Hyperemia vs. Congestion

• Hyperemia and congestion involve blood flow alterations with distinct mechanisms and clinical significance.

Hyperemia

• An active process where arteriolar dilation boosts blood flow to tissues.
• This is often triggered by increased metabolic activity or inflammation.
• Exercise induces hyperemia in skeletal muscle due to increased oxygen demand.
• Observed in inflammation (e.g., mild burns) and active organs (e.g., the GI tract during digestion).

Congestion

• A passive process resulting from reduced venous outflow, leading to blood pooling in tissues.
• It occurs when venous return is compromised due to obstruction or reduced cardiac output.
• Heart failure causes pulmonary congestion (blood backs up into the lungs) or systemic congestion (blood backs up into the liver and other organs).
• Chronic congestion can lead to tissue damage and fibrosis, such as pulmonary edema from chronic pulmonary congestion.

Examples of Congestion

Pulmonary Congestion (Post-pulmonary Edema)

• Typically occurs in left-sided heart failure; blood backs up into the lungs due to the heart’s inability to pump blood efficiently
• Increased hydrostatic pressure in pulmonary capillaries leads to fluid leakage into alveolar spaces, causing pulmonary edema
• Histological findings include hemosiderin-laden macrophages (heart failure cells) in alveolar spaces, which have phagocytized RBCs
• Can lead to dyspnea, orthopnea, and cough with pink, frothy sputum.

Chronic Passive Congestion of the Liver (Nutmeg Liver)

• Right-sided heart failure impairs venous return from the liver, leading to blood stasis in hepatic sinusoids
• Gross examination reveals a red-brown mottled appearance, resembling nutmeg
• Histologically, blood accumulates in central veins and surrounding areas
• Can impair liver function, leading to ascites, hepatomegaly, and potentially cirrhosis

Hemostasis

• Prevents or limits bleeding while facilitating vascular injury repair.
• Involves several steps to form a clot and restore vascular integrity

Steps in Normal Hemostasis

• Vasoconstriction: Immediate response to blood vessel injury, limiting blood flow via endothelin and thromboxane A2.
• Primary Hemostasis (Platelet Plug Formation):
  • Platelet Activation: Platelets adhere to exposed collagen and are activated by ADP, thromboxane A2, and collagen; they release granules and recruit more platelets.
  • Platelet Aggregation: Platelets stick together via fibrinogen bridges, mediated by glycoprotein (GP) receptors, forming a temporary plug.
• Secondary Hemostasis (Fibrin Clot Formation):
  • Coagulation Cascade: Plasma proteins activate in sequence, leading to thrombin formation.
  • Thrombin converts fibrinogen to fibrin, stabilizing the platelet plug.
• Clot Resolution and Repair:
  • Fibrinolysis: Plasmin dissolves the clot; this restores blood flow.

Defects of Hemostasis

Platelet Defects

• Platelet Dysfunction: Impaired platelet adhesion or aggregation.
  • Aspirin inhibits platelet aggregation by blocking cyclooxygenase (COX).
• Purpura: Small purple spots/bruises under the skin due to microvascular bleeding due to defects

Coagulation Factor Defects

• These defects cause problems with secondary hemostasis
• Hemophilia: Genetic condition with deficiency in coagulation factors (e.g., factor VIII in hemophilia A), leading to hemarthrosis
• Vitamin K Deficiency: Leads to defective clotting and spontaneous bleeding because it is essential for synthesizing coagulation factors.

Thrombosis

• Clot formation within blood vessels; this process can prevent normal blood flow

Types of Thrombosis

• Arterial Thrombosis: Commonly due to atherosclerosis (plaque rupture), leading to thrombus formation.
  • Can result in myocardial infarction (if it obstructs the coronary artery) or stroke (if it obstructs the cerebral artery).
• Venous Thrombosis (Deep Vein Thrombosis - DVT):
  • Typically red thrombi that form in the lower extremities due to blood stasis (e.g., prolonged immobility).
  • Can lead to pulmonary embolism if thrombi embolize to the lungs.
• Postmortem Clots: Non-adherent to vessel walls, jelly-like in appearance, and distinguishably different from premortal clots

Embolism

• Occurs when material travels through the bloodstream and obstructs a vessel.

Types of Embolism

• Venous Embolism:
  • Pulmonary Embolism: Thrombus from the venous system travels to the lungs and blocks the pulmonary artery, causing hypoxia.
  • Often originates from DVT in the legs.
• Arterial Embolism:
  • Thrombus travels from the heart or arteries and embolizes to distant organs.
  • Can lead to stroke (brain), ischemia (limbs), or affect other organs.