Urinary System: Functions and Anatomy

Urinary System Functions

• Excretion removes organic wastes from body fluids
• Elimination discharges wastes into the environment
• Homeostatic regulation maintains blood plasma volume and solute concentration
• Additional homeostatic functions exist beyond waste removal

Urinary System Components

• Two kidneys filter waste and produce urine
• Two ureters transport urine from the kidneys to the bladder
• A urinary bladder stores urine until it is excreted
• Abdominal and pelvic cavities closely interact with the gastrointestinal and reproductive systems

Kidney Characteristics

• Kidneys are smooth, bean-shaped organs with a reddish-brown color
• They lie on either side of the vertebral column on the upper posterior abdominal wall

Kidney Placement

• The upper border is near the 12th thoracic vertebra
• The lower border is near the third lumbar vertebra
• The left kidney sits 1.5 to 2 cm higher than the right kidney
• Kidneys are positioned behind the parietal peritoneum, against deep back muscles
• Connective and adipose tissue surround and hold the kidneys in place

Kidney Structure

• Kidneys feature a convex lateral surface and a concave medial side
• A medial depression leads to the hollow renal sinus
• Hilum is the chamber's entrance that accommodates blood vessels, nerves, lymphatic vessels, and the ureter
• Kidneys are located behind the abdominal cavity, in the retroperitoneal space
• The renal pelvis resides inside the renal sinus
• Calyces are large urinary tubes subdivided into major and minor calyces
• Renal papillae are small elevations projecting into the renal sinus from the renal pelvis walls
• Tiny openings from the minor calyces pierce each projection

Renal Medulla and Cortex

• The inner renal medulla consists of conical tissue called renal pyramids, exhibiting striations
• The outer renal cortex encloses the medulla
• Nephrons are located in the the outer renal cortex, giving it a granular apperance
• Nephrons are tiny tubules are the functional units of the kidneys

Kidney Homeostasis

• Kidneys help maintain homeostasis by regulating the composition, pH, and volume of extracellular fluid
• Kidneys remove metabolic wastes from the blood and dilute the with water and electrolytes, forming urine

Kidney Functions

• Kidneys secrete erythropoietin
• Kidneys help activate vitamin D
• Kidneys help maintain blood volume and pressure via secretion of the enzyme rennin

Kidney Blood Vessels

• Renal arteries arise from the abdominal aorta, supplying blood to the kidneys
• Inside the kidneys, renal arteries branch into interlobar, arcuate, and interlobular arteries
• Afferent arterioles branch from interlobular arteries and lead to the nephrons
• Renal blood returns through a series of vessels, with the renal vein joining the inferior vena cava

Nephron Basics

• Each kidney contains about 1 million nephrons
• Each nephron includes a renal corpuscle and a renal tubule

Glomerulus and Capsule

• The glomerulus is a tangled cluster of blood capillaries that constitutes a renal corpuscle
• Filtration via the glomerular capillaries initiates urine formation
• The glomerular capsule, a sac-like structure, surrounds the glomerulus
• It sits at the proximal end of a renal tubule
• The glomerular capsule receives filtered fluid from the glomerulus
• The renal tubule leads away from the glomerular capsule, coiling into the proximal convoluted tubule

Tubule Sections

• The proximal convoluted tubule dips toward the renal pelvis, forming the descending limb of the nephron loop (loop of Henle)
• The nephron loop curves, forming the ascending limb and returns to the renal corpuscle region
• Tubules coil tightly and become the distal convoluted tubule
• The distal convoluted tubules merge to form collecting ducts in the renal cortexes
• Collecting ducts pass into the renal medulla
• The tubules empty into the minor calyces via openings in the renal papillae

Arterioles and Capillaries

• Blood (minus filtered fluids) exit the glomerular capillaries through the efferent arteriole
• Efferent arterioles rise from afferent arterioles
• The efferent arteriole has a smaller diameter than the afferent arteriole
• It resists blood flow and causes blood to back up into the glomeruli, increasing glomerular capillary pressure
• Efferent arterioles branch into complex networks called peritubular capillaries that surround the renal tubules
• Blood pressure is low in the peritubular capillaries, and blood eventually enters the venous system of the kidney

Juxtaglomerular Complex

• The distal convoluted tubule contacts the afferent and efferent arterioles while passing between them
• Macula densa cells are densely packaged and narrow distal tubule epithelial cells
• The juxtaglomerular apparatus contains juxtaglomerular cells (enlarged smooth muscle cells) and the macula densa

Urine Production

• Glomerular filtration initiates the urine formation
• The glomerular capillaries filter plasma
• Most of the filtered fluid is reabsorbed into the bloodstream through the colloid osmotic pressure of the plasma
• Nephrons produce urine using two capillaries in series
• The first capillary bed filters instead of forming interstitial fluid
• Filtrate moves into the renal tubule to form urine
• Normally, glomerular filtration produces 180 liters of fluid every 24 hours
• Glomerular filtration rate (GFR) is the rate at which blood is filtered through the glomerula

