Urinary System Microanatomy- Pt 1

Urinary System: General Info

• The kidneys excrete nitrogenous waste, conserve body fluids and electrolytes, and reabsorb solutes and water.
• The ureters convey urine to the bladder.
• The urinary bladder stores urine.
• The urethra expels urine from the body.
• Urine is an ultrafiltrate of blood.
• Clinical signs of renal and non-renal disease relate to the microanatomy and function of the kidneys.
• A dog's kidney has a normal dark red/brown cortex color, while a cat's kidney has a normal pale red cortex color due to high adipose tissue components.

Blood-Urinary Barrier

• The blood-urinary barrier consists of four components: fenestration, basement membrane, endothelial cells, and podocytes.
• Proteins always travel to the basement membrane because filtration happens here.

Urine Descent

• Urine descent takes place in the following order: proximal convoluted tubules, Loops of Henle, and then distal convoluted tubules.

Kidney Functions

• The kidneys help maintain water and electrolyte homeostasis by:
  • Filtrating cellular waste from blood
  • Selectively reabsorbing water and solutes
  • Regulating fluid balance
  • Maintaining electrolyte homeostasis/acid-base balance
• The kidneys also excrete metabolic waste products, bioactive substances (including drugs), and water.
• The kidneys produce the hormones erythropoietin and renin.
• Erythropoietin is involved in targeting bone marrow cells to increase RBC (red blood cell) production.
• Kidney damage can result in anemia.
• The kidneys regulate blood pressure via the juxtaglomerular apparatus.
• The kidneys also activate vitamin D, which aids in calcium metabolism.

Kidney Structure

• The kidneys are composed of:
  • Capsule
  • Renal lobe/pyramid
  • Outer cortex
  • Inner medulla
  • Papillae/crest
  • Calices
  • Renal pelvis
• The calices are dilations of the renal pelvis.
• The renal pelvis is a dilation of the proximal ureter.
• The renal hilus is the area of the kidneys where arteries and veins enter/leave.
• It is normal to see adipose connective tissue in the renal sinus in low quantities.
• The nephron is the "functional unit" of the kidneys, composed of the renal cortex and medulla.
• The nephrons are the site of osmoregulation via:
  • Filtration of water and small molecules from blood plasma to form a filtrate
  • Selective reabsorption of most of the water and other molecules from the filtrate

Kidney Types

• Unilobular kidneys are typically found in carnivores.
• Multilobular kidneys of large ruminants lack a renal pelvis, and the kidneys' surface has each lobe distinctly outlined by deep grooves.
• Multilobular kidneys of pigs have a smooth surface.

Kidney Components

• The capsule of the kidney is composed of collagen fibers, smooth muscle, and blood vessels.
• The cortex of the kidneys is composed of:
  • Renal corpuscles
  • Distal and proximal convoluted tubules
  • Collecting tubules
  • Peritubular capillary plexuses
• Proximal convoluted tubules have a brush border.
• Distal convoluted tubules (DCT) lack a brush border.
• The medulla of the kidneys is composed of:
  • Loops of Henle
  • Collecting ducts
  • Vasa recta
• The renal pelvis is composed of:
  • Urothelium
  • Submucosae
  • Smooth muscle
  • Adventitia
• The renal corpuscle is composed of:
  • Glomerulus
  • Bowman's capsule
• The renal corpuscle has the main function of protein conservation.
• The glomerulus is loops (tufts) of capillaries with fenestrated endothelium within the bowman's capsule.
• Mesangial cells lie between fenestrated capillaries and are phagocytic, contractile, and act as support.
• The glomerulus is the site of ultrafiltration and retains proteins.
• Deep sea fish lack a glomerulus and instead have renal tubules surrounded by a well-developed renal portal system, forming primitive urine by tubular secretion.

Ultrafiltration at the Glomerulus

• Vascular pole:
  • Blood arrives at and enters the glomerular capillaries via the main afferent arteriole.
  • Blood exits at the efferent arteriole.
• Ultrafiltration at the glomerulus:
  • Blood is pushed through the filtration barrier, which is composed of:
    • Fenestrated endothelium
    • Glomerular basement membrane
    • Podocyte foot processes
  • The ultrafiltrate enters the urinary space.
• Urinary pole:
  • The ultrafiltrate enters the proximal convoluted tubule.

Filtration Barrier

• The filtration barrier is composed of:
  • Endothelium of glomerular capillary loops with fenestrations
  • Glomerular basement membrane, which is made of fused basal laminae of capillaries and podocytes
  • The glomerular basement membrane has three layers: lamina rara interna, lamina densa, and lamina rara externa
• The lamina densa of the basement membrane acts as the exclusion barrier for molecules the size of IgG and albumin.
• Podocytes with pedicles (foot processes)

Endothelium Types

• Continuous endothelium is a barrier system that controls the transfer of molecules across the cell.
• Fenestrated endothelium allows for controlled transfer of small molecules and limited amounts of protein across the fenestrae (filtration mechanism).
• Discontinued endothelium has gaps which allow for "free" transfer of plasma proteins, red and white blood cells, water, and most molecules across endothelial cells.

Urine Formation: Steps

• Step 1: Primary/glomerular filtrate is produced by ultrafiltration of blood in the renal corpuscle.
• The composition of ultrafiltrate is similar to blood plasma, but it does not contain most proteins.
• Step 2: In the renal tubules, 98% of the filtrate is reabsorbed.
• Most water and sodium, all of the glucose, and all of the amino acids from the ultrafiltrate are reabsorbed.
• Step 3: Tubular secretion of potassium, hydrogen, NH4+, bile salts, drugs, and metabolites takes place.
• Waste molecules and some water remain in the tubular system, and eventually will empty into the ureter.
• Step 4: Urine is stored in the bladder pending voiding/micturition.

Proximal Convoluted Tubule (PCT)

• The proximal convoluted tubules begin at the urinary pole of the renal corpuscle and are only located in the cortex of the kidney.
• The proximal convoluted tubule consists of a single layer of cuboidal epithelial cells with microvilli (forming the Brush border), and a basement membrane.
• The purpose of the brush border/microvilli is to increase the surface area.
• The microvilli are non-motile.
• The proximal convoluted tubules have highly metabolic active cells with many mitochondria that act as:
  • Sodium-potassium pumps
  • Aquaporins
  • Peroxisomes
  • Endosome
  • Lysosomes
• The proximal convoluted tubules function to reabsorb:
  • Glucose
  • Na+/H2O
  • Amino acids
  • Peptides
  • Low molecular weight proteins
  • Activate vitamin D
• Low molecular weight proteins may normally be in the urine to some extent, but they are undetectable.
• Otherwise, protein should not normally be detected in the urine.
• Specifically, the PCT will absorb 85% of sodium and water from the glomerular filtrate.
• Additionally, the PCT will absorb 100% of the glucose and amino acids present in the glomerular filtrate.
• The PCT has selective reabsorption of anions, cations, and urea.
• The lateral borders of the proximal convoluted tubule have inter-digitations of lateral cell processes, making the cell limits indistinct.
• The basal surface of the PCT has a folded membrane, forming basal striations.