The Urinary System

The Urinary System Chapter 23

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 • They receive some protection from ribs

Kidney Location • The right lies somewhat lower than left as it is positioned under liver • The lateral surface of each kidney is convex, while the medial is concave

External Anatomy • The adult kidney weights about 150 g (5 oz.) • Dimensions are 12 cm long, 6 cm wide, 3 cm thick

External Anatomy • 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

Anatomy of the Kidneys • The main structural and functional unit of the kidneys is the uriniferous tubule • The unit consists of a nephron and its collecting duct or tubule

Anatomy of the Kidneys • Uriniferous tubules are separated from one another by small amounts of loose areolar connective called interstitial connective tissue

Anatomy of the Kidneys • The urine forming nephron is composed of • Renal corpuscle • Proximal convoluted tubule • Loop of Henle • Distal convoluted tubule • A collecting duct (collecting tubule) • Concentrating urine

Anatomy of the Kidneys • Throughout its length the uriniferous tubule is lined by a simple squamous epithelium that is adapted for various aspects of the production of urine

Mechanisms of Urine Production • The uriniferous tubule produces urine through three interacting mechanisms • Filtration • Reabsorption • Secretion

Mechanisms of Urine Production • In filtration a filtrate of the blood leaves the kidney capillaries and enters the nephron • This filtrate resembles tissue fluid in that it contains all the small molecules of blood plasma

Mechanisms of Urine Production • As filtrate proceeds through the uriniferous tubule, the filtrate is processed into urine by the mechanisms of reabsorption and secretion

Mechanisms of Urine Production • During reabsorption, most of the nutrients, water, and essential ions are reclaimed from the filtrate, and returned to the blood of capillaries in the surrounding connective tissue • 99% of the volume of renal filtrate is reabsorbed

Mechanisms of Urine Production • As the essential molecules are reclaimed from the filtrate, the remaining wastes and needed substances contribute to the urine that ultimately leaves the body • A passive process

Mechanisms of Urine Production • Secretion is an active process which moves undesirable molecules into the tubule from the blood of surrounding capillaries

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 - Renal Corpuscle • The first part of the nephron, where the filtration occur, is the spherical renal corpuscle

Nephron - Renal Corpuscle • Renal corpuscles consist of a tuft of capillaries called a glomerulus surrounded by a cup shaped, hollow glomerular capsule (Bowman’s capsule)

Nephron - Renal Corpuscle • The glomerulus lies in the glomerular capsule like an under inflated ballon • This tuft of capillaries is supplied by an afferent arteriole and drained by an efferent arteriole

Nephron - Renal Corpuscle • The endothelium of the capillaries in the glomerulus is fenestrated (pores) and thus these capillaries are highly porous, allowing large quantities of fluid and small molecules to pass from the capillary blood

Nephron - Renal Corpuscle • The fluid passes from the capillary into the hollow interior of the glomerular capsule, the capsular space • This fluid is the filtrate that is ultimately processed into urine • 20% enters the space while 80% remains in the blood within the capillary

Nephron - Renal Corpuscle • The external parietal layer of the glomerular capsule, which is simple squamous epithelium, simply contributes to the structure of the capsule • It plays no part in the formation of filtrate

Nephron - Renal Corpuscle • The capsule’s visceral layer clings to the glomerulus and consists of unusual, branching epithelial cells called podocytes

Nephron - Renal Corpuscle • The podocytes have many branches which end in foot processes or pedicels • The processes inter digitate with one another as they surround the glomerular capillaries • The filtrate passes through the thin filtration slits into the capsular space

Nephron - Renal Corpuscle • The external parietal layer of the glomerular capsule, which is simple squamous epithelium, simply contributes to the structure of the capsule • It plays no part in the formation of filtrate

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

The Urinary System