Cardiovascular System: Blood Vessels & Hemodynamics

• L20 presents information about the cardiovascular system.

Learning Outcomes

• Focus is given to blood vessels and hemodynamics.
• Histology, innervation, and function considerations are outlined.
• Differences between arteries and veins are explored.
• Microcirculation and its functions are explained.

Classification of Blood Vessels

• The blood vascular system includes the heart, arteries, veins, and capillaries.
• The heart is a four-chambered muscular organ that pumps blood.
• Each half of the heart contains a receiving chamber called an atrium and a pumping chamber called a ventricle.

Arteries

• Arteries are large blood vessels that carry oxygenated blood away from the heart to distribute it throughout the body.
• Arteries are characterized by thick walls and a smaller lumen.
• Arteries do not have valves.
• Arteries are typically accompanied by veins and nerves, forming a neurovascular bundle.
• Arterioles are minute branches of the arteries and are visible to the naked eye.

Types of Arteries

• Large sized arteries (elastic arteries) are exemplified by the pulmonary trunk, aorta, and its branches like the brachiocephalic, subclavian, and common carotid arteries.
• Medium sized arteries (muscular arteries) include examples like the radial, popliteal, and temporal arteries.
• Smaller arteries are arterioles.

Veins

• Veins are blood vessels that return deoxygenated blood back to the heart.
• Veins are characterized by thinner walls and a larger lumen compared to arteries.
• Valves are present in veins to maintain unidirectional blood flow against gravity.
• Venae cavae, hepatic, renal, uterine, ovarian, cerebral, spinal, pulmonary, and umbilical veins lack valves.
• Venules are smaller veins.

Types of Veins

• Large sized veins include the vena cava and portal vein with their tributaries.
• Medium sized veins exist.
• Small sized veins are venules.

Capillaries and Sinusoids

• Capillaries are networks of microscopic vessels that connect arterioles with venules.
• Capillaries facilitate exchange of nutrients in intimate contact with tissues.
• Sinusoids are large, irregular vascular spaces closely surrounded by organ parenchyma.
• Sinusoids differ from capillaries: their lumen is wider and more irregular, and their walls are thinner and may be incomplete, and are found in the liver, spleen, and bone marrow.

Microscopic Structure of Arteries and Veins

• The endothelial layer is a key component.
• There are three coats (tunics): tunica intima (innermost), tunica media (middle), and tunica adventitia (outermost).

Endothelium

• The endothelium is a special type of epithelium lining the interior surface of blood and lymphatic vessels.
• It forms an interface between circulating blood or lymph in the lumen and the vessel wall.
• It is a simple squamous epithelium type.

Microscopic Structure of Arteries - Tunica Intima

• The tunica intima (innermost layer) consists of four components: endothelium (simple squamous epithelium), basal lamina, subendothelial connective tissue, and internal elastic lamina (made of elastic material).

Microscopic Structure of Arteries - Tunica Media

• The tunica media (intermediate layer) surrounds the tunica intima.
• The tunica media is comprised of elastic fibers and smooth muscles.

Microscopic Structure of Arteries - Tunica Adventitia/Externa

• The tunica adventitia/externa (outermost layer) is mainly composed of connective tissue.
• The tunica adventitia/externa is separated from the tunica media by an external elastic lamina.

Microscopic Structure of Veins

• Veins also have three layers like arteries.
• The three layers are not well defined in veins.

Microscopic Structure of a Large (Elastic) Artery

• Consists of all four components.
• The tunica media is the thickest of all three layers.
• It has high proportion of elastic fibers.
• In between elastic lamellae are layers of smooth muscle cells.
• Tunica adventitia: made up of connective tissues and relatively thinner and contains collagen fibers.

Microscopic Structure of a Medium Sized (Muscular) Artery

• Contains all four components in the tunica intima.
• Internal elastic lamina is more clearly visible.
• Tunica media contains high proportion of smooth muscle cells.
• Tunica adventitia is made of connective tissue, and it is thicker than that of the elastic artery.

Microscopic Structure of a Large Vein

• The tunica intima consists of endothelial cells, subendothelial connective tissues, and a few smooth muscle cells.
• The tunica media consists of smooth muscle cells, collagen fibers, and fibroblasts. This layer is thinner than in arteries. Difficult to distinguish between the two.
• The tunica adventitia is thicker and consists of smooth muscle cells, collagen, elastic fibers, and fibroblasts.

Microscopic Structure of a Medium Sized Vein

• The tunica intima consists of little or no sub endothelial connective tissue.
• The tunica media consists of few layers of smooth muscle cells and collagen fibres and elastic fibers.
• The tunica adventitia is thicker than the media and consists of collagen and elastic fibres.

