Vascular Resistance, Capillary Exchange, and the Lymphatic System

15.3 Resistance in the Arterioles

Mechanisms Influencing Arteriolar Resistance

Arteriolar resistance is a key determinant of blood flow and blood pressure, regulated by both local and systemic mechanisms.

• Local Control: Includes paracrine signals from vascular endothelium or nearby cells, such as nitric oxide, kinins, and histamine (potent vasodilators).

• Sympathetic Reflexes: Norepinephrine released from sympathetic neurons binds to α-receptors, causing vasoconstriction.

• Hormones: Circulating hormones like angiotensin II and atrial natriuretic peptide modulate vascular tone.

Myogenic Autoregulation

• Vascular smooth muscle can regulate its own contraction state.

• When stretched, mechanically-gated Ca2+ channels open, causing contraction to resist further stretching.

Paracrine Regulation and Hyperemia

• Active Hyperemia: Increased tissue metabolism leads to vasodilation and increased blood flow.

• Reactive Hyperemia: Follows a period of decreased blood flow; metabolites accumulate, causing vasodilation when flow resumes.

Table: Chemicals Mediating Vasoconstriction and Vasodilation

Chemical	Physiological Role	Source	Type
Vasoconstriction
Norepinephrine (α-receptors)	Baroreceptor reflex	Sympathetic neurons	Neurotransmitter
Serotonin	Platelet aggregation, smooth muscle contraction	Neurons, digestive tract	Paracrine signal
Endothelin	Local control of blood flow	Vascular endothelium	Paracrine
Vasopressin	Increases blood pressure in hemorrhage	Posterior pituitary	Neurohormone
Angiotensin II	Increases blood pressure	Plasma hormone	Hormone
Vasodilation
Epinephrine (β2-receptors)	Increases blood flow to skeletal muscle, heart, liver	Adrenal medulla	Neurohormone
Acetylcholine	Erection reflex (indirectly through NO production)	Parasympathetic neurons	Neurotransmitter
Nitric oxide (NO)	Local control of blood flow	Endothelium	Paracrine signal
Bradykinin	Increases blood flow	Multiple tissues	Paracrine signal
Adenosine	Increases blood flow to heart	Hypoxic cells	Paracrine signal
Decreased O2, increased CO2, H+, K+	Increases blood flow to match metabolism	Cell metabolism	Paracrine signal
Histamine	Increases blood flow	Mast cells	Paracrine signal
Natriuretic peptides	Reduce blood pressure	Atrial myocardium, brain	Hormone, neurotransmitter
Vasoactive intestinal peptide	Digestive secretion, relax smooth muscle	Neurons	Neurotransmitter

Sympathetic Control of Vascular Smooth Muscle

• Most systemic arterioles are innervated by sympathetic neurons (except erectile tissue).

• Norepinephrine maintains arteriolar tone via α-receptors (vasoconstriction).

• Epinephrine from the adrenal medulla binds α-receptors with low affinity, also causing vasoconstriction.

15.4 Distribution of Blood to the Tissues

Principles of Blood Distribution

Blood flow to tissues is dynamically regulated to match metabolic needs.

• Governed by local control mechanisms and homeostatic reflexes.

• Arterioles are arranged in parallel, allowing independent regulation of flow to different organs.

• Flow in the aorta equals the sum of flows in all arteries.

• Individual arterioles can adjust their own flow, with compensation by others.

• Cerebral blood flow remains nearly constant to ensure brain function.

Example: Distribution of Blood at Rest

At rest, blood is distributed according to organ needs, with the greatest proportion going to the digestive system, kidneys, and skeletal muscle.

15.5 Regulation of Cardiovascular Function

Cardiovascular Control Center (CVCC)

The CVCC in the brainstem integrates sensory input and coordinates the autonomic regulation of heart and blood vessels.

• Baroreceptor Reflex: Baroreceptors in the carotid arteries and aorta detect changes in blood pressure.

• These mechanoreceptors produce continuous (tonic) action potentials to the brainstem.

