Vessels p1

Definition of Pericytes:
- Not to be confused with parasites (like ticks).
- Functionally similar to smooth muscle cells in that they can contract and affect the diameter of capillary lumen.
Structural Characteristics of Capillaries:
- Capillaries lack a tunica media and smooth muscle.
- Pericytes compensate for absence of smooth muscle by regulating capillary diameter.

Continuous Capillaries:
- Transport Mechanisms:
- Intercellular Junctions:
  - Endothelial cells are connected by tight junctions, limiting space to small molecules, ions, and solutes.
  - Only small solutes can pass through these junctions.
- Transcytosis:
  - Process described:
  - Involves the trapping of extracellular fluid droplets within vesicles through pinched cell membranes.
  - Vesicles transport solutes across the endothelial cell and release them into the surrounding tissue.
  - Important for transferring molecules without needing to pass through the intercellular junctions.
- Summary of Continuous Capillary Features:
- Lack of large openings, presence of pericytes, tight junctions, and transcytosis facilitate selective permeability.

Fenestrated Capillaries:
- Defining Features:
- Fenestrations or Pores:
  - These capillaries are characterized by small pores in the endothelial cells facilitating enhanced filtration.
- Function:
  - Commonly found in organs like kidneys, these capillaries allow substances to be filtered from blood, relying on pressure gradients.
  - Blood pressure is higher in the capillaries, forcing solutes through fenestrations to the lower pressure outside.
Sinusoids:
- Definition:
- Specialized, modified capillaries with larger gaps than typical capillaries.
- Characteristics:
- Endothelial cells may be fenestrated.
- Allows large molecules and even blood cells to pass into surrounding tissues.
- Examples:
- Liver Sinusoid: Allows for nutrient extraction from blood passing through the liver while in close contact with liver cells.
- Bone Marrow Sinusoids: Facilitates the exit of large blood cells into circulation.

Precapillary Sphincters:
- Function:
- Smooth muscle cells that wrap around capillary entrances.
- Control blood flow through capillary beds; can close off blood supply or relax to allow blood flow.
- Flow Regulation:
- Around 75% of capillaries are closed at any given time, regulating where blood goes based on tissue needs (nutritional demands).
Metarterioles & Thoroughfare Channels:
- Metarterioles serve as transition between arterioles and capillary beds, and some function as thoroughfare channels to bypass capillary networks.

Differences Between Arteries and Veins:
- Veins have thinner walls, less smooth muscle, and are more collapsible compared to arteries.
- Mean pressure in veins is about 10 mmHg versus 120 mmHg in large arteries.
Blood Volume Distribution:
- Approximately 2/3 of blood volume is contained within veins at any moment.
Blood Flow Mechanisms in Veins:
- Low pressure means venous blood needs mechanisms to return to the heart, including the skeletal muscle pump and valves to prevent backflow.

Venules:
- Postcapillary venules are small, receiving blood from capillaries and allowing for nutrient and fluid exchange.
Muscular Venules:
- Slightly larger, possess smooth muscle and are more robust than postcapillary venules.
Medium Veins:
- Presence of valves to prevent backflow of blood; these valves are made up of flaps of endothelium.
Large Veins:
- Examples include vena cava and jugular veins that have all three tunics: tunica intima, tunica media, and tunica externa.
- Again, the tunica media is thinner than in arteries.

Venous Sinuses:
- Examples include coronary sinus and cerebral sinuses; they have very thin walls, allowing for larger volume capacity but low muscle tone.

Key Concepts:
- Blood Flow: Amount of blood received by an organ or tissue per unit time, usually in mm/min.
- Perfusion: Amount of blood flow per volume or mass of tissue per unit time, often expressed as mm blood / 100 grams of tissue/min.
Pressure and Resistance:
- Blood flow is determined by the pressure gradient (difference between arterial and venous pressure) divided by resistance (which includes vessel diameter, blood viscosity, etc.).
- Mathematical Expression: $(Flow ext{ is proportional to } rac{ ext{difference in pressure}}{ ext{resistance}})$ .

Measuring Instrument: Sphygmomanometer used to express blood pressure as systolic/diastolic (e.g., 120/80 mmHg).
Rationale for Measurement: Measurement typically taken in the brachial artery to represent pressures in the systemic circulation.
Typical Values: Normal systolic pressure is less than 120 mmHg and diastolic pressure less than 80 mmHg; elevated levels indicate hypertension.

Arterial vs. Venous Color:
- Arterial blood is bright red due to high oxygen saturation, whereas venous blood is darker red due to lower oxygen saturation after delivering oxygen to tissues.
Variation in Pulmonary Circulation: In pulmonary circulation, arterial blood is less oxygenated upon returning from the body (to lungs for oxygenation).

Regular Route Overview:
- Blood travels from heart via arteries → arterioles → capillaries → venules → veins → back to heart.
Portal Circulations:
- Blood passes through two capillary beds before returning to the heart (e.g., pituitary and hepatic portal systems).
Additional Circulatory Routes:
- Anastomoses: Connections between blood vessels; types include arteriovenous, venous, and arterial anastomoses, allowing alternate routes for blood flow, crucial during blockages.