How does our heart work?
Our heart is composed mostly of muscle cells that are somewhat similar to skeletal muscle cells yet retain certain fundamental differences. Although most of the events involved in contraction of heart (cardiac) muscle are the same as skeletal muscle, the heart is not attached to bone. Furthermore, our heart does not require the brain to tell it when to contract (beat). However, the brain certainly can play both a direct and indirect role in regulating the beating of our heart. The stimulus that invokes excitability in the heart comes from a specialized pacemaker region within our heart, called the sinoatrial node (SA node). The human heart may beat in excess of 2 billion times throughout a person’s life.
Unlike skeletal muscle, which pulls on bone when it contracts, the heart constricts in a wringing fashion when it contracts. As the heart contracts, the pressure of the blood inside the heart (ventricles) increases. This serves to propel blood out of the heart into the arteries. This increase in pressure also provides the driving force that forces blood to surge through our blood vessels.
What is the composition of blood?
The blood is comprised of two main parts, the hematocrit and the plasma, which can be assessed clinically (see Figure 2.10). Red blood cells (RBCs) are the sole component of the hematocrit and function primarily as a shuttle for oxygen. Hematocrit is the percentage of our blood that is RBCs, which is typically 40 to 45% for an adult.
Plasma is about 55 percent of our blood. Of the plasma, about 92 percent is water while the remaining 8 percent includes over 100 different dissolved or suspended substances such as nutrients, gases, electrolytes, hormones, and proteins such as albumin and clotting factors. The remaining components of our blood are the white blood cells (WBCs) and platelets, which collectively make up about 1 percent of blood. WBCs are the principal components of the human immune system and provide a line of defense against bacteria, viruses, and other intruders. Some WBCs attack foreign invaders and useless materials while others manufacture antibodies and other immune factors. Last, but certainly not least, platelets participate in the clotting of blood.
Figure 2.10. The components of our blood. The hematocrit is comprised of RBC. Roughly 90% of the plasma is water and the remaining 10% is largely proteins, electrolytes and lipoproteins
