally to thrust out)

Urinary Bladder • The bladder is very distensible and uniquely suited for its function of urine storage • It can expand for storage or collapse when empty • Empty its walls are thick and thrown into folds (rugae) • As it expands it becomes pear shaped and rises in the abdominal cavity

Urinary Bladder • The bladder can store more than 300 ml or urine without a significant increase in internal pressure • A moderately full bladder holds approximately 500 ml and can about 1000 ml at capacity • Urine is held in the bladder until release is convenient

Urethra • The urethra is a thin muscular tube that drains urine from the bladder and conveys it out of the body

Urethra • The epithelium of its mucosal lining is mostly pseudostratified columnar epithelium • Near the bladder it is transitional epithelium and near its external opening it changes to a protective squamous epithelium

Urethra • At the bladder-urethra junction a thickening of the detrusor muscle forms the internal sphincter • This voluntary sphincter keeps the urethra closed when urine is not being passed • A second sphincter, the external urethral sphincter, surrounds the urethra and is composed of skeletal muscle and thus is under voluntary control

Urethra • The levator ani muscle of the pelvic floor also serves as a voluntary constrictor of the urethra • The length and functions of the urethra differ in the two sexes • In females the urethra is 3-4 cm long and is tightly bound to the anterior vaginal wall by fibrous connective tissue

The Urinary System