The Cardiovascular System: The Blood Vessels

Overview of Blood Vessels

The cardiovascular system is composed of a network of blood vessels that transport blood throughout the body, facilitating the exchange of gases, nutrients, and wastes. Blood vessels are essential for maintaining blood flow, blood pressure, and the distribution of chemicals necessary for homeostasis.

• Arteries: Carry blood away from the heart to tissues; branch into smaller vessels as they move away from the heart.

• Capillaries: Microscopic vessels where exchange of gases, nutrients, and wastes occurs between blood and tissues.

• Veins: Carry blood from capillary beds back toward the heart; small veins merge to form larger vessels.

Key Circulatory Patterns:

• Systemic Circulation: Arteries carry oxygenated blood; veins carry oxygen-poor blood.

• Pulmonary Circulation (and fetal umbilical vessels): Arteries carry oxygen-poor blood; veins carry oxygenated blood.

Structure of Blood Vessel Walls

Blood vessels (except the smallest capillaries) have three distinct layers, or tunics, that surround the lumen (central blood-containing space):

• Tunica intima: Innermost layer; consists of endothelium, basal lamina, subendothelial tissue, and internal elastic membrane (in arteries). Provides a smooth surface to reduce friction and supports vessel wall integrity.

• Tunica media: Middle layer; composed mainly of smooth muscle cells and elastic fibers. Responsible for vasoconstriction (narrowing) and vasodilation (widening) of vessels, which regulate blood flow and pressure.

• Tunica externa (adventitia): Outermost layer; made of collagen fibers and contains nerves and lymphatic vessels. Anchors the vessel to surrounding tissues and contains the vasa vasorum (small vessels that nourish the outer layer).

Additional info: The thickness of each layer varies between arteries, veins, and capillaries, reflecting their different functions.

Comparison of Arteries, Veins, and Capillaries

Arteries, veins, and capillaries differ in structure and function, which is reflected in the composition of their walls and their roles in circulation.

Additional info: Arteries have more elastic tissue to accommodate pressure changes; veins have valves to prevent backflow due to lower pressure.

Types of Arteries

Arteries are classified based on their size and function:

• Elastic (Conducting) Arteries: Largest diameter; thick-walled; include the aorta and its major branches. Act as pressure reservoirs, expanding and recoiling as blood is ejected from the heart.

• Muscular (Distributing) Arteries: Distal to elastic arteries; deliver blood to body organs. Have a thick tunica media with more smooth muscle; active in vasoconstriction.

• Arterioles: Smallest arteries; lead to capillary beds. Control flow into capillaries via vasodilation and vasoconstriction.

Types of Capillaries

Capillaries are the smallest blood vessels and are specialized for exchange. There are three main types:

• Continuous Capillaries: Most common; found in skin, muscles, and CNS. Endothelial cells joined by tight junctions; least permeable.

• Fenestrated Capillaries: Have pores (fenestrations) that increase permeability; found in areas of active absorption or filtration (e.g., kidneys, intestines).

• Sinusoidal Capillaries (Sinusoids): Most permeable; allow passage of large molecules and blood cells; found in liver, bone marrow, and spleen.

Capillary Beds and Microcirculation

Capillaries form interweaving networks called capillary beds, which regulate blood flow between arterioles and venules. Microcirculation consists of two types of vessels:

• Vascular shunt (metarteriole-thoroughfare channel): Directly connects terminal arteriole and postcapillary venule.

• True capillaries: Branch off the shunt; regulated by precapillary sphincters (smooth muscle) that control blood flow based on tissue needs.

Structure and Function of Veins

Veins are formed when capillary beds unite to form venules, which then merge into larger veins. Veins have thinner walls, larger lumens, and lower pressure compared to arteries.

• Venules: Smallest veins; receive blood from capillaries.

• Veins: Have thin tunica media and thick tunica externa; act as blood reservoirs (contain up to 65% of blood supply).

• Valves: Prevent backflow of blood, especially in limbs.

• Specialized veins: Coronary sinus (heart) and dural sinuses (brain) are highly specialized, supported by surrounding tissues.

Blood Flow, Blood Pressure, and Resistance

Blood flow, blood pressure, and resistance are interrelated factors that determine how blood circulates through the body.

• Blood Flow (F): Volume of blood flowing through a vessel, organ, or entire circulation per unit time (mL/min). Equivalent to cardiac output (CO) for the entire vascular system.

• Blood Pressure (BP): Force per unit area exerted on vessel wall by blood; measured in mm Hg.

