08 Blood and the Cardiovascular System

Blood and its components

Blood is a fluid connective tissue that transports materials and also contributes to defense, clotting, temperature regulation, fluid balance, and pH regulation. Its liquid portion is plasma, which is mostly water and carries dissolved nutrients, hormones, salts, proteins, and wastes. Suspended in plasma are the formed elements: red blood cells, white blood cells, and platelets.

Red blood cells, also called erythrocytes, contain hemoglobin, an iron-containing protein that binds oxygen. Their flexible, biconcave shape helps them pass through narrow capillaries. White blood cells, or leukocytes, help defend the body against infection and abnormal cells; many can leave blood vessels and move into tissues where they are needed. Platelets are cell fragments that gather at damaged vessels and work with plasma clotting proteins to form a clot and help stop bleeding. Most blood cells are made in red bone marrow.

The heart and blood flow

The heart is a muscular pump with four chambers. The atria receive blood, and the ventricles pump it out. Valves help keep blood moving in one direction. The right side pumps blood toward the lungs, while the left side pumps it to the rest of the body. The left ventricle has a thicker muscular wall because it must generate enough pressure to drive blood through the systemic circuit.

The heart’s pumping produces the pressure that moves blood through vessels. Arteries carry blood away from the heart, and veins return blood to it. These terms describe direction, not oxygen content: pulmonary arteries carry relatively oxygen-poor blood to the lungs, while pulmonary veins carry oxygen-rich blood back to the heart.

Blood follows this pathway:

Body tissues → venae cavae → right atrium → right ventricle → pulmonary arteries → lung capillaries → pulmonary veins → left atrium → left ventricle → aorta → body tissues

The two linked circuits

The pulmonary circuit carries blood from the right ventricle to the lungs and back to the left atrium. In lung capillaries, carbon dioxide leaves the blood and oxygen enters it.

The systemic circuit carries blood from the left ventricle through the aorta to the body and returns it to the right atrium. In systemic capillaries, oxygen and other materials move from blood toward tissues, while carbon dioxide and some wastes enter the blood. Together, these linked circuits carry blood through the lungs for gas exchange and through the body for delivery and collection.

Exchange between blood and tissues

Capillaries have thin walls that allow substances to move between blood and nearby tissues. Oxygen and carbon dioxide diffuse down their concentration gradients. Water and dissolved substances also move across capillary walls; glucose, amino acids, and ions can use specific transport pathways.

Some fluid leaves capillaries and enters tissues, while some returns to the bloodstream. The lymphatic system collects excess tissue fluid and eventually returns it to the blood. This exchange makes capillaries the sites where blood supplies tissues and receives some of their products.

Transport of gases, nutrients, hormones, and wastes

Most oxygen travels bound to hemoglobin in red blood cells. Hemoglobin loads oxygen in the lungs and releases it in tissues for cellular respiration. Carbon dioxide produced by cells travels back to the lungs in several forms: much is converted to bicarbonate in red blood cells and carried in plasma, while some dissolves in plasma or binds to hemoglobin. In the lungs, carbon dioxide moves into the air spaces and is exhaled.

After digestion, nutrients such as glucose and amino acids enter the blood and are delivered to cells. Hormones released by endocrine glands also travel through the bloodstream to target cells, sometimes far from where the hormones were produced. Blood carries wastes, including carbon dioxide to the lungs and substances such as urea to the kidneys for removal in urine. The liver processes nutrients and helps handle or eliminate some substances.

After a meal, glucose absorbed from the small intestine enters the blood and is distributed to body cells. Insulin, which is also carried in blood, helps target cells take up glucose. The heart and vessels provide the route, and capillaries provide the exchange site.

How circulation supports the body

The cardiovascular system connects with other body systems through transport. The respiratory system supplies oxygen and removes carbon dioxide; the digestive system supplies absorbed nutrients; endocrine glands use blood to distribute hormones; and the kidneys filter wastes from blood and regulate its water and salt content. Blood’s immune cells and clotting components contribute to defense and repair.

By transporting gases, nutrients, hormones, and wastes, and by supporting fluid balance, pH regulation, temperature regulation, defense, and clotting, blood and the circulation help maintain homeostasis—the stable internal conditions cells need to function.