56 Blood Composition
What is blood?
The cardiovascular system exists to move blood, the fluid which both nourishes, and removes the waste from, cells.
Blood consists of four different components in different proportions.
| Blood component (% volume) | Description |
| Plasma (40-45%) | Forming the liquid component of blood, plasma is a mixture of water, ions, fats, and proteins. The primary purpose of the plasma is to function as a liquid medium, for heterogenous mixture of nutrients, chemical messengers, hormones, proteins, and waste products transported throughout the body. |
| Red blood cells (RBCs, 40-45%) | RBCs also known as erythrocytes, are biconcave discs (looks similar to a donut) which are specialized to deliver oxygen to cells. Hemoglobin, an iron compound, carries the oxygen and gives blood its characteristic red colour. |
| White blood cells (WBCs <1%) | WBCs, also known as leukocytes, fight infections and come in many different subtypes depending on their specific function within the body’s immune system. |
| Platelets (<1%) | Unlike RBCs or WBCs, platelets are not actually cells, but rather are small cell fragments derived from larger precursor cells. Platelets help with blood clotting (also known as coagulation) by gathering at the site of an injury and forming a clot to slow the loss of blood. |
Where do blood cells come from?
RBCs, WBCs, and platelets all develop from an immature cel called a hematopoietic stem cells. Blood cells arise from these stem cells within the bone marrow through a highly regulated process known as hematopoiesis. Because plasma is a heterogenous mixture of water, ions, fats, and proteins rather than a type of cell, it has no cellular precursor like the other blood cells.
Blood Pressure and Systemic Resistance:
Before delving into the bulk of cardiovascular anatomy, there are a few key concepts to understand which lay the foundation for how this system functions:
Heart rate (HR): the number of heart beats per minute. This measurement reflects the blood volume pumped by the heart, which often increases during exercise or in times of stress.
Blood pressure (BP): the pressure that circulating blood applies to the walls of blood vessels. Measured in millimeters of mercury (mmHg), blood pressure is reported as two numbers separated by a slash, for example,120/80 (verbally expressed as “120 over 80”). The first number is the pressure in the arteries during a heart contraction(systole) and the second number is the pressure in the arteries when the heart is relaxed (diastole).
Systemic vascular resistance (SVR): a calculated value related to the opposition encountered by the blood as it flows around the body or systemic circuit. Sometimes known as total peripheral resistance (TPR), it mainly reflects the diameter of blood vessels in the circuit where decreasesin the vessel diameter increase r the resistance. Blood circulating through the lungs (the pulmonary circuit) has a pulmonary vascular resistance (PVR) that corresponds to the SVR.
Pulmonary and systemic circuits:
Blood reaches all parts of the body by travelling in one of two circuits: the pulmonary circuit or the systemic circuit.
| Circuit | Description |
| Pulmonary | The pulmonary circuit conducts blood to and from the lungs for oxygenation. |
| Systemic | The systemic circuit carries blood everywhere in the body except the lungs. |
