📝 STANDARD DESCRIPTION:
Two parallel circuits driven by the heart:
Pulmonary Circuit (R side):
R atrium → R ventricle → Pulmonary trunk → Pulmonary arteries (carry deoxygenated blood) → Lungs → 4 Pulmonary veins (oxygenated) → L atrium.
Systemic Circuit (L side):
L atrium → L ventricle → AORTA → all body tissues → Vena cavae (cranial + caudal) → R atrium.
The aorta: ascending aorta → aortic arch → thoracic aorta → abdominal aorta → terminal bifurcation (external iliacs).
Lymphatic system = parallel return system from tissues; collects excess interstitial fluid + fats; drains into venous system at thoracic duct.
📚 DETAILED DESCRIPTION:
THE TWO CIRCULATIONS
Mammalian circulation has TWO loops in series, both pumped by the same heart:
1. PULMONARY CIRCUIT (Lesser Circulation) — Right side:
R atrium → R ventricle → Pulmonary trunk → R + L pulmonary arteries → pulmonary capillaries (gas exchange) → 4 pulmonary veins → L atrium.
• Pulmonary arteries are the only arteries carrying DEOXYGENATED blood.
• Pulmonary veins are the only veins carrying OXYGENATED blood.
• Low pressure system (~25/8 mmHg) — thin walls.
2. SYSTEMIC CIRCUIT (Greater Circulation) — Left side:
L atrium → L ventricle → Aorta → arteries → arterioles → capillaries (in tissues) → venules → veins → cranial + caudal venae cavae → R atrium.
• High pressure (~120/80 mmHg in horse and cow).
THE AORTA:
• Ascending aorta — gives off coronary arteries.
• Aortic arch — gives off:
– Brachiocephalic trunk (→ R subclavian + R + L common carotids)
– Left subclavian artery
• Thoracic aorta — descends in chest; gives intercostal & bronchio-oesophageal branches.
• Abdominal aorta — gives celiac, mesenterics, renals, gonadals, lumbars (covered in Abdomen).
• Terminal bifurcation at L6-S1 — into right + left external iliacs (with internal iliacs and median sacral).
VENOUS SYSTEM (key features):
• Veins have valves to prevent backflow (especially in limbs).
• Three pumps drive venous return: skeletal muscle pump, respiratory pump, cardiac suction.
• Most veins have arteries running with them (companion vessels).
FETAL CIRCULATION (key shunts that close at birth):
• Foramen ovale (between atria) → fossa ovalis after birth.
• Ductus arteriosus (pulmonary trunk to aorta) → ligamentum arteriosum.
• Ductus venosus (umbilical vein to caudal vena cava, bypassing liver).
• Umbilical arteries + vein through umbilical cord.
LYMPHATIC SYSTEM:
• Blind-ending capillaries in tissues → lymph vessels (with valves) → lymph nodes → larger trunks → 2 main ducts:
– Thoracic Duct (largest) — drains everything below diaphragm + left thorax + L head/neck → empties at left venous angle (junction of L jugular + L subclavian veins).
– Right Lymphatic Duct — drains R thorax + R head/neck → R venous angle.
• Cisterna chyli — sac at aortic hiatus, origin of thoracic duct.
• Functions: tissue fluid balance, fat absorption (chyle from gut), immune surveillance.
// COMPARATIVE ANALYSIS
| OX |
Rete mirabile at base of brain (carotid rete) — counter-current heat-exchange that protects brain when carotid blood is hot. Same rete in giraffe regulates pressure on rising/lowering head. |
| HORSE |
Largest heart-to-body weight ratio of domestic animals — endurance specialist. |
| BIRD |
Right aortic arch dominant (mammals: left dominant). 4-chambered heart like mammals (unlike reptiles). |
CLINICAL RELEVANCE
Patent Ductus Arteriosus (PDA): Failure of fetal ductus to close — most common congenital heart defect in dogs. Continuous 'machinery' murmur. Patent Foramen Ovale — atrial-level shunt. Lymphangitis: Infection along lymph vessels — visible 'corded' chain.
