wk 9 - hoof physiology

what is vasculature?

the intricate network of blood vessels that distribute blood throughout the body

what does the axillary artery do?

is the primary artery that supplies blood to the thoracic limb

what is the vasculature of horse limbs?

axillary artery → median artery → medial & lateral palmar digital arteries

explain the vasculature of horse limbs in detail

1. axillary artery turns into median artery which passes through carpal canal

2. at proximal to fetlock (above fetlock), the median artery branches into 2 arteries

   • medial & lateral palmar digital arteries

what is something to note about the vasculature of a horse’s limb?

• pressure changes in leg as blood moves through different foot structures

  • provides cushioning & energy dissipating effects

• there are more veins than arteries in the foot

  • all vessels are highly sensitive to vasoactive substances

explain the several valveless venous plexus systems in the hoof

• network of valveless veins

  • is basically a 2nd heart mechanism

  • relies on weight-bearing & movement to pump blood back up leg

explain how the horse’s vasculature system is categorized

• split into 3 primary plexuses that form an interconnected network

  • palmar/plantar plexus

  • coronary venous plexus

  • dorsal (laminal) venous plexus

where is the palmar/plantar venous plexus located?

deep in sole corium & on inner surface of collateral cartilages

where is the coronary venous plexus located?

situated in coronary cushion covering digital extensor tendon & lateral cartilages

where is the dorsal venous plexus located?

in lamellar corium

where do the plexus’ drain?

into digital veins

what do the laminar & coronary dermis contain?

contain arteriovenous anastomoses that’s part of pressure regulating & thermoregulatory mechanisms within foot

what is the arteriovenous anastomoses?

a system of vascular shunts that connect small arteries directly to veins bypassing capillary bed

where is the arteriovenous anastomoses located?

in coronary band, dermis, & papillae

what is the dysfunction of arteriovenous anastomoses linked to?

laminitis

explain how the dysfunction of arteriovenous anastomoses is linked to laminitis?

• in acute cases of laminitis, these shut open extensively

  • leading to run off of blood

  • causes high pressure shunting, bounding pulse, & starvation of oxygen to laminae

what does the arteriovenous anastomoses do?

• manage heat within the hoof

• responsible for blood flow or dysfunction in hoof

how does the horses’ system in lower leg work if their veins lack valves that prevent backflow?

relies on compression to send blood back to the heart

what happens during compression?

• when horse steps down

  • pressure causes digital cushion & surrounding tissues to compress venous plexus

what happens during pump action?

compression forces venous blood out of plexuses up through digital veins towards heart

what happens during filling?

• when foot lifts

• pressure releases → veins open → fill w blood via arterial pulse & gravity

what happens during concussion absorption?

hydraulic cushion created by the filled veins dissipates the concussion of landing

what is something to note about stall bound horses or non-weight bearing hoses?

• plexus system in lower legs depend on movement

  • stall bound horses or non-weight bearing horses prevent this mechanism from working

    • leads to poor circulation & edema

what is venography?

• contrast enhanced imaging

  • used to visualize the plexus for diagnosing conditions like laminitis

    • where vascular pattern in hoof is disrupted

why is it important that the plexus’ is valveless?

• allows blood to take various routes out of hoof

  • crucial for maintaining flow under diff weight-bearing pressures

what do the palmar digital nerves contain?

sensory & autonomic fibers

what do unmyelinated nerves contain?

approx 75% of sensory & 25% sympathetic fibers

what are the digital vessels innervated by?

autonomic vasomotor nerves

what are autonomic vasomotor nerves?

• components of SNS

  • control blood vessel diameter, regulate blood pressure, & blood flow distribution

what is something to note about myelinated fibers?

less numerous & convey info more rapidly to spinal cord