Anatomy Lecture 6 Revision

Use this to revise for anatomy.

The cardiovascular system

It’s made up of the heart and blood vessels and describes the blood transportation network that pumps blood to and from the heart. Blood carries oxygen, nutrients, and waste to and from the cells.

Two muscular pumps of the heart

1) Pulmonary circulation (or circuit) → Where blood goes to be oxygenated. Blood travels from the right ventricle of the heart to the pulmonary arteries to the lungs to be oxygenated and then out from the lungs into the pulmonary veins and back to the heart through to the left atrium.

2) Systemic circulation (or circuit) → Where oxygenated blood goes to supply and nourish the rest of the body. The left ventircle pumps oxygen-rich blood to the rest of the body via systemic arteries, and oxygen-poor blood travels from the systemic veins into the right atrium of the heart, where it goes to be oxygenated through the pulmonary circuit.

The 3 tunics (coats) of most blood vessels (innermost to outermost)

1) Tunica intima → Consists of an inner lining of extremely flattened epithelial cells (endothelium) that is supported by connective tissue. Capillaries only have one coat, and it’s this.

2) Tunica media → Consists of a layer of smooth muscle. This is the most variable coat, as blood vessels are able to be distinguished by the thickness of this layer relative to the lumen.

3) Tunica adventitia → An outer connective tissue layer.

Arteries

Blood vessels that transport high-pressure oxygen-rich blood to the body. They are able to be distinguished from other blood vessels because of a greater presence of elastic fibers in the tunica media, allowing arteries to accommodate higher blood pressure.

Types of artieries

1) Large elastic arteries → These are the arteries that initially receive cardiac output. In comparison to other types, these arteries have the most elastic fibers, allowing them to maintain blood pressure in the arterial systems that connect to the heart as they continue to carry blood to medium muscular arteries.

Ex: The aorta and arteries that branch from it (brachiocephalic trunk, subclavian and carotid arteries, and the pulmonary trunk and arteries).

2) Medium muscular arteries → Their walls consist chiefly of smooth muscle. They possess the ability to vasoconstrict, making their lumen smaller and propelling and circulating blood to other parts of the body, in addition to serving a thermoregulation function.

Ex: Brachial and femoral arteries.

3) Small arteries or arterioles → Are characterized by narrow lumina and thick muscular walls. The degree of filling of capillary beds and blood pressure is regulated by the degree of tonus (firmness) in the arterioles. If the tonus is too high, hypertension is caused.

Arterioles are not named and can be viewed only under magnification.

Anastomoses

Communications between multiple branches of arteries that can serve as a second mode of transport for blood to reach parts of the body distal to the blockage¹ when the main passage is blocked, forming collateral circulations that enlarge these vessels, though this takes time and is often insufficient.

1: Blockages can be caused by the compression of a joint, pathology, or surgical ligation.

Arteries that do not anastomose are called true (anatomic) terminal arteries.

Veins

Blood vessels that return oxygen-poor (deoxygenated) blood to the heart, where it is oxygenated and then circulates through arteries. The walls of the tunica media in veins are thinner than those of arteries because the blood they carry is under lower pressure.

3 sizes of veins (smallest to largest)

1) Venules → The smallest veins, venules, drain capillary beds and join similar vessels to form small veins. They are tributaries of larger veins that unite to form venous plexuses, such as the dorsal venous arch of the foot.

Small veins are unnamed.

2) Medium veins → Drain venous plexuses and accompany medium arteries. They possess venous valves that work to propel blood against gravity to prevent backflow.

Ex: Basilic and cephalic veins of the upper limbs and great and small saphenous veins of the lower limbs, in addition to other medium veins that are named according to the artery they accompany.

3) Large veins → Characterized by wide bundles of smooth muscles and a well-developed tunica adventitia.

Ex: Superior and inferior venae cavae.

The Valsalva maneuver

The expansion of veins during periods of compression or internal pressure (like taking a large breath and holding it). This happens because veins have thin walls and larger lumen diameters than arteries, promoting expansion.

Veins are more abundant than arteries, and around 80% of blood occupies the veins!

Accompanying veins

These veins surround accompanying deep arteries in an irregular branching network, surrounded by a fascial vascular sheath. This branching network forms a countercurrent heat exchanger, with the warmer arterial blood cooling the colder venous blood.

Although veins appear as one unit in illustrations for simplicity, they tend to be intertwined in groups of two or three.

Arteriovenous pump

Because accompanying veins surround deep arteries, the veins are stretched and flattened during contractions of the heart, and as arteries expand, the propulsion of blood is driven toward the heart.

Musculovenous pump

Contractions of skeletal muscle compress veins, “milking” the blood superiorly toward the heart.

Blood capillaries

Small endothelial tubes that form beds that connect venules and arterioles. Oxygen and nutrients exit into the extracellular matrix to supply and nourish cells, allowing exchange of materials with the interstitial (extracellular) fluid.

Hydrostatic pressure with relation to capillaries and Starling hypothesis

Drives blood in the arterioles to be pushed into the capillaries and allows for oxygen, nutrients, and other cellular materials to be pushed out of the capillaries and into the extracellular matrix (ECM). Capillaries are relatively impermeable, except to plasma proteins, which allow carbon dioxide and other waste to be reabsorbed into the bloodstream on the venous end of the capillary bed as a result of osmotic pressure.

Arteriovenular anastomoses (AVAs)

In some parts of the body (like in the fingers), arterioles and venules connect directly without passing through capillaries.

AV shunts play major roles in thermoregulation.

Portal venous systems

In some cases, arterial blood passes through two capillary beds before reaching the venules.

Ex: The hepatic portal system → The portal system in which nutrient-rich blood passes from the capillary beds of the digestive tract into the capillary beds or sinusoids of the liver.

Arteriosclerosis

Describes the group of diseases that cause thickening and loss in elasticity of arterial walls (hardening of the arteries).

Atherosclerosis

A form of arteriosclerosis in which fat (mainly cholesterol) accumulates in arterial walls. The subsequent narrowing of the artery can result in the formation of a thrombus¹, a type of blood clot which may occlude the artery or be flushed down as an embolus² that occludes smaller arteries

1: The process by which a thrombus is formed is called thrombosis.

2: A plug occluding a vessel.

Consequences of atherosclerosis

1) Ischemia → Reduction or loss of blood supply to an organ or region of the body.

2) Infarction → Death, or necrosis, of an area of tissue or organ resulting from ischemia.

Ex: Ischemic heart disease, myocardial infarction (MI), strokes (in the brain), and gangrene (in the distal parts of limbs).

Varicose veins

Abnormally swollen and twisted veins that stem from the loss of elasticity of venous walls. These veins dilate (expand) under the pressure of passing blood and have incompetent valves that do not meet or have been destroyed by inflammation.

These veins also occur in the presence of degenerated deep fascia. Incompetent fascia is incapable of maintaining the pressure of contracting muscles, so the musculovenous pump becomes ineffective as a result.