By Matthew M. Green and Carlos Krumpach

1
The Urinary System

• By Matthew M. Green and Carlos Krumpach

2
Introduction

• The purpose of the urinary system is to
• remove certain salts and nitrogenous wastes from
  the body,
• maintain normal concentrations of water and
  electrolytes within body fluids,
• regulate the pH and volume of body fluids,
• and help control red blood cell production and
  blood volume.
• The Urinary System consists of
• a pair of kidneys,
• a pair of tubular ureters,
• a saclike urinary bladder,
• and a tubular urethra

3
Introduction to the Kidneys

• A kidney is a reddish-brown, bean-shaped organ,
  with a smooth surface. (Adult kidneys are 12
  centimeters long, 6 centimeters wide, and 3
  centimeters thick.) They are enclosed in a
  strong, fibrous capsule.

4
Location of the Kidneys

• The kidneys lie on either side of the vertebral
  column in a depression high on the posterior wall
  of the abdominal cavity. The upper and lower
  borders of the kidneys are usually at the levels
  of the twelfth thoracic and third lumbar
  vertebrae. (The left kidney is 1.5-2.0
  centimeters higher than the right kidney.) The
  kidneys are positioned retroperitoneally, or
  behind the parietal peritoneum and against the
  deep muscles of the back. (The kidneys are held
  in place by connective tissue and adipose tissue.)

5
Kidney Structure

• The kidneys later surface is convex, while its
  medial side is concave. The resulting medial
  depression leads to the renal sinus, a hallow
  chamber. The entrance to the renal sinus is
  called the hilium, this where blood vessels,
  nerves, lymphatic vessels and the ureter enter
  the kidney. The superior end of the ureter
  expands and forms a funnel-shaped sac called the
  renal pelvis. The renal pelvis subdivides into
  two or three major calyces, which subdivide into
  several minor calyces. Renal papillae project
  into the renal sinus from its wall, and contain
  openings that lead into a minor calyx. The inside
  of the kidney is the renal medulla, which is
  composed of renal pyramids and appears striated.
  The renal cortex forms a shell around the medulla
  and dips in between renal pyramids, forming
  renal columns. Within in the renal cortex are
  nephrons, or the kidneys functional units.

6
Kidney Functions

• The kidneys help maintain homeostasis by
  regulating the composition, volume, and pH of
  extracellular fluid. They do this by forming, and
  excreting urine, a combination of metabolic
  wastes, (from the blood), water, and
  electrolytes. The kidneys also secrete the
  hormone erythropoietin, which helps control red
  blood cell production, they help activate vitamin
  D, and they help maintain blood volume and
  pressure by releasing the enzyme renin.

7
Renal Blood Vessels

• Renal arteries supply blood to the kidneys. When
  a person is at rest these arteries usually carry
  15-30 of the total cardiac out put into the
  kidneys. Renal arteries give off several branches
  called interlobar arteries, which pass between
  renal pyramids. Arcuate arteries at the junction
  between the medulla and cortex give rise to
  interlobular arteries. Afferent arterioles, the
  final branches of interlobular arteries, lead to
  nephrons. Venous blood returns in vessels that
  correspond to arterial pathways, which then lead
  to the renal vein. The renal vein leads the blood
  back to the inferior vena cava.

8
Nephrons and their Structure

• Nephrons consist of a renal corpuscle and a renal
  tubule. Fluid flows through the tubules on its
  way out of the body. Renal corpuscles are made of
  tangled clusters of blood capillaries called
  glomerulus. Glomerular capillaries filter fluid,
  which is the first step in urine formation. The
  glomerular capsule, a thin-walled, saclike
  structure surrounds the glomerulus. The
  glomerular capsule, an expansion at the proximal
  end of a renal tubule, receives the fluid the
  glomerulus filters. The renal tubule leads away
  from the capsule and becomes tightly coiled, this
  coiled portion is the proximal convoluted tubule.
  This tubule dips towards the renal pelvis, and
  becomes the descending limb of the nephron loop.
  The tubule then curves back towards the renal
  corpuscle and forms the ascending limb of the
  nephron loop. This tubule returns to the region
  of the renal corpuscle, where it is tightly
  coiled and is called the distal convoluted
  tubule. These tubules merge in the renal cortex
  to form collecting ducts, which passes through
  the renal medulla and is joined by other tubules.
  The resulting tube empties into a minor calyx
  through an opening in a renal papilla.