Processes of Urine Formation

• Tubular reabsorption moves substances from the tubular fluid into the blood within the peritubular capillary
• Kidneys reclaim the correct amounts of water, electrolytes, and glucose as required by the body
• Tubular secretion moves substances from the blood in the peritubular capillary into the renal tubule
• Substances the body must excrete are removed more quickly this way than through filtration

Urine Modification

• The urine composition is modified as it transits through the loop of Henle to the distal convoluted tubules
• This modification helps maintain fluid balance by changing urine volume due to variations in capillary blood pressure
• Increased blood pressure increases filtration and urine output
• Lowered blood pressure increases water retention
• Of the water the kidney reabsorbs, 99% is uptaken via a passive process
• Electrolytes, such as chloride and sodium, are activley transported
• Countercurrent multiplier mechanism describes how urine flows and how fluids and solutes move, across the nephron in the vasa recta
• The Vasa recta surrounds vessels that lead the arterioles in the glomerus, and vice versa

Urine Production Quantities

• The average urine production varies between 0.6 and 2.5 liters per day
• Normal urine production ranges 50-60 mL per hour
• An output of less than 30 mL per hour may indicate kidney failure

Hormones

• Several hormones affect kidney function
• Renin is produced by cells of the juxtaglomerular apparatus when blood pressure is low
• Renin starts a series of steps that contributes to the production of angiotensin II
• Angiotensin II is a kinin that causes vasoconstriction and sympathetic activation
• It signals the adrenal gland to increase aldosterone production
• Aldosterone is a steroid hormone produced by the adrenal glands
• Aldosterone increases the rate of sodium and water resorption from the tubules to the blood
• Increased aldosterone concentrations result in the retention of sodium and chloride
• Decreased concentrations result in increased urinary losses of sodium and chloride
• ADH (antidiuretic hormone) is produced by the hypothalamus but is stored in the posterior lobe of the pituitary gland
• ADH regulates the permeability of the distal convoluted tubules and the collecting ducts
• In the presence of ADH, these structures are relatively impermeable to water, resulting in relatively concentrated urine
• Lack of ADH results in the production of a large volume of very dilute urine

Ureters

• The ureters are a pair of thick-walled, hollow tubes that carry urine from the kidneys to the urinary bladder
• Each ureter wall has 3 layers
  • Mucous coat
  • Muscular coat
  • Fibrous coat
• The muscular walls propel urine
• A flap-like fold of mucous membrane covers the opening through which urine flows from each ureter into the bladder
• Folds in the mucous membrane prevents from backing up the ureters

Urinary Bladder Characteristics

• The urinary bladder is a hollow, muscular sac in the pelvis along the midline that stores urine until excretion
• It is located in the pelvic cavity behind the pubic symphysis, beneath the parietal peritoneum
• The bladder has many folds when empty that smooth out as it fills
• The trigone is a triangular internal floor of the bladder with an opening at each of its three angles
• The urinary bladder wall has four layers:
  • Mucous coat
  • Submucous coat
  • Muscular coat
  • Serous coat
• Dorsal muscle is formed by interlaced smooth muscle fibers of the muscular coat
• Part of the dorsal muscle surrounds the neck of the bladder to form the internal urethral sphincter
• The internal urethral sphincter is innervated with parasympathetic nerve fibers that function in the micturition reflex

Micturition (Urination)

• Micturition involves expelling urine from the urinary bladder
• The detrusor muscle contracts with the abdominal wall and pelvic floor muscles, and the external urethral sphincter relaxes
• The micturition reflex center in the spinal cord sends parasympathetic motor impulses to the detrusor muscle, causing it to rhythmically contract
• The bladder can hold up to 1000 mL of urine before stimulating pain receptors, with the first urge to urinate to happen at around 150 mL
• The external urethral sphincter is under conscious control
• The detrusor muscle contracts and urine flows through the urethra

Urethra

• Urine drains from the bladder to the outside of the body via the urethra
• The urethra is lined with mucous membrane and a thick layer of smooth muscle tissue
• The urethral wall has many urethral glands that secrete mucus into the urethral canal

Male Urethra

• The male urethra consists of three parts:
  • Prostatic urethra travels through the prostate gland
  • Membranous urethra from the prostate gland to the base of the penis, surrounded by the urethral sphincter
  • Spongy urethra lies within the corpus spongiosum of the penis and terminates at the external urethral orifice, which is the narrowest part
• The female urethra is about 4 cm long and fused with the anterior wall of the vagina
• The female urethra ends between the clitoris and the vagina