Circulatory System Structure and Function

Structure and Function of the Circulatory System

• The circulatory system's function is to transport blood, carrying oxygen, nutrients, and hormones to the body's tissues and remove waste products.
• Arteries typically supply structures (including bones and joints) they directly contact.
• Arteries are typically situated deep to muscles, providing protection.
• Anastomoses allow alternative pathways for blood flow if a vessel is blocked. This is called collateral circulation.
• Veins typically have valves, which prevent backflow of blood.
• The three layers of a typical vessel are tunica intima, tunica media, and tunica externa.

Learning Outcomes

• Students should be able to describe the structural similarities and differences between arteries and veins.
• Students should be able to link the structure of different types of blood vessels (conducting arteries, distributing arteries, arterioles, metarterioles, capillaries, postcapillary venules, muscular venules and collecting venules) to their function.
• Students should understand the vascular mechanisms that lead to conditions like aneurysms and arteriosclerosis.

Key Arterial Principles

• Arteries typically deliver blood to the structures they supply.
• Arteries are generally deep to muscles, providing protection.
• Anastomoses (connections between blood vessels) provide alternative pathways for blood flow if a vessel is blocked (collateral circulation).

Layers of Vessels

• Vessels have three main layers; the tunica intima (inner layer), tunica media (middle layer), and tunica externa (outer layer).
• The tunica intima is composed of endothelium.
• The tunica media is composed of smooth muscle and elastic tissue.
• The tunica externa is composed of connective tissue.

Types of Arteries

• Large elastic arteries: These have a large amount of elastic tissue in their tunica media and include the aorta. Their elasticity helps maintain a consistent blood pressure.
• Muscular (distributing) arteries: These arteries have a large amount of smooth muscle in their tunica media, allowing for vasoconstriction and vasodilation. Examples include the brachial, gastric arteries.

Arterioles

• Arterioles are small arteries.
• They have a thick tunica media, permitting precise control of blood flow to capillaries.

Metarterioles and Arteriovenous Anastomosis

• Metarterioles are vessels that connect arterioles directly to venules, bypassing the capillaries.
• Arteriovenous anastomosis refers to the network of vessels allowing blood to bypass the capillary system.

Capillary Beds

• Capillaries are the smallest vessels in the body.
• They are thin-walled to facilitate the exchange of materials between blood and tissues.
• Different types of capillaries exist (continuous, fenestrated, and sinusoidal.
• Pressure ulcers can result if pressure on tissue limits blood flow.

Postcapillary Venules

• Postcapillary venules are venules that drain from capillaries.
• They are crucial for lymphocyte migration.

Venous Pressure

• Venous pressure decreases as blood travels from the capillaries to the heart.

Key Venous Principles

• Veins hold a large proportion of the body's blood volume.
• There is more variation in the number and position of veins compared to arteries.
• The accompanying vessels pressure helps blood movement towards the heart (acting as an atriovenous pump.)

Skeletal Muscle Pump

• Contraction of skeletal muscles helps propel blood through veins by squeezing the veins.
• Valves prevent backflow of blood.

Deep and Superficial Veins

• Deep veins are situated within muscles, while superficial veins are close to the skin surface.
• Perforating veins connect superficial and deep veins.

Varicose Veins

• Varicose veins are enlarged, tortuous veins often in the legs.
• Valves in the veins can become weakened or damaged, leading to blood pooling and distension.

Deep Vein Thrombosis

• Deep vein thrombosis (DVT) is a blood clot in a deep vein, often in the leg.
• An embolus can break off and travel to the lungs, causing a pulmonary embolism.

Aneurysm

• An aneurysm is a bulge in a blood vessel wall.
• Types include saccular, fusiform and false aneurysms.

Arteriosclerosis

• Arteriosclerosis is the hardening of the arteries.
• It is a collection of conditions that thicken the arterial walls and reduce elasticity.
• Hypertrophy of smooth muscle within the tunica media and intima.
• Calcification occurs over time.

Atherosclerosis

• Atherosclerosis is a type of arteriosclerosis caused by plaque buildup in the arteries.
• Stages of plaque formation are outlined.