Blood Vessels: Structure, Function, and Circulation

Chapter 18: The Blood Vessels

Introduction to Blood Vessels

Blood vessels are essential components of the cardiovascular system, responsible for transporting blood throughout the body. They form a complex network that ensures the delivery of oxygen, nutrients, and the removal of waste products from tissues.

• Blood vessels include arteries, veins, and capillaries.

• They facilitate systemic and pulmonary circulation.

• Proper vessel function is vital for maintaining homeostasis.

Tunics (Layers) of the Blood Vessel Wall

Structure of Vessel Walls

Blood vessel walls are composed of three distinct layers, or tunics, each with specialized functions and structures. These layers provide strength, flexibility, and control over vessel diameter.

• Tunica intima: The innermost layer, consisting of:

  • Endothelium – a thin layer of simple squamous epithelial cells lining the lumen.

  • Basal lamina – supports the endothelium.

  • Subendothelial connective tissue – provides additional support.

  • Internal elastic lamina – a layer of elastic fibers allowing vessel stretch.

• Tunica media: The middle layer, primarily composed of:

  • Smooth muscle cells – regulate vessel diameter and blood pressure.

  • External elastic lamina – provides elasticity and resilience.

• Tunica externa (adventitia): The outermost layer, made of:

  • Connective tissue – protects and anchors the vessel.

  • Vasa vasorum – small blood vessels that supply the walls of larger vessels.

  • Vasomotor nerves – regulate contraction of smooth muscle.

Example: The aorta, a large artery, has a thick tunica media to withstand high pressure.

Comparing Arteries and Veins

Structural and Functional Differences

Arteries and veins differ in their structure and function, reflecting their roles in the circulatory system. Arteries carry blood away from the heart under high pressure, while veins return blood to the heart under lower pressure.

• Arteries:

  • Thicker tunica media (more smooth muscle and elastic fibers).

  • Narrower lumen compared to veins.

  • Carry oxygenated blood (except pulmonary arteries).

  • Withstand and regulate high pressure.

• Veins:

  • Thinner tunica media, thicker tunica externa.

  • Wider lumen, often collapsed in histological sections.

  • Carry deoxygenated blood (except pulmonary veins).

  • Contain valves to prevent backflow.

Example: The femoral artery has a thick wall to handle high pressure, while the femoral vein has a larger lumen and valves.

Blood Distribution in the Circulation

Relative Blood Volume in Vascular Compartments

Blood is unevenly distributed among the different types of vessels in the body, reflecting their functions in circulation.

• Systemic veins: ~55% of total blood volume (major blood reservoir).

• Systemic arteries: ~10% of total blood volume.

• Pulmonary arteries: ~5% of total blood volume.

• Pulmonary veins: ~16% of total blood volume.

• Heart chambers: ~6% of total blood volume.

Additional info: Veins act as capacitance vessels, storing blood and regulating venous return.

Plaque Formation in Arteries

Atherosclerosis and Vessel Health

Plaque formation, or atherosclerosis, is a pathological process where fatty deposits build up in the arterial walls, leading to reduced blood flow and increased risk of cardiovascular disease.

• Plaques consist of cholesterol, fatty substances, cellular waste, calcium, and fibrin.

• Can lead to narrowing (stenosis) and hardening (arteriosclerosis) of arteries.

• Major risk factor for heart attack and stroke.

Example: Coronary artery disease is caused by plaque buildup in coronary arteries.

Types of Arteries and Veins

Classification Based on Structure and Function

Arteries and veins are classified according to their size, structure, and function within the circulatory system.

• Arteries:

  • Elastic arteries (conducting arteries): Large, with abundant elastic fibers (e.g., aorta).

  • Muscular arteries (distributing arteries): Medium-sized, more smooth muscle (e.g., femoral artery).

  • Arterioles: Smallest arteries, regulate blood flow into capillary beds.

• Veins:

  • Large veins: Superior and inferior vena cava.

  • Medium veins: Named veins of limbs and organs.

  • Venules: Smallest veins, collect blood from capillaries.

Additional info: Venous valves are most abundant in limbs to prevent backflow due to gravity.

Factors that Determine Blood Pressure

Regulation of Arterial Pressure

Blood pressure is the force exerted by circulating blood on the walls of blood vessels. It is determined by several physiological factors.

• Cardiac output (CO): Volume of blood pumped by the heart per minute.

• Peripheral resistance: Resistance to blood flow in vessels, mainly arterioles.

• Blood volume: Total amount of blood in the circulatory system.

• Vessel elasticity: Ability of vessels to expand and contract.

Formula:

$\text{Blood Pressure} = \text{Cardiac Output} \times \text{Peripheral Resistance}$

Circuit Pressure

Pressure Differences in Pulmonary and Systemic Circuits

Blood pressure varies throughout the circulatory system, reflecting the functional demands of each region.

• Pulmonary circuit:

  • Pulmonary arteries: 15 mm Hg

  • Pulmonary veins: 5 mm Hg

• Systemic circuit:

  • Arteries: 120 mm Hg (systolic), 80 mm Hg (diastolic)

  • Arterioles: 80–35 mm Hg

  • Capillaries: 35–15 mm Hg

  • Venules: 15–5 mm Hg

  • Veins: 5–0 mm Hg

Additional info: The steepest pressure drop occurs in arterioles, which regulate flow into capillaries.

Calculating Mean Arterial Pressure (MAP)

MAP Formula and Example

Mean arterial pressure (MAP) is an important indicator of tissue perfusion. It is calculated using systolic and diastolic blood pressures.

