BLOOD VESSELS AND CIRCULATION
INTRODUCTION:
The heart pumps blood into vessels that vary in structure, size, and function. There are several types: arteries, arterioles, capillaries, venules, and veins.
ARTERIES AND ARTERIOLES
These blood vessels transport blood away from the heart. They vary in size and have three layers of tissue:
- Tunica adventitia: outer layer of fibrous tissue
- Tunica media: middle layer of smooth muscle and elastic tissue
- Tunica intima: inner lining of squamous epithelium called endothelium
Large arteries like the aorta have more elastic tissue in the tunica media (elastic arteries). Smaller arteries and arterioles have mostly smooth muscle to regulate diameter and blood pressure. Arteries have thicker walls than veins to withstand higher pressure.
ANASTOMOSES AND END-ARTERIES
Anastomoses are arteries that form connections between main arteries supplying an area, providing collateral circulation. End-arteries are arteries that supply a tissue exclusively. Occlusion of an end-artery can cause tissue death.
CAPILLARIES AND SINUSOIDS
Capillaries are the smallest vessels where exchange of substances occurs. They have a single layer of endothelial cells on a thin basement membrane. Certain organs like the liver and bone marrow have wider, leakier capillaries called sinusoids for faster exchange.
CAPILLARY REFILL TIME
Pressing skin causes blanching. Normal refill is < 2 seconds. Prolonged refill indicates poor perfusion or dehydration.
VEINS AND VENULES
Veins return blood at low pressure. Their walls are thinner than arteries. Some veins have valves to prevent backflow, especially in limbs. Veins are capacitance vessels, holding up to two-thirds of body blood and accommodating volume changes.
BLOOD SUPPLY
Outer layers of thick vessels are supplied by vasa vasorum; thin vessels and endothelium receive nutrients by diffusion from blood.
CIRCULATION
1. PULMONARY CIRCULATION
- Blood flows from right ventricle to lungs and back to left atrium.
- Pulmonary arteries carry deoxygenated blood to the lungs; pulmonary veins return oxygenated blood to the heart.
2. SYSTEMIC OR GENERAL CIRCULATION
- Blood pumped from left ventricle travels via the aorta to body and returns through superior and inferior venae cavae.
MAJOR BLOOD VESSELS
AORTA
The aorta is the largest artery. It begins at the left ventricle, arches, and descends through thoracic and abdominal cavities, giving off paired and unpaired branches.
Thoracic aorta
- Ascending aorta: short section, gives rise to coronary arteries.
- Arch of aorta: three branches — brachiocephalic artery, left common carotid, left subclavian.
- Descending thoracic aorta: supplies thoracic organs and walls.
Abdominal aorta
Continuation of thoracic aorta, divides into right and left common iliac arteries at L4 vertebra. Supplies abdominal organs and structures.
VENAE CAVAE
The largest veins return blood to the right atrium. Superior vena cava drains above diaphragm; inferior vena cava drains below.
CORONARY CIRCULATION
The myocardium has its own blood supply. Coronary arteries branch from the ascending aorta and supply the heart. Coronary veins collect deoxygenated blood into the coronary sinus, which empties into the right atrium.
Coronary Arteries
- Left coronary artery: anterior interventricular (LAD) and circumflex branches
- Right coronary artery: posterior interventricular and marginal branches
- Anastomoses provide collateral circulation in the myocardium.
Coronary Veins
- Great cardiac vein, middle cardiac vein, small cardiac vein, and anterior cardiac veins drain into coronary sinus.
- Coronary veins remove waste and carbon dioxide from myocardium.
OXYGEN FREE RADICALS AND REPERFUSION
Reperfusion after blockage can damage heart tissue due to oxygen free radicals. Antioxidants like vitamins E, C, beta-carotene, zinc, and selenium help neutralize these radicals. Drugs are being developed to reduce reperfusion damage.
Detailed Notes:
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