Differences Between Artery and Vein

• Arteries have a smaller overall diameter and smaller lumen; veins have larger.
• Artery walls are thicker and do not collapse post-death; vein walls are thinner and collapse.
• Blood flow is more rapid in arteries; in veins, it is slower.
• Arteries have a high oxygen content; veins have a low oxygen content.
• The tunica intima is relatively thicker in arteries; in veins, it is relatively thinner.
• In arteries, internal and external elastic laminae are well developed; in veins, they are less developed.
• The tunica media is muscular and considerably thicker in arteries; in veins, it's generally a thin muscular layer.
• The tunica adventitia is about half the thickness of media and has high elastin content in arteries; it's the thickest coat of the wall and composed mainly of collagen fibers in veins.
• Arteries have no valves, whereas veins have valves.

Terms Related to the Arterial Supply and Venous Drainage

• Arterial anastomosis is the communication between arteries or branches of arteries.
• Collateral supply involves circulation through the anastomosis. It's the supply of blood through a network of minor vessels when a major vein or artery is obstructed.
• End arteries lack anastomoses with neighbors. Examples include the central artery of the retina, central branches of cerebral arteries, arteries of the spleen, and kidney.

Deep and Superficial Veins

• Deep veins are located deep within the body, often alongside an artery having the same name (e.g., femoral vein with the femoral artery).
• Superficial veins are closer to the body surface.
• Veins have valves to maintain unidirectional blood flow against gravity.
• Since the venous pressure is low (7mmHg), the valves are invaluable in venous return, preventing backflow.

Arterio-venous Anastomosis (Shunt)

• It is the communication between an artery and a vein.
• Shunts close when an organ is active, allowing blood to circulate through capillaries.
• When an organ is at rest, blood bypasses the capillary bed and is shunted back through the arterio-venous anastomosis.
• Shunt vessels may be straight or coiled, possess a thick muscular coat, and are under sympathetic control.
• Simple structure shunts are in the skin of the nose, lips, erectile tissue of sexual organs, and the thyroid gland.

Vasa Vasorum

• It is the network of small blood vessels supplying large blood vessels.

Lymphoid Tissues

• Lymphoid tissues form a drainage system accessory to the venous system.
• They remove larger particles (proteins and particulate matter) from tissue fluid, functioning as a 'drainage system of coarse type.'
• The venous system acts as a 'drainage system of fine type'.
• The tissue fluid flowing in the lymphatic system is called LYMPH.

Different Lymphoid Tissues in the Body

• Central lymphoid tissue.
• Peripheral lymphoid organs.
• Circulating lymphocytes.
• Lymphatic follicle (nodule).

Central Lymphoid Tissue

• Bone marrow helps differentiate B-lymphocytes, which synthesize antibodies after transformation into plasma cells.
• The thymus helps in the differentiation of immunologically competent T-lymphocytes.

Peripheral Lymphoid Organs

• Lymph nodes filter lymph, removing particulate matter (carbon, bacteria, dust, cancer cells) via phagocytic action, producing lymphocytes, and generating plasma cells which then produce antibodies.
• The spleen filters blood, removing worn-out RBCs, WBCs, and platelets.

Major Lymphatic Ducts

• Filtered lymph is collected into two major trunks, the thoracic duct and right lymphatic duct.

Hemodynamics: Factors Affecting Blood Flow

• Blood flow: volume of blood that flows through any tissue in a specified time, in mL/min.
• Total blood flow: cardiac output (CO), that is the volume of blood that circulates through systemic (or pulmonary) blood vessels per minute.
• CO = heart rate (HR) x stroke volume (SV).
• Distribution of CO depends on:
  • Pressure differences that drive blood through tissue, that flows from higher to lower pressure.
  • Resistance to blood flow in specific blood vessels. A-Higher resistance means smaller blood flow.

Blood Pressure

• Contraction of ventricles generates blood pressure.
• Systolic BP is the highest pressure attained in arteries during systole.
• Diastolic BP is the lowest arterial pressure during diastole.
• Pressure falls progressively with the distance from the left ventricle.
• Blood pressure depends on the total volume of blood.

Vascular Resistance

• Vascular resistance is the opposition to blood flow due to friction.
• Resistance depends on:
• Lumen size: vasoconstriction reduces the lumen.
• Blood viscosity: the ratio of RBCs to plasma and protein increases resistance.
• Total blood vessel length; it is directly proportional to resistance.
• 400 miles of additional blood vessels occur for each 2.2lb of fat.

Venous Return

• Venous return volume of blood flowing back to the heart through systemic veins.
• Venous return occurs due to pressure from left ventricle constriction.
• A small-pressure difference (16 mmHg in venule, 0 mmHg in right ventricle) is enough.

Skeletal Muscle Pump

• The skeletal muscle pump milks blood in 1 direction due to valves
• The respiratory pump is due to pressure changes in thoracic and abdominal cavities