Urinary System Anatomy and Function

Objectives of the Urinary System

• Filter nearly 200 liters of fluid from the bloodstream
• Remove toxins, metabolic wastes, and excess ions from the body in urine

Kidneys - Anatomical Location and Structure

• Located in the retroperitoneal position
• Left kidney: around T12 to L3 vertebrae
• Right kidney: lower due to slight displacement by the liver
• Upper portions of the kidneys: protected by the eleventh and twelfth ribs
• Surrounded by a fibrous capsule
• Medial surface: concave and has a vertical cleft (renal hilum)
• Lateral surface: convex
• Renal sinus: internal cavity within kidney
• Renal pelvis: funnel-shaped tube that collects urine

Kidneys - Internal Anatomy

• Cortex: reddish-brown and granular (3 cm)
• Medulla: distinct granular texture
• Pelvis: internal cavity within kidney
• Renal columns: bands of cortical tissue that separate adjacent renal pyramids
• Renal pyramids: extensions of the medulla into the cortex
• Minor calyces: cup-shaped areas that enclose the papillae
• Major calyces: subdivisions of the renal pelvis

Nephron - Functional Unit of the Kidneys

• Renal corpuscle: spherical structure consisting of glomerulus and glomerular capsule (Bowman's capsule)
• Renal tubule: long tubular passageway that begins at the renal corpuscle
• Proximal convoluted tubule: located in the cortex
• Nephron loop/Loop of Henle: located partially in the medulla
• Distal convoluted tubule: located in the cortex
• Collecting duct: receives filtrate from many nephrons and delivers urine into minor calyces via papillae

Blood Vessels of Nephron

• Nephron capillary bed: glomerulus and peritubular capillaries
• Juxtamedullary nephrons: associated with special capillaries called the vasa recta
• Cortical nephrons: not associated with vasa recta

Classes of Nephron

• Cortical nephrons: 85% of the nephrons in the kidneys, located in the cortex

• Juxtamedullary nephrons: located near the cortex-medulla junction, associated with vasa recta### Kidney Structure and Function

• Peritubular capillaries cling closely to adjacent renal tubules and empty into nearby venules.

• The kidneys receive 20-25% of total cardiac output, or 1200ml of blood per minute.

• Kidney receives blood through the renal artery and exits through the renal veins.

• Renal plexus provides the nerve supply of the kidney and its ureter.

Juxtaglomerular Complex (JGC)

• Each nephron has a JGC, which regulates the rate of filtrate formation and systemic blood pressure.
• The JGC consists of 3 populations of cells:
  • Macula densa (distal convoluted tubule): a salt sensor that generates paracrine chemical signals to control vital kidney functions.
  • Granular/Juxtaglomerular cells: smooth muscle cells that synthesize, store, and secrete the enzyme renin.
  • Extraglomerular mesengial cells: involved in renal autoregulation of blood flow to the kidney and regulation of systemic blood pressure through the renin-angiotensin system.

Urinary Tract

• The urinary tract consists of the ureters, urinary bladder, and urethra.
• Ureters are pair of muscular tubes that continue from the renal pelvis and penetrate the posterior wall of the urinary bladder at an oblique angle.
• The urinary bladder is a hollow, muscular organ that can contain up to 1 liter of urine.
• The bladder wall has three layers: mucosa, muscularis, and adventitia.

Urethra

• The male urethra is approximately 20 cm long and has three regions: prosthetic urethra, membranous urethra, and spongy urethra.
• The female urethra is shorter, around 3-5 cm, and extends from the bladder to the vestibule.
• The external urethral orifice is near the anterior wall of the vagina.

Kidney Function and GFR

• Homeostatic functions of the urinary system include:
  • Regulating blood volume and blood pressure
  • Regulating plasma ion concentrations
  • Helping stabilize blood pH
  • Conserving valuable nutrients
  • Assisting liver to detoxify poisons
  • Regulating osmolarity
• Kidney functions include:
  • Concentrating filtrate by glomerular filtration
  • Absorbing and retaining valuable materials for use by other tissues
• The basic processes of urine formation are:
  • Filtration
  • Tubular reabsorption
  • Tubular secretion

Forms of Membrane Transport

• Filtration: occurs at the renal corpuscle, where glomeruli produce about 180 L of filtrate per day (70 times plasma volume)
• Water and solute reabsorption: primarily occurs along proximal convoluted tubules
• Active secretion: primarily occurs at proximal and distal convoluted tubules### Glomerular Filtration
• Glomerular filtration is a passive process driven by hydrostatic pressure, which forces water and small solutes through membrane pores.
• The glomerular membrane has three components: capillary endothelium, lamina densa, and filtration slits.
• The filtration slits are the finest filters, preventing the passage of most small plasma proteins.
• Net filtration pressure (NFP) is the pressure needed to filter blood plasma from the glomerulus into the capsular space.
• NFP is calculated by the formula: NFP = GBHP - CHP - BCOP.

Glomerular Filtration Rate (GFR)

• GFR is the amount of plasma ultrafiltrate produced by the kidneys per minute.
• GFR averages 125 ml/min for males and 105 ml/min for females.
• Any factor that alters filtration pressure alters GFR.
• GFR is regulated by three levels of control: autoregulation, hormonal regulation, and autonomic regulation.

Regulation of GFR

• Autoregulation: local regulation of GFR through myogenic mechanisms and tubuloglomerular feedback.
• Hormonal regulation: initiated by the kidneys to regulate GFR through hormones such as angiotensin II and atrial natriuretic peptide (ANP).
• Autonomic regulation: regulated by the sympathetic division of the autonomic nervous system (ANS).
• Strong stimulation (e.g., exercise or hemorrhage) causes afferent arterioles to constrict, reducing GFR.

Reabsorption and Secretion

• Reabsorption: the process of recapturing useful materials from the filtrate into the bloodstream.
• Most of the filtered water (99%) and solutes are reabsorbed, especially in the proximal convoluted tubules (PCT).
• Reabsorption occurs through both active and passive processes.
• Small proteins and peptides are reabsorbed by pinocytosis.
• Reabsorption routes include paracellular reabsorption (50% of reabsorbed material) and transcellular reabsorption.

Secretion

• Secretion: the transfer of materials from the blood into the tubular fluid.
• Purpose: to eliminate toxic wastes and regulate blood pH.
• Eg., H+, K+, NH4-, creatinine, drugs.
• Secretion occurs through both active and passive processes.

Transport Mechanism

• Solutes move in one direction through the transport mechanism.
• Involves combination of diffusion, osmosis, channel-mediated diffusion, and carrier-mediated transport.
• Primary active transport uses ATP, e.g., Na+/K+ pumps.
• Secondary active transport involves facilitated diffusion, symporters, and antiporters.

Renal Transport Systems

• Lots of transporter proteins are involved in the transport of different molecules/ions.
• Each transporter protein has a maximum transport (TM) capacity, where transport saturates.