Module 16: Urinary System

Functions of the urinary system

Store/excrete urine, regulate blood volume/pressure, ion/acid-base balance, erythrocyte production.

Location of the kidneys

Retroperitoneal, T12-L3, right lower due to liver.

Size of the kidneys

11-14 cm long, 6 cm wide, 4 cm thick, fist-sized.

External protection of the kidneys

Fibrous capsule, perirenal fat, renal fascia, pararenal fat, ribs.

Fibrous capsule

Dense connective tissue covering the kidney.

Perirenal fat

Adipose layer for shock absorption around the kidney.

Renal fascia

Connective tissue anchoring kidneys to abdominal wall.

Pararenal fat

Additional adipose cushioning around kidneys.

Renal hilum

Entry/exit for renal artery, vein, pelvis, and nerves.

Renal cortex

Outer, lighter kidney region with renal corpuscles.

Renal medulla

Inner region with renal pyramids and columns.

Renal columns

Connective tissue extensions separating renal pyramids.

Renal pyramids

Medullary structures containing nephrons.

Renal papilla

Tip of pyramid, drains into minor calyx.

Minor calyx

Collects urine from renal papilla.

Major calyx

Merges minor calyces, drains to renal pelvis.

Renal pelvis

Funnels urine from calyces to ureter via peristalsis.

Blood supply to the kidneys

Renal arteries from descending aorta.

Blood drainage from the kidneys

Renal veins to inferior vena cava.

Path of renal arterial blood flow

Renal artery → segmental → interlobar → arcuate → cortical radiate → afferent arterioles.

Path of renal venous drainage

Peritubular capillaries/vasa recta → cortical radiate → arcuate → interlobar → renal vein.

Nephron

Functional unit of the kidney, filters blood to form urine.

Cortical nephrons

Nephrons with short loops of Henle in cortex.

Juxtamedullary nephrons

Nephrons with long loops extending into medulla.

Renal corpuscle

Glomerulus and glomerular capsule, filters blood.

Glomerulus

High-pressure capillary bed for filtration.

Glomerular capsule

Surrounds glomerulus, captures filtrate.

Parietal layer of the glomerular capsule

Simple squamous epithelium, outer layer.

Visceral layer of the glomerular capsule

Podocytes with pedicels, surrounds glomerulus.

Podocytes

Cells with pedicels forming filtration slits in glomerular capsule.

Pedicels

Finger-like extensions of podocytes forming filtration slits.

Filtration slits

Gaps between pedicels allowing filtrate passage.

Filtration membrane

Fenestrated capillaries, basement membrane, filtration slits.

Fenestrated capillaries

Glomerular capillaries with pores for filtration.

Filtration membrane filtration

Filters small molecules (4-8 nm), repels negatively charged substances.

Proximal convoluted tubule (PCT)

Tortuous tubule reabsorbing ~67% water, Na+, K+.

Loop of Henle

Descending/ascending limbs, reabsorbs ~15% water, Na+.

Descending loop of Henle

Simple squamous, water-permeable, reabsorbs water.

Ascending loop of Henle

Thick cuboidal, reabsorbs Na+/Cl-, creates hypoosmotic filtrate.

Distal convoluted tubule (DCT)

Reabsorbs ~10-15% water, Ca++, secretes waste.

Collecting tubules

Merge nephron filtrate, lead to collecting ducts.

Collecting ducts

Merge in medulla, regulated by ADH for water reabsorption.

Role of ADH in collecting ducts

Inserts aquaporins for water reabsorption.

Filtrate

Fluid filtered by glomerulus, modified into urine.

Tubular fluid

Filtrate being modified in nephron tubules.

Juxtaglomerular apparatus (JGA)

Macula densa and juxtaglomerular cells, regulates filtration.

Macula densa

DCT cells monitoring Na+ in filtrate.

Juxtaglomerular cells

Afferent arteriole cells regulating filtration via contraction.

Peritubular capillaries

Surround nephron tubules, recover solutes/water.

Vasa recta

Capillaries around juxtamedullary nephron loops, recover water.

Ureters

~30 cm tubes, transport urine from renal pelvis to bladder.

Ureter's epithelial lining

Transitional epithelium with goblet cells for mucus.

How ureters move urine

Peristalsis via longitudinal/circular smooth muscle.

What prevents ureteral reflux

Oblique entry into bladder wall.

Urinary bladder's function

Stores urine (~300-400 mL capacity).

Bladder's epithelial lining

Transitional epithelium, stretches with filling.

Detrusor muscle

Smooth muscle in bladder wall, contracts for micturition.

Trigone

Triangular area between ureteral openings and urethra.

Urethra's function

Transports urine (and semen in males) to exterior.

Female urethra's length

~4 cm, higher UTI risk due to shortness.

Male urethra's length

~20 cm, includes prostatic, membranous, spongy regions.

Prostatic urethra

Passes through prostate, receives sperm/secretions.

Membranous urethra

Passes through perineum, invested by sphincters.

Spongy urethra

Passes through penis, exits at tip.

Internal urethral sphincter

Involuntary smooth muscle, relaxes during micturition.

External urethral sphincter

Voluntary skeletal muscle, controlled by pudendal nerve.

Micturition

Urination, involves detrusor contraction and sphincter relaxation.

Triggers of micturition reflex

Bladder stretch receptors at ~150 mL, signals to sacral spinal cord.

Nerves controlling micturition

Hypogastric (sympathetic), pelvic (parasympathetic), pudendal (somatic).

Role of the pudendal nerve

Controls voluntary external urethral sphincter.

Vesicoureteral reflux

Urine backflow into ureters, increases UTI risk.

Erythropoietin

Hormone produced by kidneys, stimulates red blood cell production.

Renin's role in the urinary system

Regulates blood pressure, produced by juxtaglomerular cells.

Kidney's blood flow percentage

Receives ~25% of cardiac output at rest.