Structure and Functions of Blood Vessels

Complete study notes covering the histology of vessel walls, classification of the vascular tree, hemodynamics, capillary exchange mechanisms, and major systemic vessels.

---

2.1 Introduction

Blood vessels form a closed system of tubes that transport blood between the heart and body tissues. There are five main types: arteries, arterioles, capillaries, venules, and veins. Together, they regulate blood flow, blood pressure, and the exchange of substances between blood and tissues.

2.2 General Structure of Blood Vessels

Most blood vessels share a common three-layered (tunica) wall structure:

---

2.3 Arteries

Arteries carry blood away from the heart. They are classified by size and structure into three categories:

2.3.1 Elastic (Conducting) Arteries

• Largest arteries: Aorta, pulmonary trunk, brachiocephalic, common carotid, subclavian, common iliac arteries.
• Structure: Tunica media contains abundant elastic fibers (lamellae) with less smooth muscle.
• Function: Conduct blood from heart to medium-sized arteries; elastic recoil maintains continuous blood flow during diastole (Windkessel effect).

2.3.2 Muscular (Distributing) Arteries

• Medium-sized arteries: Radial, ulnar, femoral, tibial, coronary arteries.
• Structure: Tunica media contains more smooth muscle (40-60% of wall thickness) and less elastic tissue.
• Function: Distribute blood to specific organs and tissues; vasoconstriction and vasodilation regulate blood flow to organs.

2.3.3 Arterioles

• Smallest arteries: 10-100 um diameter.
• Structure: Tunica media has 1-3 layers of smooth muscle cells.
• Function: Major resistance vessels; regulate blood flow into capillary beds; primary site of peripheral vascular resistance.
• Precapillary sphincters: Rings of smooth muscle that control capillary blood flow.

---

2.4 Capillaries

Capillaries are the smallest blood vessels (5-10 um diameter) and form extensive networks (capillary beds) between arterioles and venules. They are the primary site of exchange between blood and tissues.

2.4.1 Structure

• Wall composed of a single layer of endothelial cells on a basement membrane.
• No tunica media or tunica externa.
• Total surface area: Approximately 600 m² (enormous exchange capacity).
• Total length: Approximately 96,000 km if all capillaries were placed end-to-end.

2.4.2 Types of Capillaries

2.4.3 Capillary Exchange Mechanisms

• Diffusion: Primary mechanism for exchange of gases, nutrients, and waste products.
  — Lipid-soluble: (O2, CO2, steroid hormones) pass directly through endothelial membranes.
  — Water-soluble: (glucose, amino acids, ions) pass through intercellular clefts or fenestrae.
• Transcytosis: Transport of large molecules (proteins, hormones) via vesicles (endocytosis + exocytosis).
• Bulk Flow (Filtration and Reabsorption): Driven by hydrostatic and osmotic pressure differences (Starling forces).
  — At arterial end: net filtration (fluid moves out into interstitium).
  — At venous end: net reabsorption (fluid moves back into capillary).
  — Approximately 85% of filtered fluid is reabsorbed; 15% returns via lymphatic vessels.

---

2.5 Veins and Venules

Veins carry blood toward the heart. They have larger lumens and thinner walls than corresponding arteries.

2.5.1 Venules

• Smallest veins: 10-100 um; formed by union of capillaries.
• Postcapillary venules: Major site of leukocyte extravasation during inflammation.
• Tunica media is very thin (1-2 layers of smooth muscle).

2.5.2 Veins

• Medium and large veins have all three tunics, but tunica media is thinner than in arteries.
• Tunica externa is the thickest layer in large veins.
• Luminal diameter is larger than corresponding arteries.
• Blood pressure is low (approximately 10-15 mmHg in vena cava).
• Blood volume capacity is high (approximately 60% of total blood volume at rest).

Venous Return Mechanisms:

1. Skeletal muscle pump: Contraction of surrounding muscles compresses veins, pushing blood toward the heart (one-way valves prevent backflow).
2. Respiratory pump: During inspiration, thoracic pressure decreases and abdominal pressure increases, drawing blood toward the heart.
3. One-way valves: Prevent backflow of blood, especially in lower limbs.
4. Gravity: Aided by upright posture and venous tone.

---

2.6 Comparison of Blood Vessels

---

2.7 Major Blood Vessels of the Body

2.7.1 Aorta and Major Arteries

• Ascending aorta: Gives rise to right and left coronary arteries.
• Aortic arch:
  — Brachiocephalic trunk -> right subclavian + right common carotid
  — Left common carotid artery
  — Left subclavian artery
• Thoracic aorta: Bronchial, esophageal, mediastinal, posterior intercostal arteries.
• Abdominal aorta:
  — Celiac trunk (liver, stomach, spleen)
  — Superior mesenteric artery (small intestine, proximal colon)
  — Renal arteries (kidneys)
  — Inferior mesenteric artery (distal colon, rectum)
  — Common iliac arteries -> external iliac (lower limb) + internal iliac (pelvis)

2.7.2 Major Veins

• Superior vena cava: Drains head, neck, upper limbs, and thorax. Formed by union of right and left brachiocephalic veins.
• Inferior vena cava: Drains abdomen, pelvis, and lower limbs. Formed by union of right and left common iliac veins. Receives hepatic veins, renal veins, gonadal veins.
• Hepatic portal system: Unique system: veins -> capillaries -> veins. Hepatic portal vein carries nutrient-rich blood from GI tract, spleen, pancreas to liver for processing before entering systemic circulation.

---

2.8 Vascular Tone and Regulation

Vascular tone is the baseline contraction of smooth muscle in vessel walls, maintained by sympathetic nervous system activity.

Vasoconstriction:

• Mediated by alpha-1 adrenergic receptors on vascular smooth muscle.
• Increases peripheral resistance and blood pressure.
• Other mediators: endothelin-1, angiotensin II, vasopressin (ADH).

Vasodilation:

• Decreased sympathetic activity; mediated by beta-2 receptors (skeletal muscle).
• Local factors: decreased O2, increased CO2, increased H+, increased K+, increased adenosine, increased temperature (active hyperemia).
• Endothelial factors: nitric oxide (NO), prostacyclin (PGI2).
• Decreases peripheral resistance and blood pressure.

---

Visual Reference Summary

---

SUMMARY — STRUCTURE AND FUNCTIONS OF BLOOD VESSELS

Key Structures:

• All vessels have three layers (tunica intima, media, externa) except capillaries.
• Arteries: thick-walled, high pressure, elastic or muscular types.
• Capillaries: single-layer endothelium; site of exchange (continuous, fenestrated, sinusoidal).
• Veins: thin-walled, large lumen, low pressure, contain valves, capacity reservoirs.

Key Functions:

• Arteries: transport blood away from heart; regulate distribution via vasoconstriction/dilation.
• Capillaries: exchange of gases, nutrients, wastes, fluids between blood and tissues.
• Veins: return blood to heart; act as blood reservoirs; venous return aided by muscle pump, respiratory pump, and valves.