Circulatory and Lymphatic Systems

Introduction

The circulatory and lymphatic systems are essential for transporting nutrients, gases, and waste products throughout the body, as well as for maintaining fluid balance and immune defense. This study guide covers the structure and function of blood vessels, the regulation of blood flow and pressure, and the organization of the lymphatic system.

Key Vocabulary

• Arteries: Blood vessels that carry blood away from the heart to the tissues.

• Arterioles: Small branches of arteries that lead to capillaries.

• Blood Pressure: The force exerted by circulating blood on the walls of blood vessels.

• Bulk Flow: The movement of fluid and solutes together due to a pressure gradient.

• Capillaries: The smallest blood vessels where exchange of gases, nutrients, and wastes occurs between blood and tissues.

• Fetal Circulation: The circulatory system of a fetus, which differs from postnatal circulation due to the presence of shunts and the placenta.

• Hepatic Portal: A system of veins that carries blood from the digestive organs to the liver.

• Lymph Nodes: Small, bean-shaped structures that filter lymph and house immune cells.

• Lymphatic System: A network of vessels and nodes that returns excess tissue fluid to the bloodstream and provides immune defense.

• Net Bulk Flow: The overall movement of fluid into or out of capillaries, determined by the balance of hydrostatic and osmotic pressures.

• Pulmonary Circuit: The part of the circulatory system that carries blood between the heart and lungs.

• Veins: Blood vessels that carry blood toward the heart.

• Venules: Small vessels that collect blood from capillaries and join to form veins.

Structure and Function of Blood Vessels

Arteries and Veins

• Arteries have thick, muscular walls to withstand high pressure from the heart and transport oxygenated blood (except in the pulmonary circuit).

• Veins have thinner walls and valves to prevent backflow, carrying deoxygenated blood back to the heart (except in the pulmonary circuit).

• Arteries and veins are often paired together in the body for efficient circulation and temperature regulation.

Capillaries and Capillary Beds

• Capillaries are the smallest blood vessels, consisting of a single layer of endothelial cells to facilitate exchange.

• A capillary bed is a network of capillaries supplying a tissue region, allowing for efficient exchange of gases, nutrients, and wastes.

• Blood velocity is slowest in capillaries, maximizing exchange time.

Blood Pressure and Bulk Flow

• Blood pressure is highest in arteries and decreases through arterioles, capillaries, venules, and veins.

• Bulk flow across capillary walls is governed by hydrostatic and osmotic pressures, described by Starling's Law:

• Where = capillary hydrostatic pressure, = interstitial hydrostatic pressure, = capillary oncotic pressure, = interstitial oncotic pressure.

• Net bulk flow depends on the balance of these pressures at the arterial and venous ends of capillaries.

Regulation of Blood Flow and Pressure

Arterial Flexibility

• Arteries flex (expand and contract) to accommodate changes in blood pressure and maintain continuous blood flow.

• This elasticity helps dampen the pulse pressure generated by the heart.

Capillary Exchange

• Capillaries allow the movement of molecules by diffusion, filtration, and reabsorption, not just by pressure but also by concentration gradients.

• Examples include oxygen and carbon dioxide exchange, glucose uptake, and waste removal.

Lymphatic System

Structure and Distribution

• The lymphatic system consists of lymphatic vessels, lymph nodes, and associated organs (e.g., spleen, thymus).

• Lymph nodes are distributed throughout the body, especially in the neck, armpits, groin, and abdomen.

Functions

• Returns excess interstitial fluid to the bloodstream, maintaining fluid balance.

• Filters lymph to remove pathogens and debris.

• Houses lymphocytes and supports immune responses.

Lymphatic Regulation of Fluid

• The lymphatic system collects fluid that leaks from capillaries and returns it to the venous circulation.

• Blockage or dysfunction can lead to swelling (edema) of tissues or lymph nodes.

Special Circulatory Features

Hepatic Portal System

• The hepatic portal system carries nutrient-rich blood from the digestive tract to the liver for processing before it enters the general circulation.

• This allows the liver to detoxify substances and regulate nutrient levels.

Fetal Circulation

• Fetal circulation includes unique structures (e.g., foramen ovale, ductus arteriosus) that bypass the non-functioning fetal lungs.

• Oxygen and nutrients are obtained from the placenta, not the lungs or digestive tract.

• After birth, these shunts close and the circulatory pattern changes to the adult form.

Clinical Relevance

Coronary Artery Blockages

• Blockages in the coronary arteries can deprive the heart muscle of oxygen, leading to heart attacks (myocardial infarction).

• This is dangerous because the heart requires a constant supply of oxygenated blood to function.

Summary Table: Blood Vessel Types and Functions

Additional info:

• Starling's Law of Capillaries describes the movement of fluid across capillary membranes based on hydrostatic and oncotic pressures.

• Fetal circulation is adapted to bypass the lungs and liver, which are not fully functional before birth.