• Formula:

$\text{MAP} = \text{Diastolic Pressure} + \frac{1}{3}(\text{Systolic Pressure} - \text{Diastolic Pressure})$

• Example: For a blood pressure of 120/80 mm Hg:

$\text{MAP} = 80\ \text{mm Hg} + \frac{1}{3}(120\ \text{mm Hg} - 80\ \text{mm Hg}) = 93\ \text{mm Hg}$

The Skeletal Muscle Pump

Assisting Venous Return

The skeletal muscle pump is a mechanism that aids the return of blood to the heart, especially from the lower limbs.

• Contraction of skeletal muscles compresses veins, pushing blood toward the heart.

• Venous valves prevent backflow during muscle relaxation.

• Essential for maintaining venous return during standing and exercise.

Effects of the Autonomic Nervous System (ANS) on Blood Pressure

Neural Regulation of Vessel Tone

The ANS regulates blood pressure by controlling the diameter of blood vessels and heart rate.

• Sympathetic stimulation increases heart rate and causes vasoconstriction, raising blood pressure.

• Parasympathetic stimulation decreases heart rate and promotes vasodilation, lowering blood pressure.

• Neurotransmitters involved include norepinephrine (NE) and epinephrine (E).

The Baroreceptor Reflex

Response to Blood Pressure Changes

Baroreceptors are stretch-sensitive receptors located in the carotid sinuses and aortic arch. They help maintain stable blood pressure by initiating reflexes.

• High blood pressure: Baroreceptors increase firing, stimulating the parasympathetic system to lower heart rate and cause vasodilation.

• Low blood pressure: Baroreceptors decrease firing, stimulating the sympathetic system to increase heart rate and cause vasoconstriction.

Additional info: The baroreceptor reflex is rapid and helps protect against sudden changes in posture.

Blood Pressure Maintenance

Homeostatic Mechanisms

Blood pressure is maintained by short-term neural and long-term hormonal mechanisms.

• Short-term: Baroreceptor reflex, chemoreceptor reflexes.

• Long-term: Renin-angiotensin-aldosterone system (RAAS), antidiuretic hormone (ADH), and atrial natriuretic peptide (ANP).

The Structure of a Capillary

Capillary Anatomy

Capillaries are the smallest blood vessels, specialized for exchange of gases, nutrients, and waste products between blood and tissues.

• Composed of a single layer of endothelial cells and a thin basal lamina.

• Diameter just large enough for red blood cells to pass through in single file.

• No tunica media or externa.

Capillary Exchange Mechanisms

Transport Across Capillary Walls

Capillary exchange occurs through several mechanisms, allowing substances to move between blood and interstitial fluid.

• Diffusion: Movement of small molecules (O2, CO2) down their concentration gradients.

• Transcytosis: Transport of larger molecules via vesicles.

• Bulk flow: Movement of water and solutes driven by pressure differences.

Blood Flow through a Capillary Bed

Microcirculation Dynamics

Blood flow through capillary beds is regulated by precapillary sphincters and arterioles, ensuring efficient exchange.

• Precapillary sphincters open and close to control flow.

• Arterioles dilate or constrict in response to tissue needs.

• Capillary beds are sites of nutrient and gas exchange.

Exercising Muscles Get More Blood Flow

Local Regulation of Blood Supply

During exercise, blood flow to active muscles increases due to vasodilation of arterioles and feed arteries.

• Local metabolic factors (e.g., increased CO2, decreased O2) cause vasodilation.

• Ensures adequate delivery of oxygen and nutrients.

Hydrostatic and Osmotic Pressure Gradients

Forces Driving Capillary Exchange

Fluid movement across capillary walls is governed by hydrostatic and osmotic pressures.

• Hydrostatic pressure: The force exerted by fluid pressing against the vessel wall; pushes fluid out of capillaries.

• Osmotic pressure: The force exerted by solutes drawing water into the capillaries.

• Water moves from areas of lower solute concentration to higher solute concentration (osmosis).

Comparison Table:

Pressure Type	Direction of Fluid Movement	Main Effect
Hydrostatic Pressure	Out of capillary	Filtration
Osmotic Pressure	Into capillary	Reabsorption

Net Filtration Pressure (NFP) in Capillaries

Calculating Fluid Movement

Net filtration pressure determines the direction and amount of fluid movement across capillary walls.

• Formula:

$\text{NFP} = (\text{Capillary Hydrostatic Pressure} - \text{Interstitial Hydrostatic Pressure}) - (\text{Capillary Osmotic Pressure} - \text{Interstitial Osmotic Pressure})$

• Positive NFP: Net movement out of capillary (filtration).

• Negative NFP: Net movement into capillary (reabsorption).

Major Systemic Arteries and Veins

Key Vessels in the Body

The systemic circulation includes major arteries and veins that supply and drain blood from all body regions.

• Major arteries: Aorta, carotid, subclavian, brachial, femoral, iliac, etc.

• Major veins: Superior/inferior vena cava, jugular, subclavian, brachial, femoral, iliac, etc.

• Regional arteries and veins are named for the body part they supply or drain.

Additional info: Pulse points are locations where arteries are close to the skin and can be palpated (e.g., radial, carotid).

Blood Vessels of the Head, Neck, Abdomen, and Limbs

Regional Vascular Anatomy

Specific arteries and veins serve the head, neck, abdomen, and limbs, ensuring targeted blood supply and drainage.

• Head and neck: Carotid arteries, jugular veins, dural sinuses.

• Abdomen: Abdominal aorta, hepatic portal vein, renal arteries/veins.

• Upper limb: Subclavian, axillary, brachial, radial, ulnar arteries and veins.

• Lower limb: Femoral, popliteal, tibial, saphenous arteries and veins.

Example: The brachial artery is a common site for measuring blood pressure.