chapter 19: blood vessels

Lumen

The inside space, cavity, or channel within a tubular structure like blood vessels or organs

Vein and artery in neck

External jugular : vein

Common carotid : artery

Veins in arms

1. Cephalon vein

2. Basilic vein

3. Median cubital vein

Arteries in arms

1. Brachial

2. Radial

3. Ulnar

Arteries in the abdomen

Mesentric arteries

Arteries in the leg

1. Femoral

2. Poplitel

3. Posterior tibial

Veins in the leg

1. Femoral

2. Great saphenous

Layers of blood vessels

1. Intima

2. Media

3.Externa

Tunica Intima

The innermost layer of a vessel wall

• Composition: Endothelium (simple squamous epithelium)

• Functions: selectively permeable barrier, secretes chemicals, repels blood cells and platelets, and inflammation

• only layer containing capillaries

Tunica Media

• The middle layer of a vessel wall

• Composition: Smooth muscle, elastic tissue, and collagen (amount varies)

• Functions: Strengthens vessels to prevent rupture and facilitates vasomotion (vasoconstriction and vasodilation)

Tunica Externa

The outermost layer of a vessel wall

• Composition: Loose connective tissue

• Functions: Protects and anchors the vessel; provides a passage for nerves and lymphatic vessels

Vaso Vasorum

"blood vessel of blood vessels"; small vessels found in the tunica externa that supply larger blood vessels

Arteries

Vessels designed to withstand surges of pressure that carry blood away from the heart.

- thicker walls and more smooth muscle than veins

Elastic Arteries

largest diameter arteries (e.g., aorta, pulmonary trunk)

• Composition: Elastic tunica media

• Functions: Reduce the effects of blood pressure surges by expanding during systole and recoiling during diastole

Muscular Arteries

Thick-walled arteries (e.g., brachial artery, femoral artery).

• Composition: Thick smooth muscle

• Functions: Deliver blood to specific organs and adjust flow based on demand

Arterioles (arteries)

Small vessels that lead into capillary beds

• Composition: Variable smooth muscle layers and little elastic tissue

• Functions: Primary controllers of blood flow; most responsible for peripheral resistance (PR) and significantly affecting blood pressure

Capillaries (Exchange Vessels)

Microscopic connections between arteries and veins

• consisting of a single layer of endothelial cells and a sparse basal lamina

• Their diameter allows only a single RBC to pass at a time to slow velocity for exchange

Veins (Capacitance Vessels)

Vessels that carry blood towards the heart

• They have thinner walls, larger lumens, and often contain valves to prevent backflow

• At rest, they act as blood reservoirs, containing about 60% of the body's blood volume

Anastomoses

Branching of vessels that provide collateral (alternative) pathways for blood flow

Cerebral Arterial Circle (Circle of Willis)

A ring or pentagon-shaped structure of arteries at the base of the brain where the internal carotid and vertebral artery pathways connect

Continuous Capillaries

cells are tightly held together with intercellular clefts that allow small molecules to pass while holding back large molecules

• Found in most tissues

• May contain pericytes

Fenestrated Capillaries

Contain fenestrations (pores) for greater permeability of water-soluble substances (no formed elements)

• kidneys and endocrine glands

Sinusoids

Irregular, blood-filled spaces with large fenestrations and an incomplete basement membrane, allowing formed elements (like blood cells) to pass

• Found in the liver, bone marrow, and spleen

Venules

Include post-capillary venules (extremely porous) and muscular venules (thin tunica media and externa)

Post-capillary venules

Smallest venous vessels that collect blood directly from capillary beds

• endothelial layer and basement membrane with scattered pericytes, lacking smooth muscle

• Extremely porous (high permeability)

Muscular Venules

Small blood vessels that collect blood from capillaries and transition into larger veins

• thin tunic media and externa, endothelium of simple squamous cells

• Follow postcapillary venules

Medium veins

All three tunics, thick media, large lumen, valves

Venous Sinuses

Veins with thin walls, large lumens, and no smooth muscle, meaning they cannot perform vasoconstriction (e.g., coronary sinus)

