Blood Vessels Overview

Blood Vessels Overview

• Blood vessels are categorized into arteries, capillaries, and veins.
• Arteries carry oxygenated blood (except pulmonary and umbilical) away from the heart.
• Capillaries are exchange vessels for gas, nutrient, and waste exchange with tissues.
• Veins carry deoxygenated blood (except pulmonary and umbilical) towards the heart.
• All blood vessels (except capillaries) have three layers called tunics.
  • Tunica intima: Innermost layer, made of endothelium for smooth blood flow.
  • Tunica media: Middle layer, composed of smooth muscle for controlling vasoconstriction/vasodilation.
  • Tunica externa: Outer layer, made of collagen fibers; anchors the vessel.

Arteries

• Elastic arteries (e.g., aorta) are large and contain elastin for maintaining continuous blood flow.
• Muscular arteries are smaller and deliver blood to specific organs via active vasoconstriction.
• Arterioles are the smallest arteries and regulate blood flow into capillaries by controlling resistance and blood pressure.

Capillaries

• Capillaries are thin-walled (endothelium and basement membrane), enabling efficient exchange.
• Continuous capillaries are the least permeable, found in tissues like skin and muscles.
• Fenestrated capillaries have pores for increased permeability, found in organs needing high filtration (e.g., kidneys).
• Sinusoidal capillaries are the most permeable, having large gaps for bulk exchange, found in tissues like the liver and bone marrow.
• Capillary beds are networks of capillaries between arterioles and venules, with blood flow controlled by precapillary sphincters.

Veins

• Veins have thin walls and large lumens; they act as blood reservoirs, storing approximately 65% of the blood.
• Venous valves prevent backflow, especially in limbs.
• Clinical Notes:
  • Varicose veins are dilated veins due to incompetent valves.
  • Deep vein thrombosis are clots in deep veins, leading to possible complications like pulmonary embolism (a clot traveling to the lungs).

Blood Flow, Pressure, and Resistance

• Blood flow (F) is the volume of blood flowing through a vessel.
• Blood pressure (BP) is the force exerted by blood on vessel walls.
• Resistance (R) is the opposition to flow, significantly influenced by vessel diameter.
• Flow is directly proportional to the pressure gradient and inversely proportional to resistance (F = ΔP/R).

Systemic Blood Pressure

• Arterial blood pressure:
  • Systolic pressure (approx. 120 mmHg) – highest pressure during ventricular contraction.
  • Diastolic pressure (approx. 80 mmHg) – lowest pressure during relaxation.
  • Pulse pressure is the difference between systolic and diastolic pressure.
• Capillary blood pressure is low to prevent rupture and facilitate efficient exchange.
• Venous blood pressure is low but aided by the muscular and respiratory pumps.

Blood Pressure Regulation

• Short-term regulation:
  • Neural controls: Baroreceptors (responding to stretch) and chemoreceptors (responding to O₂, CO₂, and pH) affect blood pressure.
  • Hormonal controls: Epinephrine and norepinephrine increase blood pressure, while atrial natriuretic peptide (ANP) decreases it.
• Long-term regulation: Renal mechanisms adjust blood volume via urine output and the renin-angiotensin-aldosterone system.

Tissue Perfusion and Blood Flow

• Tissue perfusion delivers oxygen/nutrients and removes waste, adjusted based on tissue needs (e.g., increased blood to muscles during exercise).
• Autoregulation:
  • Metabolic controls: Local chemical changes (O₂ and CO₂) affect blood flow.
  • Myogenic controls: Vascular smooth muscle responds to pressure changes.

Capillary Exchange

• Bulk flow across capillaries depends on:
  • Hydrostatic pressure (HP): Pushes fluid out of capillaries.
  • Colloid osmotic pressure (OP): Pulls fluid into capillaries.
  • Net filtration pressure (NFP): Positive at the arterial end for filtration, negative at the venous end for reabsorption.

Clinical Notes

• Edema is caused by increased hydrostatic pressure or decreased colloid osmotic pressure, resulting in fluid buildup in interstitial spaces.
• Atherosclerosis is plaque buildup in arteries, causing hypertension, heart attacks, and stroke.
• Aneurysms are weakened and ballooning arterial walls, risking rupture.

Developmental Aspects and Circulatory Pathways

• Fetal circulation:
  • Foramen ovale connects atria, bypassing non-functional lungs.
  • Ductus arteriosus connects pulmonary trunk to aorta.
• Vascular aging: Increased risk of varicose veins and atherosclerosis.

This quiz provides a comprehensive overview of blood vessels, detailing their categories: arteries, capillaries, and veins. Learn about the structure of each type, including the three tunics found in blood vessels and their specific functions in circulation.