3.2 Circulation: Resistance, Venous Return, Coronary Flow, Microcirculation & Fetal Circulation

Why this area matters

The TOS lists "venous return, coronary circulation, systemic resistance and blood pressure regulation, microcirculation and lymphatics and fetal and neonatal circulation". These topics explain why fluids help a patient in shock, why tachycardia is dangerous in coronary disease, why a filariasis patient's leg swells, and why a newborn's ductus closes. They also set up hypertension and heart failure in Medicine, and congenital heart disease in Pediatrics.

Vascular Resistance, Hemodynamics & Autonomic Control

Poiseuille's Law & Vascular Resistance

Fluid flow through the cylindrical systemic circulation is described by Poiseuille's equation:

$R = \frac{8\eta L}{\pi r^4} \quad \text{and} \quad Q = \frac{\Delta P \cdot \pi r^4}{8\eta L}$

Where $R$ is vascular resistance, $\eta$ is blood viscosity (primarily determined by hematocrit), $L$ is vessel length, and $r$ is vascular internal radius. Resistance is inversely proportional to the fourth power of the radius ($r^4$):

• Halving the arteriolar radius ($r / 2$) increases local vascular resistance by $2^4 = 16\text{-fold}$, reducing flow to $1/16$th at constant pressure.
• Arterioles possess the thickest muscular media relative to lumen diameter, accounting for roughly 50% to 70% of total peripheral resistance (TPR). Arteriolar vasoconstriction and vasodilation serve as the primary physiologic throttle for blood pressure regulation.

Mean Arterial Pressure (MAP)

$\text{MAP} \approx \text{Diastolic BP} + \frac{1}{3}(\text{Systolic BP} - \text{Diastolic BP}) = \text{CO} \times \text{TPR} + \text{CVP}$

Where Pulse Pressure is $\text{SBP} - \text{DBP}$. Central venous pressure (CVP) is normally near $0\text{--}4\text{ mmHg}$, so $\text{MAP} \approx \text{CO} \times \text{TPR}$.

The Arterial Baroreceptor Reflex

1. Receptors & Innervation:
   • Carotid Sinus: Located at the bifurcation of the common carotid artery; transmits afferents via the sinus nerve of Hering into the Glossopharyngeal Nerve (CN IX). Highly sensitive to both increases and decreases in blood pressure.
   • Aortic Arch: Receptors embedded in the aortic wall transmit afferents via the Vagus Nerve (CN X). Primarily responsive to acute elevations in arterial pressure.
2. Central Integration:
   • Afferent signals terminate in the Nucleus Tractus Solitarius (NTS) in the dorsal medulla.
   • The NTS excites the caudal ventrolateral medulla (CVLM), which tonically inhibits the Rostral Ventrolateral Medulla (RVLM)—the primary sympathetic preganglionic drive center.
   • The NTS also directly excites the nucleus ambiguus and dorsal motor vagal nucleus, driving parasympathetic efferent outflow.
3. Orthostatic Response:
   • Sudden standing pools 500 to 1,000 mL of blood in lower extremity capacitance veins, decreasing venous return, EDV, stroke volume, and MAP.
   • Reduced carotid sinus stretch decreases baroreceptor firing to the NTS.
   • Reduced NTS stimulation disinhibits the RVLM and withdraws vagal tone. Sympathetic discharge surges: alpha-1 mediated arteriolar vasoconstriction (raising TPR), alpha-1 mediated venoconstriction (restoring venous return), and beta-1 mediated tachycardia and inotropy. MAP is normalized within seconds.
4. Cushing's Reflex (Cushing's Triad):
   • Massive increase in intracranial pressure (ICP) compresses cerebral microvessels, producing severe brainstem ischemia.
   • Local ischemic accumulation of $CO_2$ and lactic acid stimulates an extreme sympathetic surge from the RVLM, driving severe arterial hypertension (systolic BP often $> 200\text{ mmHg}$) to restore cerebral perfusion pressure.
   • Carotid and aortic baroreceptors sense this extreme hypertension and trigger profound reflex bradycardia via the vagus nerve.
   • Brainstem compression impairs respiratory pacemakers, causing irregular, ataxic respirations.

