Blood Flow and the Control of Blood Pressure: Advanced Human Physiology Study Notes

Blood Vessels and Their Structure

Endothelium and Vascular Smooth Muscle

The blood vessels are composed of several layers, each with specialized functions in regulating blood flow and pressure.

• Endothelium: A single layer of epithelial cells lining the interior of blood vessels. It secretes paracrines that regulate blood pressure, vessel growth, and absorption of materials.

• Vascular Smooth Muscle: Most vessels have smooth muscle arranged in circular or spiral layers. This muscle controls vessel diameter:

  • Vasoconstriction: Narrowing of the vessel lumen, increasing resistance and blood pressure.

  • Vasodilation: Widening of the vessel lumen, decreasing resistance and blood pressure.

  • Muscle Tone: Blood vessels maintain a state of partial contraction (tone) at all times.

Types of Blood Vessels

• Arteries: Carry blood away from the heart. Their elastic walls act as a pressure reservoir, storing pressure from heart contractions and maintaining continuous blood flow.

• Arterioles: Small branches of arteries that direct blood flow to tissues by constricting or dilating. They are the main site of variable resistance in the systemic circulation.

  • Metarterioles: Vessels partially surrounded by smooth muscle, connecting arterioles to capillaries.

  • Precapillary Sphincters: Rings of smooth muscle that regulate blood flow into capillary beds.

• Capillaries: The primary site of exchange between blood and tissues. Surrounded by pericytes that regulate permeability.

• Venules and Veins: Carry blood back to the heart. They have large lumens and valves to prevent backflow, acting as a volume reservoir (holding about 60% of blood volume).

Angiogenesis

Angiogenesis is the growth of new blood vessels after birth. It is crucial for tissue growth, repair, and in pathological conditions such as tumor development and coronary heart disease (where new vessels may form around blockages).

Blood Pressure and Its Regulation

Blood Pressure Gradients

Blood pressure is highest in the arteries and lowest in the veins, creating a pressure gradient that drives blood flow.

• Systolic/Diastolic Pressure: Normal value is 120/80 mm Hg.

• Pulse: The pressure wave felt when the ventricles contract and force blood into the aorta.

• Pulse Pressure: The difference between systolic and diastolic pressure.

$\text{Pulse Pressure} = \text{Systolic Pressure} - \text{Diastolic Pressure}$ $\text{Pulse Pressure} = 120\,\text{mm Hg} - 80\,\text{mm Hg} = 40\,\text{mm Hg}$

• Venous Return: Aided by large lumens, valves, the skeletal muscle pump, and the respiratory pump.

Mean Arterial Pressure (MAP)

MAP is the average pressure in the arteries, weighted closer to diastolic pressure because diastole lasts longer than systole.

$\text{MAP} = \text{Diastolic Pressure} + \frac{1}{3}(\text{Systolic Pressure} - \text{Diastolic Pressure})$ $\text{MAP} = 80\,\text{mm Hg} + \frac{1}{3}(120 - 80)\,\text{mm Hg} = 93\,\text{mm Hg}$

• Hypotension: Abnormally low blood pressure; can impair blood flow to the brain, causing dizziness or fainting.

• Hypertension: Abnormally high blood pressure; can damage vessel walls, leading to hemorrhage or stroke.

Determinants of Mean Arterial Pressure

• Cardiac Output (CO): The volume of blood pumped by the heart per minute.

• Peripheral Resistance: The resistance to blood flow offered by the arterioles.

• Distribution of Blood: Between arteries (11%) and veins (60%).

• Total Blood Volume: Influences overall pressure.

$\text{MAP} \propto \text{Cardiac Output} \times \text{Peripheral Resistance}$

Regulation of Arteriole Resistance

Local and Systemic Control

• Local Control: Matches blood flow to tissue metabolic needs.

  • Myogenic Autoregulation: Vascular smooth muscle responds to stretch by constricting, maintaining constant flow.

  • Paracrines: Substances like nitric oxide (NO) and adenosine cause vasodilation.

  • Active Hyperemia: Increased blood flow in response to increased metabolic activity.

