Understanding the Internal Environment, Blood, and Circulation
The Internal Environment
The internal environment, through which cells obtain nutrients and eliminate waste products from metabolic reactions, is formed by interstitial plasma. This liquid medium, located in the spaces between cells, must be continually renewed to ensure cells receive adequate nutrients and waste products are removed.
Blood: Composition and Functions
Blood is a viscous fluid with a salty taste that circulates within the circulatory system. It performs several crucial functions:
- Transports nutrients and oxygen to all cells.
- Removes waste products generated by cell metabolism.
- Participates in the body’s defense mechanisms.
- Transports hormones, essential molecules for controlling bodily functions.
- Contributes to temperature regulation.
Blood Plasma
Blood plasma, yellowish in color, consists primarily of water in which various molecules are dissolved, including minerals, nutrients, waste products, proteins, and hormones.
Blood Cells
Blood cells are of three main types: red blood cells, white blood cells, and platelets.
- Red Blood Cells (RBCs): The most numerous blood cells, RBCs are disc-shaped and lack a nucleus. They contain hemoglobin, a red pigment containing iron, which carries oxygen from the lungs to all body cells.
- White Blood Cells (WBCs): Fewer in number than RBCs, WBCs are involved in defending the body against invading microbes. There are various types of leukocytes, including granulocytes (with granular cytoplasm and a multi-lobed nucleus) and agranulocytes.
- Platelets (Thrombocytes): These are cell fragments containing substances that facilitate blood coagulation.
Types of Blood Vessels
Arteries
Arteries are large, elastic vessels that carry blood away from the heart.
Capillaries
Capillaries are microscopic blood vessels that branch from arterioles and are found throughout body tissues. Their walls consist of a single layer of flat cells, facilitating exchange of substances.
Veins
Veins are formed when venules gather capillaries. These thicker vessels carry blood back to the heart.
The Heart: Structure and Function
The heart is a hollow, muscular organ with a thick wall of muscle called the myocardium. The inner surface is lined with a thin layer of cells called the endocardium, and the outer surface is covered by a double membrane called the pericardium. The heart is divided into two halves, right and left, with no communication between them. Each half is further subdivided into an upper chamber (atrium) and a lower chamber (ventricle). An atrioventricular valve is located between the atrium and ventricle on each side. The valve between the right atrium and ventricle is called the tricuspid valve, while the corresponding valve on the left is the bicuspid or mitral valve. Several veins enter the atria: two vena cava into the right atrium and four pulmonary veins into the left atrium. Two arteries exit the ventricles: the pulmonary artery from the right ventricle and the aorta from the left ventricle. Semilunar valves are located at the beginning of these two arteries.
Heart Movements: Systole and Diastole
- Atrial Systole: The atria contract, pumping blood into the ventricles through the open atrioventricular valves.
- Ventricular Systole: The ventricles contract, forcing blood through the pulmonary artery and aorta as the semilunar valves open.
- Diastole: The heart relaxes, and blood from the vena cava and pulmonary veins enters the atria. The blood cannot return to the ventricles.