• Resistance (R): Opposition to flow; mainly due to friction between blood and vessel walls (peripheral resistance).

Sources of Resistance:

• Blood viscosity (thickness)

• Blood vessel length

• Blood vessel diameter (most significant)

Relationship:

Blood flow is directly proportional to the pressure gradient and inversely proportional to resistance.

Blood Pressure in Different Vessels

Blood pressure decreases as blood flows from arteries through capillaries to veins. The pressure gradient drives blood flow from high to low pressure areas.

• Arterial Blood Pressure: Systolic (ventricular contraction, ~120 mm Hg), Diastolic (ventricular relaxation, ~80 mm Hg), Pulse pressure (difference between systolic and diastolic), Mean arterial pressure (MAP) calculated as:

• Capillary Blood Pressure: Drops to ~17 mm Hg; low pressure prevents rupture and facilitates exchange.

• Venous Blood Pressure: Very low (~16 mm Hg); aided by muscular pump, respiratory pump, and sympathetic venoconstriction.

Regulation of Blood Pressure

Maintaining adequate blood flow requires cooperation between the heart, blood vessels, kidneys, and the brain. Blood pressure is regulated by:

• Cardiac Output (CO): (Stroke Volume × Heart Rate)

• Peripheral Resistance (PR): Influenced by vessel diameter, blood viscosity, and vessel length.

• Blood Volume: Controlled by kidneys (long-term regulation).

Overall relationship:

Short-term regulation: Neural and hormonal controls alter vessel diameter and cardiac output.

Long-term regulation: Renal mechanisms adjust blood volume (direct and indirect via renin-angiotensin-aldosterone system).

Hypertension and Hypotension

Hypertension: Sustained high blood pressure (≥140/90 mm Hg); often asymptomatic initially but can lead to heart failure, vascular disease, renal failure, and stroke. Classified as:

• Primary (Essential) Hypertension: No identifiable cause; risk factors include genetics, diet, obesity, age, diabetes, stress, and smoking.

• Secondary Hypertension: Due to identifiable disorders (e.g., kidney disease, endocrine disorders).

Hypotension: Low blood pressure (below 90/60 mm Hg); usually not a concern unless it leads to inadequate tissue perfusion. Can be acute (e.g., shock) or chronic (e.g., Addison's disease).

Circulatory Shock: Condition where blood vessels are inadequately filled, leading to poor circulation and tissue damage. Types include hypovolemic (blood/fluid loss), vascular (extreme vasodilation), and cardiogenic (pump failure).

Regulation of Blood Flow Through Tissues

Tissue perfusion is regulated to meet the metabolic needs of tissues, facilitate gas exchange, absorption, and urine formation.

• Intrinsic Controls (Autoregulation): Local changes in vessel diameter in response to metabolic needs (e.g., low O2, high CO2 cause vasodilation).

• Extrinsic Controls: Neural and hormonal mechanisms adjust vessel diameter to maintain systemic blood pressure.

Capillary Exchange Mechanisms

Capillary exchange involves diffusion and bulk flow, allowing nutrients, gases, and wastes to move between blood and tissues.

• Diffusion: Movement of molecules down concentration gradients (O2 and nutrients from blood to tissues; CO2 and wastes from tissues to blood).

• Bulk Flow: Movement of fluid driven by hydrostatic and osmotic pressures; determines relative fluid volumes in blood and interstitial space.

Routes Across Capillary Walls:

• Diffusion through endothelial membranes (lipid-soluble substances)

• Movement through intercellular clefts (water-soluble substances)

• Movement through fenestrations (water-soluble substances)

• Transport via vesicles (large substances)

Hydrostatic and Osmotic Pressures in Capillary Exchange

Fluid movement across capillary walls is determined by the balance of hydrostatic and osmotic pressures:

• Capillary Hydrostatic Pressure (HPc): Pushes fluid out of capillary (arterial end ~35 mm Hg; venous end ~17 mm Hg).

• Interstitial Fluid Hydrostatic Pressure (HPif): Usually negligible (close to zero).

• Capillary Colloid Osmotic Pressure (OPc): Pulls fluid into capillary (~26 mm Hg).

• Interstitial Fluid Osmotic Pressure (OPif): Usually low.

Net Filtration Pressure (NFP): Determines the direction of fluid movement:

Filtration occurs at the arterial end (fluid leaves capillary); reabsorption occurs at the venous end (fluid returns to capillary).

Summary Table: Blood Vessel Types and Functions