Human Body Systems: Circulation, Fluids, and Excretion

Internal Environment: The Body's Inner Sea

The internal environment refers to the set of fluids that surround the cells of an organism. From these fluids, cells extract the nutrients and oxygen they need.

Key Components of the Internal Environment

• Interstitial Fluid: This fluid, also known as tissue fluid, performs defensive and nutritive functions, remaining between the cells.
• Blood: Responsible for the transport of substances, blood circulates through blood vessels.
• Lymph: A fluid formed from plasma and interstitial fluid, lymph flows through lymphatic vessels.

The Blood Circulatory System

The blood circulatory system distributes nutrients and oxygen throughout the body while collecting waste products of cell metabolism.

Blood Vessels: Pathways of Circulation

Blood vessels are tubes that circulate blood to all parts of the organism. They include:

• Arteries: These vessels conduct blood from the heart to the organs of the body.
• Veins: These vessels carry blood from various organs back to the heart.
• Capillaries: Microscopic vessels that form a network, connecting arteries and veins at their terminations.

Blood: Composition and Vital Functions

Blood is a red, viscous fluid with a salty taste. It performs several vital functions:

• Nutrition: Transports nutrients from the gastrointestinal digestive system to the body's cells.
• Respiration: Delivers oxygen to the cells and carries carbon dioxide away.
• Excretion: Collects waste products and excreta for removal.
• Defense: Transports the body's defense cells, playing a crucial role in the immune system.
• Thermoregulation: Helps regulate body temperature.

The Heart: The Body's Central Pump

The heart is a muscular organ, roughly the size of a fist, responsible for pumping blood through the blood vessels.

Heart Chambers and Valves

• Mitral Valve: This valve communicates the left atrium and ventricle, composed of leaflets.
• Tricuspid Valve: This valve connects the right atrium and right ventricle, also formed by leaflets.
• Atria: The upper chambers of the heart where veins deliver blood.
• Ventricles: The lower chambers of the heart from which arteries carry blood away.

Heart Function: Contraction and Relaxation

The functioning of the heart consists of two coordinated movements that occur in the atria and ventricles: a contraction (systole) and a relaxation (diastole).

Double Circulation: Pulmonary and Systemic Circuits

The human body utilizes a double circulation system:

• Pulmonary Circuit: This circuit occurs between the heart and the lungs. Blood, rich in carbon dioxide, is pumped to the lungs to release CO2 and pick up oxygen.
• Systemic (General) Circuit: This circuit occurs between the heart and other organs of the body. It carries oxygenated blood to all cells and returns deoxygenated blood to the heart.

The Lymphatic System: Fluid Balance and Immunity

The lymphatic circulatory system collects excess fluid that remains between the cells, returning it to the blood. It also plays a role in transporting fats from the intestine.

Components of the Lymphatic System

• Lymphatic Capillaries: Tiny, blind-ended vessels distributed throughout all tissues.
• Lymphatic Vessels: Larger vessels with a structure similar to veins, collecting lymph from capillaries.
• Lymph Nodes: Small, bean-shaped structures that occur along the lymphatic vessels, filtering lymph and housing immune cells.

Excretion: Removing Metabolic Waste

Excretion is the process by which waste substances from the blood, resulting from cellular metabolism, are eliminated from the body.

Main Excretory Organs

• Kidneys
• Lungs
• Liver
• Skin (via sweat glands)