Blood Vessel Lecture Notes

Blood Vessels: Lower Body, Abdomen, and Legs

Blood Vessel Structure

• Three layers:

  • Tunica interna (intima): Simple squamous epithelium lining the lumen.

  • Tunica media: Smooth muscle and elastic tissue.

  • Tunica externa (adventitia): Dense irregular connective tissue.

• Arteries have thicker walls than veins due to blood pressure.

• Veins have a thinner tunica media and a wider lumen, which facilitates easier blood flow.

Types of Arteries and Veins

• Arteries:

  • Elastic Arteries: Increase in elastic tissue in the tunica media (e.g., aorta).

  • Muscular Arteries: Increase in smooth muscle, allowing for autonomic control of blood distribution.

    • Blood can be shunted away from organs not in use (e.g., digestive system during fight or flight).

  • Capillaries: Site of gas and nutrient exchange. They are porous and leaky.

• Veins:

  • Venules: Small veins exiting organ systems.

  • Medium-Sized Veins: Drain venules.

  • Large Veins: (e.g., superior and inferior vena cava).

Tissue Makeup of Blood Vessel Layers

• Tunica Interna (Endothelium):

  • Simple squamous epithelium continuous throughout all vessel types.

• Tunica Media:

  • Elastic Arteries: High amount of elastic tissue for expansion and recoil due to high blood pressure.

  • Other Arteries: Less elastic tissue as blood pressure decreases further from the heart.

  • Veins: Little to no elastic tissue.

  • Smooth Muscle: Consistent throughout to control blood flow.

• Capillaries:

  • Only a simple layer of cells for gas exchange.

• Tunica Externa (Adventitia):

  • Dense irregular connective tissue.

Venous Flow

• Veins have lower blood pressure and must work against gravity.

• Factors aiding venous return:

  • Venous Valves: Folds of the intima layer that act as one-way valves to prevent backflow.

  • Skeletal Muscle Pump: Muscle contractions squeeze veins and push blood upward.

  • Abdominal Thoracic Pump (Respiratory Pump): Breathing changes the volume and pressure in the thoracic and abdominal cavities, aiding blood flow.

    • Inhalation: Diaphragm contracts, increasing thoracic volume and decreasing pressure, while decreasing abdominal volume and increasing pressure.

    • Exhalation: Diaphragm relaxes, reversing the pressure gradients.

Types of Capillaries

• Based on the organ's relationship with blood.

  • Continuous Capillaries: Least porous; for gas and nutrient exchange. Found in skin and muscle.

  • Fenestrated Capillaries: More porous due to fenestrations (pores) for fluid exchange. Found in kidneys and small intestines.

  • Sinusoidal Capillaries: Most porous; for cell exchange. Found in bone marrow, liver, and spleen.

Capillary Beds

• Networks of capillaries, not just single vessels.

• Metarterioles: Regulate entry into capillaries via precapillary sphincters.

• True Capillaries: Site of gas exchange.

• Precapillary Sphincters: Circular muscles (tunica media) that control blood flow into capillaries.

  • Open in active organs to supply nutrients.

  • Close to shunt blood away from less active organs (regulated by the autonomic nervous system).

• Vascular Shunt: Always remains open for a base level of nutrient exchange. This ensures there's a minimum supply of nutrients to all parts of the body.

• Variable blood flow within the capillary bed based on organ needs.

Anastomoses

• Networks of blood vessels.

  • Arterial Anastomoses: Networks on the arterial side.

  • Venous Anastomoses: Networks on the venous side.

  • Examples: Papillary plexus and cutaneous plexus in the skin for thermal regulation and nourishment.

  • Cerebral arterial circle to service blood to the brain.

  • Cardiac anastomosis to ensure the heart gets the required oxygen.

• Arteriovenous Anastomoses: Connect arteries to veins, bypassing capillary beds.

  • Lined with precapillary sphincters.

  • Act as detours when capillary beds are shut down.

  • Maintain blood flow even when capillaries are closed.

  • Dynamic systems controlled by smooth muscle in the tunica media.

Blood Pressure and Blood Flow

• Blood pressure comes from the heart (systolic/diastolic).

  • Systolic: Pressure during ventricular contraction.

  • Diastolic: Pressure during ventricular relaxation.

• Blood pressure decreases further from the heart.
  $Blood \; Vessel \; Diameter \; \propto \; Blood \; Flow$

• Larger diameter = less resistance = more blood flow.

• Smaller diameter = more resistance = less blood flow.

• Capillaries have small diameters to slow blood flow for gas exchange.

• Veins increase in diameter to pick up speed, aided by venous valves, skeletal pump, and respiratory pump.

Fetal Circulation

• Bypassing the lungs:

  • Foramen Ovale: Hole in the interatrial septum connecting the right and left atria.

    • Allows blood to bypass the right side of the heart and go directly to the left.

    • Closes after birth to become the fossa ovalis.

  • Ductus Arteriosus: Connects the pulmonary trunk to the aorta.

    • Allows blood to bypass the lungs.

    • Closes after birth to become the ligamentum arteriosum.

• Bypassing the liver:

  • Ductus Venosus: Connects the umbilical vein to the inferior vena cava.

    • Allows blood to bypass the hepatic portal system since the liver is still developing.

    • Closes after birth to become the ligamentum venosum.

Hepatic Portal System

• Splenic vein (from the spleen).

• Gastric vein (from the stomach).

• Inferior mesenteric vein (from the large intestine).

• Superior mesenteric vein (from the small intestine).

• Blood from these veins goes to the liver first for processing before returning to the inferior vena cava.