The Circulatory System: A Comprehensive Guide

1. The Circulatory System

The circulatory system carries substances via the blood, which circulates through the blood vessels and is pumped by the heart.

1.1 Blood

Human blood is a thick, red liquid composed of plasma and blood cells.

Types of Blood Cells:

• Red Blood Cells: Small cells without a nucleus. They are the most numerous in the body and contain iron. This protein helps transport oxygen and carbon dioxide.
• White Blood Cells: Larger and less numerous. They defend our bodies against pathogens and tumor cells.
• Platelets: Are not true cells, but pieces of cytoplasm. They help our bodies to clot.

The Functions of Blood:

• Transport Substances: Transport nutrients and waste substances.
• Regulates Body Temperature: Maintain body temperature by distributing heat throughout the body.
• Defends the Body: White blood cells form part of the body's defense mechanisms against pathogens and tumor cells. They also help our body to clot.

1.2 Blood Vessels

Types of Blood Vessels:

• Arteries: Blood vessels with thick, elastic walls. They carry blood from the heart to the organs and branch into arterioles.
• Veins: Blood vessels with thinner and less elastic walls than arteries. They have valves, which prevent blood from flowing backward. They are formed by small veins called venules.
• Capillaries: Microscopic blood vessels that reach the cells. They join the arterioles and the venules to form a closed circuit. Their walls are very thin and allow nutrients, gases, and waste to be exchanged between the blood and the cells.

1.3 The Heart

• Pumps the Blood: The heart pumps blood through the blood vessels. If the heart stopped working, cells would not receive any oxygen or the nutrients they need to carry out vital functions.

Anatomy of the Heart:

• The heart is a muscular, hollow organ located in the rib cage between the lungs.
• It is divided into two halves, right and left, separated by the septum. Each half has two cavities:
• The Atria (Upper Chambers): The pulmonary veins end in the left atrium. The venae cavae, where blood from the rest of the body's veins flows, end in the right atrium.
• The Ventricles (Lower Chambers): Joined to the atria via a set of valves. The left valve is the mitral valve, and the right one is the tricuspid valve.
• The pulmonary artery leaves the right ventricle and carries blood to the pulmonary alveoli, where gas exchange takes place.
• The aorta leaves the left ventricle and distributes blood containing oxygen. Large arteries that leave the heart have valves called semilunar valves, which prevent blood from flowing back.

1.4 The Cardiac Cycle

• The heart has special muscle tissue called cardiac muscle, which allows it to contract and dilate to pump blood. The systole and diastole are called the cardiac cycle.
• A cardiac cycle lasts less than one second.
• During a cardiac cycle, blood enters the heart through the arteries. However, the blood vessels remain full of blood at all times, as this is a continuous cycle.

The Stages of the Cardiac Cycle:

• Diastole: In the first stage of the cardiac cycle, the atria are relaxed and dilated. This allows blood to enter. It flows to the right atrium and to the left atrium from the pulmonary veins.
• Atrial Systole: The atria contract, and blood flows into the ventricles from the right atrium to the right ventricle through the tricuspid valve and from the left atrium to the left ventricle through the mitral valve. Valves prevent blood from flowing back into the atria.
• Ventricular Systole: The ventricles contract, forcing blood into the arteries through the semilunar valves: blood leaves the right ventricle through the pulmonary artery, and the left ventricle through the aorta. Blood flows intermittently and is perceived as the arterial pulse.

2. Blood Flow

Blood flow is the circulation of blood, pumped by the heart through the circulatory system.

Humans have a double and complete circulatory system. It runs through two circuits:

Pulmonary Circuit

The pulmonary circuit is the path blood takes between the heart and lungs. Blood is pumped by the right side of the heart.

How it Works:

Oxygen-depleted blood, carrying carbon dioxide picked up from all body organs, reaches the right atrium and flows into the right ventricle, where it exits to the pulmonary artery. The pulmonary artery branches off into two arteries. Once they reach the lungs, the arteries divide further, following branches of the bronchioles. When blood passes through the alveoli, it releases carbon dioxide and picks up oxygen. Once this gas exchange has taken place, blood flows into the pulmonary veins, which carry it to the left atrium of the heart.

Systemic Circuit

, blood delivers oxygen and  nutrients to the cells and picks up carbon dioxide and waste.