  • Reactive Hyperemia: Increased blood flow following a period of reduced perfusion.

• Sympathetic Reflexes: Tonic release of norepinephrine maintains vascular tone.

• Hormonal Control: Hormones such as atrial natriuretic peptide and angiotensin II affect blood volume and resistance via the kidneys.

Distribution of Blood to Tissues

The body can selectively alter blood flow to different organs based on need, a key aspect of cardiovascular regulation.

Cardiovascular Control Center (CVCC)

• Located in the medulla oblongata of the CNS.

• Ensures adequate blood flow to the brain and heart by maintaining MAP.

Baroreceptor Reflex

• Baroreceptors: Stretch-sensitive mechanoreceptors in the carotid arteries and aorta.

• Tonically active and respond rapidly to changes in blood pressure.

• Adjust cardiac output and peripheral resistance within two heartbeats.

• Orthostatic Hypotension: A drop in blood pressure upon standing, triggering the baroreceptor reflex.

Exchange at the Capillaries

Types of Capillaries

• Continuous Capillaries: Most common; found in muscle, connective, and neural tissue.

• Fenestrated Capillaries: Have large pores for rapid fluid exchange; found in kidneys and intestines.

• Sinusoids: Large, leaky capillaries in the liver, bone marrow, and spleen; allow passage of blood cells and proteins.

Capillary Exchange Mechanisms

• Diffusion: Movement of small solutes down their concentration gradients.

• Transcytosis: Vesicular transport of larger molecules across the endothelium.

• Bulk Flow: Mass movement of fluid due to pressure gradients.

  • Filtration: Fluid movement out of capillaries, driven by hydrostatic pressure.

  • Absorption: Fluid movement into capillaries, driven by colloid osmotic pressure.

$\text{Net Filtration Pressure} = (\text{Capillary Hydrostatic Pressure}) - (\text{Colloid Osmotic Pressure})$

The Lymphatic System

Functions and Structure

• Returns excess interstitial fluid to the blood.

• Provides sites for immune surveillance and cell division.

• Lymph Vessels: One-way vessels with large lumens and valves, relying on smooth muscle contraction and skeletal muscle pump for flow.

• Lymph: Fluid that originated as plasma, became interstitial fluid, and then entered lymphatic vessels.

• Lymph Nodes: Bean-shaped structures that filter lymph before it returns to circulation.

Edema

• Swelling caused by excess fluid in tissues.

• Can result from increased capillary hydrostatic pressure (e.g., heart failure), decreased plasma protein concentration, or increased interstitial proteins (e.g., inflammation).

Cardiovascular Disease

Risk Factors and Pathology

• Risk Factors: Smoking, obesity, and genetic predisposition.

• Atherosclerosis: Accumulation of lipids and calcium beneath the endothelium, leading to vessel narrowing and reduced elasticity.

• Hypertension: Chronic high blood pressure, often due to hereditary factors (primary hypertension) or secondary to other diseases (secondary hypertension).

• Baroreceptors may adapt to higher pressures, reducing their sensitivity and contributing to sustained hypertension.

Summary Table: Types of Blood Vessels

Vessel Type	Main Function	Key Features
Arteries	Carry blood away from heart	Thick walls, elastic, pressure reservoir
Arterioles	Regulate blood flow to tissues	Variable resistance, smooth muscle
Capillaries	Exchange of gases, nutrients, wastes	Thin walls, pericytes, slowest flow
Venules	Collect blood from capillaries	Thin walls, converge to form veins
Veins	Return blood to heart	Large lumens, valves, volume reservoir

Key Equations

• $\text{Pulse Pressure} = \text{Systolic Pressure} - \text{Diastolic Pressure}$

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

• $\text{MAP} \propto \text{Cardiac Output} \times \text{Peripheral Resistance}$

• $\text{Net Filtration Pressure} = (\text{Capillary Hydrostatic Pressure}) - (\text{Colloid Osmotic Pressure})$

Additional info:

• Baroreceptor adaptation to chronic hypertension can reduce their effectiveness in regulating blood pressure.

• Active and reactive hyperemia are important for matching tissue perfusion to metabolic needs.