Venous return

Venous return equals cardiac output in a steady state. It is driven by the gradient between the mean systemic filling pressure (about 7 mmHg) and right atrial pressure, divided by the resistance to venous return:

$$\text{Venous return} = \frac{P_{\text{msf}} - P_{\text{RA}}}{R_{\text{VR}}}$$

Venous return rises with:

• transfusion or venoconstriction, which increase mean systemic filling pressure;
• the skeletal muscle pump;
• the respiratory pump, because inspiration lowers intrathoracic pressure.

It falls with hemorrhage, venodilation (nitrates, spinal anesthesia) and high right atrial pressure (tamponade, right heart failure). Where the cardiac function curve meets the venous return curve gives the operating cardiac output and right atrial pressure.

Coronary circulation

The left coronary artery (left anterior descending and circumflex) and the right coronary artery supply the myocardium. Whichever artery gives the posterior descending branch defines dominance; the right is dominant in about 80% of people. Flow is regulated by local metabolism:

• Left ventricular flow occurs mainly in diastole, because systolic contraction compresses intramural vessels. Tachycardia shortens diastole and reduces subendocardial perfusion.
• Resting oxygen extraction is already about 70% to 75%, so higher demand must be met by vasodilation. Adenosine, nitric oxide, CO2, H+ and hypoxia all dilate coronary vessels.
• The subendocardium is most vulnerable to ischemia, which is why demand ischemia first causes ST depression.

Microcirculation, Starling forces and lymphatics

Fluid movement across a capillary follows the Starling equation:

$$J_v = K_f\left[(P_c - P_i) - \sigma(\pi_c - \pi_i)\right]$$

• Capillary hydrostatic pressure is about 30 mmHg at the arterial end and 10 mmHg at the venous end.
• Plasma oncotic pressure is about 28 mmHg and comes mainly from albumin.
• There is a small net filtration. The lymphatics return the excess, roughly 2 to 3 L per day, together with filtered protein.

Long-term blood-pressure regulation

Pressure is controlled at several speeds:

• Seconds: the baroreceptor reflex (above), chemoreceptor reflexes, and the CNS ischemic response (the Cushing reflex).
• Minutes to hours:
  • Renin is released by juxtaglomerular cells when renal perfusion falls, when the macula densa senses low NaCl, or when beta-1 sympathetic nerves fire.
  • Renin converts angiotensinogen to angiotensin I, and ACE (mainly in the lungs) forms angiotensin II.
  • Angiotensin II constricts arterioles (including the efferent glomerular arteriole), releases aldosterone and ADH, and stimulates thirst.
  • Atrial and B-type natriuretic peptides oppose this system when the heart is stretched.
• Days and longer: the kidney's pressure natriuresis adjusts sodium and water excretion until arterial pressure returns to its set point. This renal-body fluid feedback is the dominant long-term controller, which is why most sustained hypertension involves the kidney.

Fetal and neonatal circulation

Oxygenated blood from the placenta (PO2 about 30 to 35 mmHg, around 80% saturated) travels in the umbilical vein. That makes it the most oxygenated fetal blood.

1. About half bypasses the liver through the ductus venosus into the inferior vena cava.
2. In the right atrium, this stream is directed through the foramen ovale into the left atrium, left ventricle and ascending aorta, so it supplies the brain and heart.
3. Superior vena cava blood enters the right ventricle and pulmonary trunk. Because fetal pulmonary vascular resistance is high, most of it passes through the ductus arteriosus into the descending aorta.
4. Two umbilical arteries return deoxygenated blood to the placenta.

At birth:

• Lung expansion and rising PO2 drop pulmonary resistance.
• Clamping the cord raises systemic resistance.
• Left atrial pressure now exceeds right atrial pressure, which closes the foramen ovale functionally.
• Higher oxygen and falling prostaglandin E2 constrict the ductus arteriosus within the first day or so; anatomic closure takes weeks.

Indomethacin or ibuprofen helps close a patent ductus arteriosus in a preterm infant. Prostaglandin E1 keeps the ductus open in ductal-dependent lesions until surgery.