Circulatory System Overview

Circulatory System

• Functions: Transports gases between sites of external and internal respiration.
• Also transports: products of digestion, hormones, waste products, cells and chemicals of the immune system, and heat.

Two Vascular Systems

• 1. Cardiovascular system: blood, vessels, and heart.
• 2. Lymphatic system: lymphatic vessels and lymph.

The Lymphatic System

• Lymphatic vessels and lymph.
• Cervical lymph nodes.
• Lymphatics of the mammary gland.
• Cisterna chyli.
• Lumbar lymph nodes.
• Lymphatics of the upper limb.
• Pelvic lymph nodes.
• Lymphatics of the lower limb.
• Inguinal lymph nodes.
• Thoracic duct.
• Thymus.
• Axillary lymph nodes.
• Spleen.
• etc...

Cardiovascular System

• Formed elements of blood:

  • Blood plasma: fluid component.
  • Erythrocytes (red blood cells): carry hemoglobin.
  • Leukocytes (white blood cells): immune response.
  • Thrombocytes (platelets): important for clot formation.
  • Plasma is 95% water, proteins, glucose, clotting agents, electrolytes...

• Do all vertebrates have red blood cells and hemoglobin?: No.

  • 16 species of crocodile icefish do not have red blood cells or hemoglobin.
  • However, they have a larger heart.
  • They have no myoglobin in skeletal muscles.
  • They can live in waters down to 28°F with antifreeze proteins.
  • They also have no swim bladders, low-density bone, and low metabolism.

• Does the blood passing through a shark heart differ from that passing through a dog's (or pigeon's) heart?: Yes.

  • Shark: all low-O2 blood, one stream.
  • Dog: 2 streams (one low-O2, the other high-O2).

• Arteries vs Veins:

  • Arteries: take blood away from heart.
  • Veins: bring blood back to heart.

• Pulmonary arteries (mammals), blood to lungs, low O2.

• Ventral aorta (shark), blood to gills, low O2.

• Blood vessels:

  • Highest blood pressures in arteries, and walls with high elastin fiber content, absorbing sudden energy surges (pulse), and stretching.
  • Elastic recoil moves blood along smoothly.
  • Arteriosclerosis, losing elasticity due to deposits, and potentially causing heart to work harder (unable to recoil) or causing smaller vessels to rupture.

• Microcirculation:

  • Arterioles-capillary beds-venules.
  • Arterioles & Venules: smooth muscle and a bit of elastin.
  • Capillaries: site of exchange.
  • Thin walls, with mostly epithelium, for gas, nutrient, waste, water, ion, and heat transport.

• Controls arterial flow:

  • Vasoconstriction/Vasodilation: contraction/relaxation of smooth muscles to narrow/expand lumen.
  • Capillaries represent huge volume.
  • Danger if all open: shock/trauma = vasodilation, drop in blood pressure.
  • Epinephrine (adrenaline) - vasoconstriction.

• Precapillary sphincters:

  • Regulate flow to capillary beds.
  • Adjust to activity levels.
  • Under nervous + hormonal control.

• Capillary Beds:

  • Overlap, redundancy.
  • Tissues with multiple beds.
  • With sphincters: adjust blood flow to activity level.

• Microcirculation:

  • Blood to active tissues.
  • Thanksgiving: 'food coma'.
  • Running w/o digesting: ischemia.
  • Heat transfer: cooling - blood to skin, e.g., Jackrabbit ears.
  • Ectotherm: adjusting behavior (warming vs cooling).

• Veins:

  • Large volume.
  • Thinner walls.
  • Little elasticity, 'baggy'.
  • Reserve volume: up to 70% of blood may be in veins at one time.

• Veins (blood flow product):

  • One-way valves: prevent backflow.
  • Skeletal muscle: forces blood through system.

• Activity moves more blood from reserve into arterial vessels.

• With long necks:

  • Cardiovascular system adjusts with head movements to maintain flow/pressure.
  • Aortic pressure adjusts with head movements.
  • Pooling prevented likely by vasoconstriction (raised) and vasodilation (lowered).
  • Blood affects kidneys.

• Circulation systems:

  • Single circulation: blood passes through heart once during complete circuit (e.g., shark, perch).
  • Double circulation: blood passes through heart twice during complete circuit (e.g., pigeon, mammals).

• Cardiovascular system:

  • Does blood passing through a shark heart differ from that passing through a dog?: yes
    • Sharks: all low-O2 blood, one stream.
    • Dogs: 2 streams (one low-O2 and one high-O2).

• Renovation:

  • Evolution through modifications of previously existing structures.

• Blood vessels: formed from mesoderm, formed early (48 hours in chick).

• Arterial vessels: follow similar embryonic pathways; diverge through late stages of development.

  • Single ventral aorta from heart.
  • Six pairs of branchial arteries.
  • Paired dorsal aortae.
  • Basic pattern, embryos, sharks, most fish.
  • Each arch with afferent (& efferent) branchial arteries.