9
Blood Supply of a Nephron

• The capillaries that form a glomerulus all come
  from arterioles. Blood, after passing through the
  glomerular capillaries, enters an efferent
  arteriole, (that is smaller than the afferent
  vessel). The arteriole resists blood flow, which
  raises blood pressure in the glomular capillary.
  The efferent arteriole branches off into a freely
  interconnecting network of capillaries called the
  peritubular capillary system, that surrounds the
  renal tubule. After flowing through the
  peritubular capillary system, the blood rejoins
  from other branches and enters the venous system
  of the kidney.

10
Juxtaglomerular Apparatus

• The structure of densely packed epithelial cells
  at the point of contact between the distal
  convoluted tubule and efferent and afferent
  arterioles makes up the macula densa. Within the
  walls of arterioles are juxtaglomerular cells,
  that with the cells of the macula densa form the
  juxtaglomerular apparatus. This apparatus
  controls the secretion of renin.

11
Urine Formation

• Urine formation begins with glomerular
  filtration, which is the process of the
  glomerular capillaries filtrating plasma. Tubular
  reabsorption, which moves substances from tubular
  fluid back into the blood within the peritubular
  capillary, and tubular secretion, which is the
  reverse process of tubular absorption, also aid
  in forming urine.

12
Glomerular Filtration

• Urine formation begins with water and certain
  dissolved substances being filtered out. The
  resulting glomerular filtrate is sent into
  glomerular capsules.

13
Filtration Pressure and Filtration Rate

• The hydrostatic pressure of blood and the osmotic
  pressure of plasma influence the pressure of
  glomerular capillaries, and capsules. The net
  filtration pressure is the net pressure for
  forcing substances out of the glomerulus. The net
  filtration pressure is responsible for the
  kidneys filtration rate.

14
Regulation of Filtration Rate

• The glomerular filtration rate is an example of a
  negative feedback mechanism. The filtration rate
  can also be regulated by sympathetic nervous
  system reflexes. Juxtaglomerular cells also
  control the rate by releasing renin, which reacts
  with the plasma protein angiotensinogen to form
  angiotensin I. A second enzyme called angiotensin
  converting enzyme, converts angiotensin I to
  angiotensin II. Angiotensin II is a plasma
  protein that helps maintain sodium balance, water
  balance, and blood pressure in the body.

15
Tubular reabsorption and Secretion

• Tubular reabsorption is the process in which
  filtered substances are returned to the blood
  stream. Sodium and water can be reabsorbed
  multiple times making them more concentrated.
  Tubular secretion is when substances move from
  the bloods plasma in the peritubular capillary to
  fluid in the renal tubule.

16
Regulation of Urine Concentration and Volume

• Aldosterone and antidiuretic hormone may
  stimulate the additional reabsorption of sodium
  and water. These changes in sodium and water
  excretion in response to these hormones are the
  final adjustments the kidney makes before
  urination.

17
Urea, Uric Acid Excretion, and Urine Composition

• Urea is a by-product of amino acid catabolism,
  uric acid is a result of the metabolism of
  organic bases in nucleic acids. Both of these
  by-products are excreted in urine. Urine
  composition reflects the amount of water and
  solutes that the kidney must eliminate or retain.

18
Urine Elimination

• From the kidneys, urine passes through the renal
  pelvis, and a ureter conveys it to the urinary
  bladder and reference plate. The urethra then
  excretes urine to the outside.

19
The Ureters, The Urinary bladder, Micturition and
The Urethra

• The ureters are 25 centimeters long, they begin
  as the renal pelvis, and connect to the urinary
  bladder. The urinary bladder is a hollow,
  distensible, muscular organ that stores urine and
  forces it out into the urethra. The detrusor
  muscle is a muscle that prevents the bladder from
  emptying until a certain pressure is reached.
  Micturition is the process that conveys urine
  from the urinary bladder to the outside of the
  body.

20
Medical Conditions

• Dysuria is a condition that causes painful
  urination.
• Nephrolithiasis, which is a kidney stone.
• Cystis, which is an inflammation of the urinary
  bladder.

21
Work Cited

• Butler, Jackie. Lewis, Ricki. Shier, David, ed.
  Holes Essentials of Human Anatomy and
  Physiology. New York McGraw- Hill.2006. 454-471.
  Print.