Anatomy and Physiology - Urinary System

Urinary tract

Ureters, urinary bladder, and urethra; they transport urine from kidneys to outside body

Processes that occur as filtrate is converted to urine

• Elimination of metabolic wastes

• Regulation of ion levels (ex: Na+, K+, Ca2+)

• Regulation of acid-base balance: alters levels of H+/hydron (acid) and HCO3-/bicarbonate (base)

• Regulation of blood pressure

• Elimination of biologically active molecules: hormones, drugs

• Formation of calcitriol: increases absorption of calcium from small intestine

• Secretes erythropoietin (EPO) in response to low blood oxygen

  • Stimulates red bone marrow to increase erythrocyte production

• Potential to engage in gluconeogenesis to maintain glucose levels

  • During prolonged fasting or starvation

Kidney structure

Against the posterior abdominal wall & are retroperitoneal (posterior to the parietal peritoneum)

• Only the anterior surface is covered by the parietal peritoneum

• Tissue layers (inner → outer): fibrous capsule, perinephric fat, renal fascia, and paranephric fat

• Two distinct regions: outer renal cortex and an inner renal medulla

• Concave medial border (hilum) where vessels, nerves, and the ureter connect to it

Peritoneum

Serous membranes lining the internal walls of the abdominopelvic cavity and covering most of the organs within the cavity

Nephron

Functional filtration unit of the kidney that mostly resides in the renal cortex; composed of:

• Renal corpuscle: includes the glomerulus and glomerular capsule/Bowman’s capsule

• Renal tubule: includes the proximal convoluted tube (PCT), nephron loop/loop of Henle, and distal convoluted tubule (DCT)

• Not part of the nephron but associated with it are the afferent arteriole, efferent arteriole, collecting tubule, and collecting duct

Glomerulus (renal corpuscle)

Thick tangle of blood capillary loops

• Blood enters by afferent arteriole and exits by an efferent arteriole

• Primary location of blood filtration

Fluid flow through the renal tubule

1. First region is the PCT, originating at the renal corpuscle

2. Bends sharply to form a nephron loop that juts into the renal medulla

3. Re-enters the renal cortex at the DCT

4. Drain into a connecting tubule

5. Multiple collecting tubules empty into larger collecting ducts

Types of nephrons

Functions are filtration and reabsorption

• Cortical: located almost completely in the cortex with short nephron loops that barely penetrate into the medulla; 85% of nephrons

• Juxtamedullary: lie close to the corticomedullary junction and have relatively long nephron loops that extend deep into the medulla; remaining 15%

  • Additional functions of urine concentration and electrolyte balance

Juxtaglomerular apparatus

Formed by the physical contact between the afferent arteriole and the adjacent DCT

Regulates urine production and blood pressure

Composed of:

• Extraglomerular mesangial cells

• Macula densa of DCT

• Granular cells of affertent arteriole

Fluid flow through the urinary system

1. As filtrate: capsular space (between glomerulus and glomerular capsule)

2. As tubular fluid: PCT

3. Descending limb of nephron loop

4. Ascending limb of nephron loop

5. DCT

6. Collecting tubules

7. Collecting ducts

8. As urine: papillary duct

9. Minor calyx (part of renal sinus)

10. Major calyx (part of renal sinus)

11. Renal pelvis (part of renal sinus)

12. Ureter

13. Urinary bladder

14. Urethra

Filtrate vs. tubular fluid vs. urine

Essentially plasma (water + solutes) minus large solutes (e.g., most proteins)

• Filtrate enters the PCT → called tubular fluid

• Leaves the collecting ducts → called urine

Process of urine formation

• Glomerular filtration: movement of substances from the blood within the glomerulus into the capsular space to form filtrate

• Tubular reabsorption: movement of substances from the tubular fluid back into the blood

• Tubular secretion: movement of substances from the blood into the tubular fluid

(Tubule is focused on fine-tuning filtrate, ISF, and blood concentrations)

Peritubular capillaries and vasa recta

Tubular reabsorption and secretion occur between the tubules and the peritubular capillaries + vasa recta

Specialized blood vessels in the kidney that play a critical role

in maintaining the medullary osmotic gradient and preventing its washout

Branches from efferent arteriole and merges into venules

• Peritubular capillaries: surround the DCT and PCT; provide oxygen and nutrients in the cortex

• Vasa recta: surround the nephron loop; provide oxygen and nutrients in the medulla

Filtration membrane

Filtrate is made in renal corpuscle when blood plasma is forced across the filtration membrane under pressure (produced by difference in afferent and efferent arteriole pressures)

3 layers (inner → outer)

• Fenestrated endothelium of a glomerular capillary: blocks formed elements

• Basement membrane of a glomerular capillary: blocks large proteins

• Visceral layer of glomerular capsule (membrane-covered filtration slits): blocks small proteins

Podocytes

Specialized epithelial cells in the kidneys that play a critical role in glomerular filtration

Spaces between the pedicels (footlike processes) form the membrane-covered filtration slits of the filtration membrane