Human Circulatory System and Blood Physiology

The Internal Environment and Nutrition

All cells need to live in an internal environment that provides them with all the food and nutrients they need and allows cells to release waste substances. It is formed of interstitial plasma, and the plasma has to be continually renewed thanks to the blood, which transports nutrients and removes waste products. The circulatory system connects the other three systems involved in nutrition.

Blood Composition

• Blood Plasma (55%): Made up of water with minerals (90%), nutrients (glucose, amino acids, vitamins, etc.), waste products (urea, uric acid, CO2), proteins, and dissolved hormones.
• Blood Cells (45%): Includes erythrocytes (98-99%), leukocytes (granulocytes and agranulocytes), and platelets (1-2%).

Blood Groups and the ABO System

The different types of blood are determined by the presence of certain proteins in the plasma. The most well-known grouping system is the ABO System, but erythrocytes have another specific protein known as the Rh factor.

Essential Functions of Blood

Blood carries hormones, contributes to the regulation of body temperature, and plays a role in the fight against infections.

The Mechanics of Blood Circulation

In multicellular organisms, blood must reach all the organs. To do this, they use ducts (blood vessels) through which blood circulates and a motor that pumps it (the heart). The circulation is double and complete.

Types of Blood Vessels

• Arteries: They carry oxygenated blood from the heart to other organs. They have wide muscular walls and are elastic.
• Capillaries: These are microscopic blood vessels located in all the organism's tissues. They are formed by walls that have a single layer of cells.
• Veins: They carry deoxygenated blood from other tissues to the heart. They have thinner walls than arteries and possess valves that stop blood from going backwards.

Anatomy of the Heart

The heart is a hollow muscular organ. It has two parts that are completely separate: the left and the right. Each half is subdivided into two chambers: the atrium (upper smaller chamber) and the ventricle (larger lower chamber). It is formed by a thick wall called the myocardium. A double membrane, the pericardium, covers its exterior, while the endocardium covers its interior.

The left part carries oxygenated blood and the right part carries deoxygenated blood. Between the ventricles and arteries, there are the sigmoid valves that prevent the backward flow of the blood.

Heartbeat and Cardiac Metrics

The heartbeat is the movement that pumps the blood from the veins into the atria; from there, it goes into the ventricles and is finally released into the arteries.

• Heart Rate: The number of times that the heart beats per unit of time.
• Cardiac Output: The volume of blood pumped by the heart per minute.
• Blood Pressure: The pressure applied by the blood on the artery walls.

The dilation of the aortic artery due to the entry of blood is transmitted to the entire arterial system as a wave, which is known as the pulse.