Overview
This lecture explains the meaning of blood pressure readings, differences between arteries, veins, and capillaries, and the physiological basis for systolic and diastolic pressure.
Types of Blood Vessels
- Arteries carry blood away from the heart; veins carry it back to the heart.
- Arteries usually carry high-oxygen blood, except the pulmonary artery, which carries low-oxygen blood to the lungs.
- Veins usually carry low-oxygen blood, except the pulmonary vein, which returns high-oxygen blood from the lungs.
- Capillaries connect arteries and veins and are the primary site for gas exchange.
Structure of Blood Vessels
- Blood vessels have three layers: outer connective tissue, middle smooth muscle, and inner epithelial tissue.
- Arteries have thick walls and a narrow lumen to handle high pressure from the heart.
- Veins have thin walls, a wide lumen, and one-way valves to prevent backflow, helping return blood at low pressure.
- Capillaries have a single thin epithelial layer to allow efficient gas exchange.
Blood Flow and Pressure
- The heart directly pumps blood through arteries at high pressure; veins rely on smooth muscle, body movement, and valves.
- Arteries experience high pressure; veins operate at low pressure and need valves to prevent backward flow.
Blood Pressure Readings
- A typical blood pressure reading has two numbers, e.g., 120/80 mmHg, both measuring arterial pressure.
- Systolic pressure (top number) is arterial pressure during heart contraction (heartbeat).
- Diastolic pressure (bottom number) is arterial pressure between heartbeats (heart relaxation).
- Normal blood pressure: systolic < 120 mmHg and diastolic < 80 mmHg.
- Hypertension: systolic > 130 mmHg or diastolic > 80 mmHg.
- Chronically high blood pressure increases risk of heart attack and stroke; very low pressure can cause fainting.
Blood Pressure Throughout Circulation
- Blood pressure is highest in the aorta and arteries, drops through arterioles and capillaries, and is lowest in veins and vena cavas.
- Vein blood flow depends on smooth muscle, movement, and valves—pressure is close to zero by the time it reaches the vena cava.
Key Terms & Definitions
- Artery — vessel carrying blood away from the heart.
- Vein — vessel carrying blood back to the heart.
- Capillary — tiny vessel for gas exchange between blood and tissues.
- Lumen — space inside a blood vessel where blood flows.
- Systolic pressure — arterial pressure during heart contraction.
- Diastolic pressure — arterial pressure during heart relaxation.
- Hypertension — high blood pressure above the normal range.
Action Items / Next Steps
- Review the definitions and differences between arteries, veins, and capillaries.
- Memorize normal and hypertensive blood pressure cutoffs.
- Study the relationship between the structure and function of blood vessels.