B3.2 Transport

Lumen Diameter: Capillaries have a very small lumen (around 5-10 µm) that forces red blood cells to move in single file, reducing diffusion distance between blood and tissues.
Branching: Capillaries form extensive networks that increase surface area for exchange and ensure that all cells in tissues are close to a capillary for diffusion.
Wall Thickness: Capillary walls are only one cell thick (endothelium), reducing diffusion distance for oxygen, carbon dioxide, nutrients, and waste products.
Fenestrations: Some capillaries have fenestrations (pores) that allow for the exchange of larger molecules, like proteins, and water-soluble substances between the blood and tissues, especially in areas like the kidneys and intestines.

Feature	Arteries	Veins
Diameter	Smaller than veins	Larger than arteries
Wall Thickness	Thicker to withstand high pressure	Thinner, lower pressure
Lumen Size	Narrower than veins	Wider to accommodate low pressure
Wall Layers	Three layers (tunica externa, tunica media, tunica intima)	Same three layers but thinner
Muscle and Elastic Fibers	Abundant to maintain pressure and allow for recoil	Less abundant as pressure is lower
Valves	Absent	Present to prevent backflow of deoxygenated blood
Functions	Arteries carry oxygen-rich blood away from the heart to the body.	Veins carry oxygen-poor blood back to the heart from the body.4o

Identifying in Micrograph: Arteries have thicker walls and a more rounded appearance, while veins appear more collapsed with thinner walls and larger lumens.

Function: Arteries transport oxygenated blood (except pulmonary arteries) from the heart to tissues at high pressure.
Three Layers of Arterial Walls:
- Tunica Intima: Thin inner lining of endothelial cells; smooth to reduce friction.
- Tunica Media: Middle layer of smooth muscle and elastic fibers; thick to withstand and maintain pressure.
- Tunica Externa: Outer layer of connective tissue providing structural support.
Wall Thickness: Thick walls prevent the artery from rupturing under high pressure.
Lumen Size: Narrow to maintain high pressure.
Muscle and Elastic Fibers: These help maintain blood pressure by allowing the artery to stretch and recoil with each heartbeat.

Unit of Measurement: Pulse rate is measured in beats per minute (bpm).
Methods for Determining Heart Rate:
1. Manual Method: Counting beats at a pulse point (e.g., radial artery) for 15 seconds and multiplying by four.
2. Electronic Monitors: Use of heart rate monitors or electrocardiograms (ECG) to track beats per minute.