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Blood is the fluid of the circulatory system, responsible for carrying many substances, including dissolved respiratory gases, throughout the body.
The heart serves as a pump, facilitating blood movement through various vessels.

Characterized by short, open-ended vessels.
Blood flows freely through the cavity, bathing organs directly with hemolymph (a fluid equivalent to blood).

In contrast to open systems, closed circulatory systems maintain blood within vessels.

Closed circulatory systems can be classified based on the number of heart chambers, reflecting evolutionary adaptations of different animal groups.

Example: Fishes and larval amphibians possess a two-chambered heart.
Blood is pumped from the heart to the gills for oxygenation and then distributed to the body.

Example: Adult amphibians and most nonavian reptiles possess a three-chambered heart.
In these animals, blood mixes in one chamber before being delivered simultaneously to the lungs and the rest of the body.

Example: Both birds and mammals are equipped with a four-chambered heart.
Blood is first pumped to the lungs for oxygenation and then returns to the heart before it is circulated to the rest of the body.

Blood has diverse roles in the body, primarily consisting of cells suspended in a liquid extracellular matrix known as plasma.

Plasma is mainly composed of water, but also contains antibodies and many other dissolved substances.
It serves the purpose of carrying red blood cells, white blood cells, and platelets.

Red blood cells are designed to carry oxygen to body tissues.
They contain hemoglobin, a protein that binds oxygen at respiratory surfaces and releases it at various body tissues.

These cells are integral to the immune system, involved in provoking inflammation and destroying microbial threats among other functions.

Platelets are cellular fragments that play a crucial role in the initiation of blood clotting processes.

The heart propels blood through three primary types of blood vessels:
1. Arteries: carry blood away from the heart and branch into smaller arterioles.
2. Capillaries: the smallest blood vessels, where the exchange of water and dissolved substances with interstitial fluid occurs.
3. Veins: collect blood returning to the heart from the capillaries through venules.

The muscular heart is responsible for pumping thousands of liters of blood through the body daily.
The heart’s wall is primarily composed of myocardium, a thick layer of cardiac muscle.

Surrounding the heart is the pericardium, a protective sac that anchors the heart while allowing it to beat without obstruction.

The heart consists of four chambers: the atria and ventricles.
- Atria receive blood from veins.
- Ventricles are responsible for pumping blood into arteries.

The right side of the heart deals with oxygen-poor blood:
- Receives deoxygenated blood from the body and pumps it to the lungs for oxygenation.
The left side of the heart deals with oxygen-rich blood:
- Receives oxygenated blood from the lungs and pumps it to the body.

Right Atrium: Receives oxygen-depleted blood from the body through the superior and inferior vena cava.
Right Ventricle: Receives blood from the right atrium and pumps it into the lungs via the pulmonary arteries, where gas exchange occurs.
Pulmonary Veins: Carry oxygen-rich blood back to the heart from the lungs into the left atrium.
Left Atrium: Receives oxygenated blood from the pulmonary veins.
Left Ventricle: Pumps oxygen-rich blood into the aorta, distributing it to the entire body.

Blood pressure experiences a decrease as it travels through the circulatory system.
Blood velocity is the lowest in the capillaries due to the high total cross-sectional area, which facilitates the exchange of materials between blood and tissues.