Ch 20.Cardiovascular Review – Vessel Structure, Hemodynamics & Fetal Circulation

Blood Vessel Structure

• Three-layered wall (“tunics”)
  • Tunica intima – endothelium + sub-endothelial CT (mentioned indirectly)
  • Tunica media – muscular & elastic layer; "Which one has muscle? — the middle one."
  • Largest component in arteries; determines vasoconstriction/-dilation.
  • In large conducting arteries (aorta, pulmonary trunk) rich in elastic fibers → called elastic (conducting) arteries — “special feature about the largest arteries … allow elastic stretch.”
  • Tunica externa (adventitia) – collagen, vasa vasorum.
• Fenestrated capillaries – “fenestrations = holes” that facilitate exchange (renal glomerulus, endocrine glands, intestinal mucosa).

Capillary Dynamics & Portal Systems

• Portal system – two capillary beds in series before venous return (e.g., hepatic portal system).
  • Liver filters blood → supplies itself → returns to heart.
• Pre-capillary sphincters
  • Ring of smooth muscle at capillary entrance.
  • “Control blood flow into capillaries; open when tissue needs nutrients, close when needs are met.”
  • When closed, blood bypasses capillary bed via metarteriole / thoroughfare channel (pronounced “met-ar-tee-riole”).
• Filtration vs re-absorption across capillary wall
  • At arterial end: P{c} \,(\text{hydrostatic}) > \pi{c} \,(\text{colloid osmotic}) → net filtration of plasma into tissue.
  • At venous end: \pi{c} > P{c} → net re-absorption back to blood.
  • Formal equation: \text{NFP} = (P{c}-P{if}) - (\pi{c}-\pi{if}).
• Edema management
  • Loss of plasma proteins ↓\pi_{c} → swelling.
  • Give IV human albumin to raise oncotic pressure & elevate legs to aid venous return.

Control of Micro-circulation

• Angiogenesis – chronic hypoxia (↓O₂) stimulates new vessel growth to improve perfusion.

Blood Pressure Fundamentals

• Systolic pressure (SBP) – peak during ventricular contraction; mnemonic “systolic = sky/up.”
• Diastolic pressure (DBP) – minimum during ventricular relaxation; mnemonic “diastolic = down/dying.”
• Typical textbook value: 120/80\ \text{mmHg}.
• Two values exist because cuff records pressure while heart contracts and relaxes.

Peripheral Resistance & Blood Viscosity

• Peripheral resistance (PR) = friction that opposes flow.
  • Depends on three variables (slide 74):
  1. Vessel length – longer path, ↑PR.
  2. Blood viscosity – “thickness; dehydration makes thicker.”
  3. Lumen radius – smaller radius ↑PR (recall R \propto 1/r^{4} from Poiseuille).
• Plaque, cholesterol, etc. roughen wall → ↑contact → ↑PR.

Fluid Balance & Hypervolemia

• Excess fluid intake or renal failure → hypervolemia.
  • ↑Volume (not viscosity) elevates BP.
  • IV fluids must be balanced with urine output; watch for lack of diuresis.
• Fluid-electrolyte balance revisited in renal Chapter 25.

Chemoreceptor Regulation & Acid–Base Balance

• Peripheral & central chemoreceptors monitor
  • ↑CO₂, ↓pH (acidic), ↓O₂ → reflex ↑HR, ↑BP, divert blood to lungs.
• CO₂ + H₂O ⟶ H₂CO₃ ⟶ H⁺ + HCO₃⁻ → more CO₂ → lower pH.
• Acid–base terms
  • Respiratory acidosis – hypoventilation, CO₂ retention.
  • Metabolic acidosis – excess acids from GI absorption or renal failure.
• Compensation hierarchy
  • If metabolic problem ⇒ respiratory system increases ventilation.
  • If respiratory problem ⇒ kidneys excrete/retain H⁺ / HCO₃⁻.

Major Arteries, Bifurcations & Organ Supply (diagram references)

• Aorta bifurcates ("bifurcate = split in two") into right & left common iliac arteries supplying legs.
• Named branches (from labeling exercise)
  • Renal artery → kidney.
  • Splenic artery → spleen.
  • Others implied on diagram (celiac trunk, mesenterics, etc.).

Fetal Circulation Peculiarities

• Chambers pump differently; foramen ovale shunts blood right → left atrium (ventricles less active).
• Umbilical vein
  • Carries oxygenated blood toward fetal heart (vein by definition flows to heart).
• Umbilical arteries (pair)
  • Carry deoxygenated blood away from fetus back to placenta.
  • Naming is based on direction, not oxygen content → “arteries/veins switched re: oxygen.”
• Other developmental issues
  • Foramen ovale closure failure; leukocyte & immune problems; insufficient pulmonary surfactant in premature lungs.

Key Clinical / Practical Implications & Examples

• Giving albumin for edema.
• Raising legs for venous return when valves fail.
• Watching IV fluids in anuric (non-urinating) patients.
• Recognizing respiratory vs metabolic acidosis origins for appropriate therapy.

Memory Aids & Classroom Metaphors

• Fenestrations = “holes/hoses.”
• Systolic = sky; diastolic = down/dying.
• Lumen described as the “hole in the needle.”
• “Missy, where’s your AI notes?” – anecdote emphasizing importance of written study aids.