• Lymphatic System

  • 2nd part of the circulatory system with its own vessels, tissues, and fluid.
  • 3 functions
    • Recapture fluid lost from cardiovascular system (interstitial fluid).
    • Absorb fats.
    • Remove and destroy harmful invaders.

• Lymph:

  • Mostly water with electrolytes and proteins
  • Capillaries leaky
  • Hydrostatic pressure pushes fluid out.
  • Osmotic pressure helps recapture.
  • 10% remains in the tissues

• Lymphatic vessels:

  • Walls like veins, little muscle, one-way valves.
  • Rely on body movements: breathing, muscle contraction, arteries.
  • Some vertebrates have smooth muscle pumps.

• Lacteals: special vessels around digestive tract to absorb lipids.

• Lymphatic tissue: free cells - leukocytes (Lymphocytes - produce antibodies, and Macrophages - attack foreign bodies), lymph nodes

  • Located along lymph vessels.
  • Fluid percolates through.
  • Lymph nodes swell with infection (bubonic plague).
  • Cancer treatment may involve examination and/or removal of lymph nodes if necessary.

• Venous Systems

  • Veins: Very variable in arrangement within species.
  • Functions
    • Drain body tissues (systemic, jugular)
    • Drain lungs (pulmonary veins)
    • Hepatic portal system (extension from the lungs)
    • Renal portal system (extension from the lungs)
  • Vitelline veins
    • First to form in embryos
    • Drain from yolk, gut, and into your heart.
    • Bring nutrients from yolk.
    • Brings O2 rich blood in amniotes
    • Gets incorporated into the liver and hepatic portal system.
  • Primitive
    • Paired anterior/posterior cardinal veins.
    • Paired common cardinal veins.

• Portal Systems

  • Vascular channel; begins in one set of capillaries, runs to another, without going through the heart.
  • Hepatic
    • All vertebrates
    • Connects digestive tract to liver.
    • Transports nutrients to liver for storage and processing.
    • Transports toxic stuff to be destroyed.
  • Renal
    • All vertebrates except mammals.
    • Connects caudal blood (from tail) to kidney.
    • Subcardinal or renal vein drains kidneys.
    • Processed by kidney.

• Heart

  • Hagfish: unusual in having accessory structures, initially a simple tubular structure, with four chambers in a row.
  • Heart rate under nervous & hormonal control.
  • Starling reflex: greater stretching of cardiac muscle increases contraction
  • Heart beat: contraction intrinsic to cardiac muscle: (intercalated disks connect cells and coordinate contractions).
  • SA node (sinoatrial node): Acts as a pacemaker, near right atrium.
  • Cardiac muscle cells that act neuron-like (AV node, Purkinje fibers)
  • Heart of a Lungfish: modified heart structures to allow switching between gill breathing and lung breathing
  • Heart of amphibians: similar to lungfish, with fully divided atria, undivided ventricle, trabeculae, and a spiral valve.
  • Hearts of birds and mammals: more straight-forward structure. 2 atria and 2 ventricles
  • Apnea (in diving birds and mammals); aerobic metabolism increases, microcirculation alters blood flow to needed organs/tissues.

• Heart of reptiles

  • Complex and plumbing (3 aortae).
  • Accommodates apnea, time periods of non-breathing during submersion, hibernation/aestivation, and quiet periods.
  • Single ventricle but partially divided, with a shunt.
  • Muscular ridge separates cavum venosum and cavum pulmonale.
  • Can tune blood volume to the lungs.
    • Crocodile
      • Ventricles fully divided
      • Foramen of Panizza
        • Connects left and right systemic arches.
        • Shunts high-O2 blood when breathing to body
        • Limited use during apnea.

• Heart Trivia

  • Entire blood goes through heart in one minute
  • Heart could empty a pool in a week
  • Last organ operated on
  • Broken heart (is it real?): Takotsubo Cardiomyopathy; a syndrome that causes your heart to change shape (emotionally stressful situations).

• Heat transfer

  • Microcirculation moves blood to/away from skin.
    • With exercise or high environmental temps, blood shunted to skin for cooling.
  • Rete (adjacent networks of arteries and veins): Acts as heat block with countercurrent flow. Important in aquatic vertebrates (e.g., duck feet, dolphin fins).
  • Pampiniform plexus (sperm production): countercurrent heat exchange.
  • Carotid rete: Keeps brain from overheating. Blood returning from nose/turbinates passes by carotids and cools. (without in rabbits).

• Major shifts in circulation at birth for placental mammals:

  • Umbilical vein: from placenta (high O2 + nutrients)
  • Ductus venosus
  • Foramen ovale
  • Ductus arteriosus
  • Umbilical arteries: to placenta (low O2).
  • Patent Foramen Ovale (PFO):
    • In ~ 25-33% of adults; foramen ovale never closes completely.
    • Pressure largely keeps closed, but some leakage
    • Increases risk of stroke, migraines, decompression illness, high altitude pulmonary edema (once considered unlikely).