Blood Vessel I

Blood Vessels Overview

  • Arteries (efferents): Carry blood away from the heart.

  • Veins: Carry blood back towards the heart.

  • Capillaries: Major sites of exchange; connect arteries and veins.

Blood Pressure

  • Blood pressure is highest in arteries near the heart and decreases as blood moves through the system.

Vessel Layers (Tunics)

  • Tunica interna: Inner layer, simple squamous epithelium (endothelium).

  • Tunica media: Middle layer, smooth muscle (controls diameter); arteries have elastic tissue for handling pressure.

  • Tunica externa: Outer layer, anchors vessel, contains nutrient blood vessels (vasa vasorum) and nerves.

Artery vs. Vein

  • Arteries: Thicker walls, higher pressure, elastic, pulsatile.

  • Veins: Thinner walls, less stretchy, collapse when empty, have valves.

  • Capillaries: Only tunica interna (endothelium) for exchange.

Valves in Veins

  • Prevent backflow in low-pressure veins, especially in legs; rely on muscle contraction to move blood.

Arterioles and Venules

  • Arterioles: Small arteries that feed capillary beds; control blood distribution via smooth muscle.

  • Venules: Small veins that receive blood from capillaries, some exchange with tissues.

Capillary Beds

  • Blood flow regulated by arterioles and precapillary sphincters; metarterioles can bypass capillary networks.

Anastomoses

  • Alternative pathways for blood flow; arteriovenous shunts, arterial and venous anastomoses.

Types of Capillaries

  • Continuous: Standard exchange, most tissues.

  • Fenestrated: Increased permeability via fenestrations (pores); kidneys, endocrine tissues, intestines.

  • Sinusoids: Very large gaps for large molecules/cells; bone marrow, liver, spleen.

Blood Distribution

  • Most blood (84%) in systemic circuit; veins act as a reservoir.

Blood Flow Routes

  • Classic: Heart → artery → capillary bed → vein → heart.

  • Portal system: Two capillary beds in series (e.g., hypophyseal portal system).

  • Arteriovenous anastomosis: Blood bypasses capillary bed.

  • Arterial/venous anastomoses: Alternate routes between arteries/veins.

Blood Pressure Regulation

  • Baroreceptors in aortic arch and carotid sinus detect pressure changes.

  • High blood pressure: Parasympathetic activation (slows heart), vasodilation (reduced sympathetic).

  • Hypertension: Sustained high blood pressure (140/90), managed with diet, exercise, stress management, and medication.

Venous Return

  • Skeletal muscle pump: Muscle contraction aids this.

  • Thoracic pump: Breathing affects pressure.

  • Cardiac suction: Ventricle systole creates suction.