Vasculature and Blood Pressure microanatomy Physiology and Pathology Summary

Histological Characteristics of Blood Vessels

• Tunica Intima: Comprised of endothelium in all vessel types. Arteries also contain connective tissue and elastic fibers. Veins include valves.
• Tunica Media: Dominated by smooth muscle cells ($SMC$). Arteries ($+++$) have the highest concentration, followed by veins ($++$) and arterioles ($++$). Capillaries lack this layer.
• Tunica Externa: Comprised of connective tissue and vasa vasorum. Veins have the most substantial connective tissue ($+++$) compared to arteries ($++$).

Types of Capillaries and Lymphatics

• Continuous Capillaries: Found in most tissues throughout the body.
• Fenestrated Capillaries: Located in the kidneys.
• Sinusoid Capillaries: Found in the liver, bone marrow, and spleen.
• Lymphatic Vessels: These differ from capillaries by being more permeable and possessing valves to prevent backflow.

Pathogenesis of Atherosclerosis

• Nature of Disease: Atherosclerosis is an inflammatory disease involving endothelial dysfunction and cell migration.
• Cellular Involvement: Includes Monocytes, Macrophages (foam cells), $B\text{-Cells}$, $T\text{-Cells}$, Dendritic cells, and Smooth muscle cells ($SMC$).
• Process of Atherogenesis:
  • Endothelial injury or damage occurs due to risk factors.
  • Adhesion molecules (Selectins, Integrins) promote monocyte and platelet adherence.
  • Monocytes transmigrate across the endothelium and release chemoattractants and growth factors.
  • $SMC$ proliferation and fibroblast production are stimulated.
  • Fat-laden macrophages (foam cells) and lipids deposit in the tunica intima and media.
  • A plaque forms with a collagen cap, bulging into the lumen and restricting blood flow.
• Risk Factors: Male sex over 50, post-menopausal female sex, genetic predisposition, high blood pressure, elevated $LDL$ levels, uncontrolled diabetes, and smoking.

Dynamics of Capillary Exchange

• Capillary Hydrostatic Pressure ($CHP$): The driving force for filtration, pushing fluid out of the capillary. Typically $35\text{ mm Hg}$ at the arterial end and $18\text{ mm Hg}$ at the venous end.
• Blood Colloid Osmotic Pressure ($BCOP$): Caused by suspended blood proteins (primarily serum albumin) that draw water into the capillary. It remains consistent at approximately $25\text{ mm Hg}$.
• Net Filtration Pressure ($NFP$): Calculated as $NFP = CHP - BCOP$.
  • Filtration: Occurs when $CHP > BCOP$ ($NFP = +10\text{ mm Hg}$ at the arterial end).
  • Absorption: Occurs when $BCOP > CHP$ ($NFP = -7\text{ mm Hg}$ at the venous end).
• Homeostatic Deviations: Dehydration increases $BCOP$. Liver failure or a decrease in albumin levels decreases $BCOP$, potentially leading to edema.

Components and Regulation of Blood Pressure

• Arterial Pressures:
  • Systolic Pressure ($SP$): Peak pressure during systole.
  • Diastolic Pressure ($DP$): Minimum pressure at the end of diastole.
  • Pulse Pressure ($PP$): $SP - DP$.
  • Mean Arterial Pressure ($MAP$): An indication of organ perfusion, calculated as $MAP = DP + \frac{PP}{3}$ or $MAP = \frac{2\text{DP} + \text{SP}}{3}$.
• Factors Affecting BP: Peripheral resistance (vessel diameter, blood viscosity, vessel length), Cardiac Output ($CO$), Blood Volume, and Vessel Compliance ($\text{Compliance} = \frac{dV}{dP}$).
• Hypertension Categories:
  • Elevated: $SP = 120-129\text{ mm Hg}$ and $DP < 80\text{ mm Hg}$.
  • Stage 1: $SP = 130-139\text{ mm Hg}$ or $DP = 80-89\text{ mm Hg}$.
  • Hypertensive Crisis: $SP > 180\text{ mm Hg}$ and/or $DP > 120\text{ mm Hg}$.

Neural and Hormonal Control Mechanisms

• Vasomotor Center: Located in the medulla; transmits sympathetic impulses to innervate $SMC$ of arterioles.
• Baroreceptor Reflex: Mechano/stretch receptors in the carotid sinus and aortic arch. A decrease in stretch triggers a compensatory sympathetic response to increase heart rate and vasoconstriction.
• Chemoreceptor Reflex: Responds to decreases in $PO_2$ and $pH$ or increases in $PCO_2$. Central chemoreceptors in the medulla are primarily sensitive to $PCO_2$.
• Renin-Angiotensin-Aldosterone System ($RAAS$): Drop in blood pressure triggers the kidneys to release Renin, converting Angiotensinogen to Angiotensin I, which $ACE$ converts to Angiotensin II. This results in vasoconstriction, thirst, and Aldosterone secretion (salt/water retention).
• Hormonal Targets:
  • Antidiuretic Hormone ($ADH$): Stimulated by high osmolarity, targets kidney collecting tubules.
  • Atrial Natriuretic Peptide ($ANP$): Released by the right atrium in response to high blood volume to trigger arteriole dilation in kidneys.

Questions & Discussion

• Which ion channels are active in the plateau phase of myocardial action potentials? Calcium channels open, some potassium channels close while others remain open, and sodium channels are closed.
• What causes stroke volume to increase? A decrease in afterload will increase stroke volume.
• Which factor causes pulmonary edema in left-sided heart failure? Hydrostatic pressure in vessels increases, backing up blood into pulmonary vessels.
• Expected findings for a patient with smoking history, barrel-shaped chest, and high blood pressure ($159/83\text{ mm Hg}$)? Hypercapnia and lower blood pH due to respiratory compromise.
• What is expected in a patient who is unconscious and pale with blood pressure $80/60\text{ mm Hg}$ after a gunshot wound? Compensatory baroreceptor activation seeking to increase heart rate and peripheral resistance.