Large Veins

Vessels with all three layers and a thick tunica externa, but no valves

Blood Flow (ml/min)

The volume of blood moving through a tissue in a given time; equivalent to cardiac output

Blood Pressure (mmHg)

The force blood exerts against a vessel wall. Clinically, this refers to systemic arterial pressure (e.g., 120/80)

Blood Perfusion (ml/min/g)

The specific flow of blood through the capillary beds

• acts as an "on/off switch" for nutrient exchange

Resistance

Friction encountered by blood through vessels

• which opposes flow

Blood Viscosity

The "thickness" of blood

• directly proportional to resistance and oppositely proportional to flow

• decreases with anemia

• increases with blood doping or dehydration

Blood Vessel Length

The distance blood must travel

• directly proportional to resistance and oppositely proportional to flow

Blood Vessel Diameter

The most influential factor for resistance and friction

• controlled by the Sympathetic Nervous System (SNS)

Laminar Flow

Normal, smooth, silent, and efficient blood flow

Turbulent Flow

Non-smooth flow caused by obstructions like plaques, heart murmurs, or valve stenosis

Pulse Pressure (PP)

Calculated as SBP – DBP

• measures pressure surges and indicates the force of each heartbeat and artery health (average is 40)

Mean Arterial Pressure (MAP)

average of pressures throughout the cardiac cycle: calculated as DBP + 1/3 (PP). It is the most important index of perfusion; it must be 60 or higher for organs to receive enough blood

Skeletal Muscle Pump

mechanical venous return mechanism where contracting muscles (like the calves/gastrocnemius, the "second heart") press against veins to propel blood toward the heart

Respiratory Pump

mechanical venous return mechanism where deep breathing changes pressure in the abdomen and thorax to squeeze blood toward the heart

Sympathetic Vasoconstriction

neural (involuntary) mechanism where the sympathetic nervous system signals smooth muscle in veins to constrict to help return blood to the heart

Medulla Oblongata

brainstem's "autopilot" for the cardiovascular system

• containing the Vasomotor Center (controls diameter) and the Cardioacceleratory/inhibitory centers (regulate heart rate)

Vasomotor Center

Controls vessel diameter vis sympathetic NS

Cardioacceleratory/inhibitory Center

Regulates heart rate using sympathies NS

Baroreflex

negative feedback mechanism using pressure-sensitive mechanoreceptors in the carotid sinuses and aortic arch to reduce HR, CO, and BP during acute surges (e.g., standing up quickly)

Chemoreflex

response to chemical changes sensed by receptors (↓pH, ↓O2, ↑CO2)

• primary role is adjusting respiration

• secondary role is using vasomotor changes to increase perfusion

Vasoconstriction

Vessels diameter narrows

Vasodilation

Vessels diameter widen

Vasomotor changes

Changes in diameter of blood vessels

Diffusion

primary mode of exchange where substances move from high to low concentration

• Small solutes (O2) diffuse through endothelial cells

• large solutes (proteins) pass through fenestrations or gaps

Transcytosis

form of vesicular transport used to move specific macromolecules across the endothelial cell

Pinocytosis

non-specific form of transcytosis for "in-between" sized molecules

Exocytosis

final step of vesicular transport where a vesicle fuses with the cell membrane to release contents into the interstitial fluid or bloodstream

Modes of capillary exchange

1. Diffusion

2. Vesicular transport

   • transcytosis

• Pinocytosis

• exocytosis

Mechanisms of venous return

1. Skeletal muscle pump

2. Respiratory pump

3. Sympathetic vasoconstriction

Determinants of Blood Pressure

1 total peripheral resistance

2 cardiac output

3 blood volume

Redirection of Blood Flow

Pressure downstream decreases

Pressure upstream increase

Blood takes the path of least resistance

Capillary exchange of nutrients

Blood to capillary to interstitial space to tissue