Urinary System

Urinary System

  • Filters 200L of blood daily, removing toxins, metabolic wastes, excess ions, etc.

  • Regulates volume and chemical makeup of blood

  • Maintains the proper balance btwn water and salts; acids and bases

  • Important system for excretion—removal of cellular waste

  • Urine = trash

  • Kidneys are in retroperitoneal area

What is Excretion?

  • Removing cellular waste from the body

  • Also performed by other systems (respiratory system excretes CO2)

  • Urinary system excretes urine

Other Renal Functions

  • Gluconeogenesis during prolonged fasting

  • Production of renin to inc BP and erythropoietin to stimulate RBC production

  • Activation of vitamin D to form calcitriol to inc blood calcium

Urinary System is Retroperitoneal

  • Kidneys

    • Form urine from blood substances and transport it to the ureters

    • The surrounding fibrous capsule prevents infection

  • Ureters—transport urine from kidneys to bladder by peristalsis

  • Urinary bladder—stores urine

  • Urethra—transports urine from bladder → outside of body

Urinary Bladder

  • Collapsible, smooth muscle sac (detrusor muscle) lined w/ transitional epithelium

  • Trigone—a triangular area outlined by the openings for the ureters and the urethra

    • Clinically important bc infections tend to persist in this region

Blood Supply

  • ~1/4 (1200 ml) of systemic cardiac output flows through the kidneys each minute

  • Arterial flow into/venous flow out of the kidneys follow similar paths

    • Vasa recta are only found on the nephron loop of the juxtamedullary nephrons, which dip far down into the medulla, shown in the next figure

The Nephron

  • Structural and functional units that form urine, consisting of

    • Glomerulus—a tuft of capillaries assocaited w/ a renal tubule

    • Glomerular (Bowman’s) capsule)—cup-shaped end of a renal tubule that completely surrounds the glomerulus

  • Renal corpuscle—the glomerulus and its Bowman’s capsule (the combination)

  • Glomerular endothelium—fenestrated endothelium that allows solute-rich, virtually protein-free filtrate to pass from the blood into the glomerular capsule

    • Fenestrated—has holes in it

      • Like a pasta strainer; holds the solids back but lets the fluid flow freely

    • Filtrate—fluid that is removed from the blood and is modified to form urine

    • Both glomerular capillaries and pertibuluar capillaries are fenestrated

Nephron Filtration Membrane

  • Filter that lies btwn the blood and the interior of the glomerular capsule

  • Composed of three layers

    • Fenestrated endothelium of the glomerular capillaries

    • Visceral membrane of the glomerular capsule (podocytes)

    • Basement membrane composed of fused basal laminae of the other layers

Nephron Renal Tubule

  • Proximal convoluted tubule (PCT)—composed of cuboidal cells w/ numerous microvilli and mitochondria

    • Reabsorbs water and solutes from filtrate and secretes substances into it

      • Reabsorption—mvmt of substances from the filtrate to the blood

      • Secretion—mvmt of substances from the blood to the filtrate

Nephron Renal Tubule

  • Nephron loop (loop of Henle)—a hairpin-shaped loop of the renal tubule

    • Proximal part is similar to the proximal convoluted tubule

    • Proximal part is followed by the thin segment (simple squamous cells) and the thick segment (cuboidal to columnar cells)

    • Descending limb allows water reabsorption, while the ascending limb allows NaCl reabsorption

  • Distal convoluted tubule (DCT)—cuboidal cells wi/o microvilli that function more in secretion than reabsorption

Kidney Nephrons Form Urine

  • Over 1 million nephrons (filtering stations) per kidney

  • Renal corpuscle is location of blood filtration (blood comes in from renal artery/arterioles)

  • Be mindful of the coloring; urine tubules are supposed to be creme/tan/beige
Nephron Urine Formation

  • Glomerular filtration—filtration of blood plasma by the glomerular capillaries

    • Filtrate (which will be modified to become urine) is similar to blood plasma but lacks large proteins

  • Tubular reabsorption—selective body reabsorption of substances such as water, amino acids, glucose, etc., by the kidney tubule back to the blood

    • Water is reabsorbed by osmosis

    • All other substances reabsorbed by diffusion, passive, or active transport

  • Tubular secretion—rapid removal of toxins (i.e., ammonia, hydrogen ions) from blood plasma to the kidney tubule for removal through the urine

    • Ammonia (NH3) is a byproduct of protein processing

      • The liver detoxifies it to form urea, which is filtered out by the kidney

Urine Flow

  • Urine flow—renal pyramid (renal cortex, renal medulla), minor calyx, major calyx, renal pelvic, ureter, urinary bladder, urethra

  • Urination is referred to as micturition/diuresis and is a reflex controlled by the spinal cord and pons

  • Male urethra ~18 cm

  • Female urethra ~4 cm

Male Urethra

  • Fibromuscular tube from urinary bladder to end of penis

  • Lined w/ mucous membrane

  • Nearby prostate gland may affect urination

  • Internal urethral sphincter (smooth muscle) at base of bladder

  • External urethral sphincter (skeletal muscle) at base of prostate gland

Female Urethra

  • Much shorter than male urethra

  • Location of opening may vary greatly - may even be slightly inside the vaginal opening

  • Internal urethral sphincter (smooth muscle) at base of bladder

  • External urethral sphincter (skeletal muscle) about halfway down urethra

Urine Chars

  • 0.6 - 2.5L excreted per day (more w/ diuretics)

  • Clear to amber in color (urochrome pigment)

  • Acidic odor

  • pH 4.5 - 8.0 (avg 5.5 - 6.5)

    • Wide range may prevent kidney stones (renal calculi)

  • Composed of water, urea, salts, and minerals

    • Very small amounts of albumin protein and blood or body cells

  • Urine is trash; a healthy person should NOT dump nutrients or any other substances that are needed by the body

  • Diuretics—cause incd urination

Hormones Affecting Urine Concentration

  • Aldosterone

    • From the adrenal glands

    • Causes Na+ reabsorption and corresponding water reabsorption in the DCT, which incs BP and results in concentrated (darker) urine

  • Antidiuretic hormone (ADH)

    • From the pituitary gland

    • Acts on the DCT and collecting duct to cause water reabsorption and prevent blood dehydration, resulting in concentrated urine

  • Dark urine is an indication of dehydration in the body