Ch 15a Blood Flow and Control of Blood Pressure: Structure and Function of Blood Vessels

Blood Flow and Control of Blood Pressure

Overview of Chapter Topics

This chapter introduces the anatomy and physiology of the cardiovascular system, focusing on blood vessels, blood pressure, resistance, and the regulation of cardiovascular function. Key concepts include the structure and function of arteries, veins, and capillaries, mechanisms of blood flow, and factors influencing blood pressure.

• Types of blood vessels

• Blood pressure

• Resistance in the arterioles

• Distribution of blood to tissues

• Regulation of cardiovascular function

• Exchange at the capillaries

• The lymphatic system

• Cardiovascular disease

The Blood Vessels

Arteries and Veins: Structure and Function

Blood vessels are classified based on their structure and function. Arteries and veins have distinct roles in the circulatory system.

• Arteries: Carry blood away from the heart. They have elastic walls and thick layers of vascular smooth muscle, allowing them to act as pressure reservoirs that maintain blood flow during ventricular relaxation.

• Veins: Carry blood back to the heart. They have thin walls of vascular smooth muscle and act as volume reservoirs, accommodating varying amounts of blood.

Example: The aorta is the largest artery, receiving blood directly from the left ventricle and distributing it to systemic circulation.

Functional Model of the Cardiovascular System

The cardiovascular system consists of a closed circuit of blood vessels, including arteries, arterioles, capillaries, venules, and veins. Blood flows from the heart through arteries, arterioles, capillaries, and returns via venules and veins.

• Arteries: Pressure reservoir

• Arterioles: Adjustable resistance

• Capillaries: Site of exchange

• Venules and Veins: Volume reservoir

Example: Systemic veins serve as an expandable volume reservoir, while elastic systemic arteries maintain blood flow during ventricular diastole.

Comparative Structure of Blood Vessels

Blood vessels differ in diameter, wall thickness, and composition, which determines their function.

Vessel Type	Diameter	Wall Thickness	Endothelium	Elastic Tissue	Smooth Muscle	Fibrous Tissue
Artery	0.1–103 mm	1.0 mm	High	High	High	Moderate
Arteriole	10–100 μm	6.0 μm	Moderate	Low	High	Low
Capillary	4–10 μm	0.5 μm	High	None	None	None
Venule	10–100 μm	1.0 μm	Moderate	None	Low	Low
Vein	0.1–103 mm	0.5 mm	Moderate	Low	Low	High

Additional info: Capillaries are the only vessels with a single cell-thick wall, facilitating efficient exchange of substances.

Exchange Takes Place in the Capillaries

Structure and Function of Capillaries

Capillaries are specialized for exchange due to their thin walls and lack of smooth muscle and elastic tissue.

• One cell-thick layer of endothelial cells on a basal lamina allows for rapid diffusion of gases, nutrients, and waste.

• Pericytes are closely associated with many capillaries, helping to decrease capillary permeability and can differentiate into endothelial or smooth muscle cells.

• Absence of vascular smooth muscle and elastic tissue reinforcement facilitates exchange.

Example: Oxygen and nutrients diffuse from blood in capillaries to surrounding tissues, while waste products move into the blood for removal.

Blood Flow, Pressure, and Resistance

Principles of Blood Flow

Blood flow in the cardiovascular system is governed by pressure gradients and resistance.

• Blood flows if a pressure gradient (ΔP) is present.

• Blood flows from areas of higher pressure to areas of lower pressure.

• Blood flow is opposed by resistance (R) of the system.

• Three factors affect resistance: radius of the blood vessels, length of the blood vessels, and viscosity of the blood.

• Flow is usually expressed in liters or milliliters per minute (L/min or mL/min).

• Velocity of flow is usually expressed in centimeters per minute (cm/min) or millimeters per second (mm/sec).

• The primary determinant of velocity (when flow rate is constant) is the total cross-sectional area of the vessel(s).

Key Equations:

• Flow ($Q$) is proportional to the pressure gradient ($ΔP$) and inversely proportional to resistance ($R$): $Q = \frac{ΔP}{R}$

• Mean Arterial Pressure (MAP): $MAP = Diastolic\ Pressure + \frac{1}{3}(Systolic\ Pressure - Diastolic\ Pressure)$

Example: If the pressure in the aorta is 120 mmHg and in the vena cava is 5 mmHg, blood will flow from the aorta to the vena cava.

Clinical Relevance: Blood Pressure Measurement

Blood pressure is estimated using a sphygmomanometer. Abnormal values indicate hypotension (lower than normal MAP) or hypertension (higher than normal MAP).

• Valves in veins ensure one-way flow and prevent backflow.

• Maintaining normal blood pressure is essential for tissue perfusion and organ function.

Example: Hypertension is a major risk factor for cardiovascular disease.