• Changes in blood pressure are reflected as changes in the frequency of action potentials.

• The baroreceptor reflex is always functioning to maintain homeostasis.

15.6 Exchange at the Capillaries

Structure and Function of Capillaries

Capillaries are the primary sites for exchange between blood and tissues, with structure optimized for this function.

• Most cells are within 0.1 mm of a capillary, ensuring efficient exchange.

• Capillary density correlates with tissue metabolic activity.

• Capillaries have the thinnest walls: a single layer of flattened endothelial cells supported by a basal lamina.

Types of Capillaries

• Continuous Capillaries: Have tightly joined endothelial cells; found in muscle, connective tissue, and the blood-brain barrier.

• Fenestrated Capillaries: Have large pores; found in kidneys, intestines, and endocrine glands.

• Sinusoids: Modified capillaries with large gaps; found in bone marrow, liver, and spleen.

Velocity of Blood Flow

• Blood flow velocity is lowest in capillaries due to their large total cross-sectional area, facilitating exchange.

Capillary Exchange Mechanisms

Diffusion and Transcytosis

• Exchange between plasma and interstitial fluid occurs via:

  • Paracellular pathway (between cells)

  • Endothelial transport (through cells)

• Small dissolved solutes and gases move by diffusion, depending on lipid solubility and concentration gradients.

• Larger solutes and proteins are transported by vesicular transport (transcytosis).

Bulk Flow: Filtration and Absorption

• Bulk flow is the mass movement of fluid due to hydrostatic or osmotic pressure gradients.

• Filtration: Fluid movement out of capillaries, driven by hydrostatic pressure ($P_H$).

• Absorption: Fluid movement into capillaries, driven by colloid osmotic pressure ($\pi$), also called oncotic pressure.

• Capillaries have plasma proteins ($\pi_{cap} = 25$ mm Hg); interstitial fluid has few ($\pi_{IF} = 0$ mm Hg).

• Net pressure determines direction of bulk flow:

$P_{NET} = P_H - \pi$

• Net filtration at arterial end ($P_H > \pi$)

• Net absorption at venous end ($P_H < \pi$)

15.7 The Lymphatic System

Functions and Structure

The lymphatic system returns excess fluid and proteins from the interstitial space to the circulatory system, absorbs fats, and serves as a filter for pathogens.

• Allows one-way movement of interstitial fluid into the circulatory system.

• Lymph: The fluid transported by lymphatic vessels.

• Lymph capillaries are blind-ended; lymph vessels have semilunar valves and empty into venous circulation.

• Lymph nodes filter lymph and participate in immune responses.

• Edema: Accumulation of fluid in the interstitial space due to inadequate lymph drainage or excessive filtration.

Key Terms

• Perfusion

• Elastic recoil

• Pressure reservoir

• Arterioles

• Vascular smooth muscle

• Vasoconstriction / Vasodilation

• Microcirculation

• Metarterioles

• Precapillary sphincters

• Driving pressure

• Systolic / Diastolic pressure

• Pulse pressure

• Mean arterial pressure (MAP)

• Myogenic autoregulation

• Active / Reactive hyperemia

• Atrial natriuretic peptide

• Angiotensin II (ANGII)

• Cardiovascular control center (CVCC)

• Baroreceptors

• Continuous / Fenestrated capillaries

• Sinusoids

• Bulk flow

• Absorption / Filtration

• Colloid osmotic pressure ($\pi$)

• Hydrostatic pressure ($P_H$)

• Lymph / Lymph nodes / Lymph capillaries

Learning Outcomes

• Define myogenic autoregulation and its role in local blood flow.

• List and describe major paracrine molecules in local blood flow control.

• Describe hormonal and neural control of blood vessel diameter.

• Explain local and long-distance signaling in blood flow regulation.

• Describe types of capillaries and their locations.

• Explain why blood flow velocity is lowest in capillaries.

• Explain diffusion and transcytosis in capillary exchange.

• Explain forces influencing capillary filtration and absorption.

• Describe lymphatic system anatomy and function, and its relation to circulatory and immune systems.