Blood Circulation: Functions of the Heart and Blood Cells

The Vital Role of Blood in Circulation

Blood is a red liquid that circulates through the blood vessels. It transports nutrients and oxygen to cells and collects carbon dioxide and other wastes.

Blood Composition: Plasma and Cells

Blood is made up of plasma and blood cells.

Blood Plasma Explained

Blood plasma is a yellowish liquid that carries nutrients, hormones, enzymes, and waste products from cells.

Types of Blood Cells

• Red blood cells (RBCs or erythrocytes): These contain hemoglobin, which binds oxygen in the lungs and transports it through the bloodstream to the capillaries, where it is released to reach all cells.
• White blood cells (leukocytes): These defend us from infectious agents like bacteria, viruses, and various parasites that enter through the skin or natural orifices of the body. The most important types are neutrophils, monocytes, and lymphocytes.
• Platelets (thrombocytes): These are involved in blood clotting.

Blood Vessels: Arteries, Veins, Capillaries

There are three types of blood vessels:

• Arteries: Send blood from the heart to all organs.
• Veins: Carry blood from the organs back to the heart.
• Capillaries: Their walls are formed by only a single layer of endothelium, facilitating exchange.

The Heart: Structure and Function

The heart is a hollow, slightly conical organ located between the lungs. It is enveloped by a membrane called the pericardium. Its walls are formed by the myocardium, the muscle that allows the heart to contract and relax rhythmically.

Heart Anatomy

The heart is divided into two halves. Each half has two chambers: an atrium (upper chamber) and a ventricle (lower chamber). The mitral and tricuspid valves allow blood flow from the atria into the corresponding ventricle and prevent movement in the contrary direction. At the start of the pulmonary artery and aorta are the semilunar valves, which prevent blood backflow into the ventricles.

How the Heart Pumps Blood (Heart Function)

1. Blood enters the atria.
2. The atria contract (atrial systole).
3. Blood passes into the ventricles.
4. The ventricles contract (ventricular systole).
5. Blood exits the heart.

The Journey of Blood: Circulation Paths

The general route of blood within the circulatory system is: Heart → Arteries → Arterioles → Capillaries → Venules → Veins → Heart.

Pulmonary and Systemic Circuits

• Lesser or Pulmonary Circulation: This circuit is established between the heart and the lungs. Its function is the exchange of respiratory gases (oxygen and carbon dioxide).
• Greater or Systemic Circulation: This circuit takes place between the heart and the rest of the body. It carries oxygen and nutrients to all tissues and removes waste products generated by the cells.

Common Circulatory System Diseases

Some common conditions affecting the circulatory system include:

• Respiratory disease (affecting gas exchange)
• Hypertension (high blood pressure)
• Varicose veins
• Stroke
• Pulmonary embolism
• Aneurysm
• Myocardial infarction (heart attack)
• Blood coagulation disorders