A cat's heart beats 110 to 140 times per minute, sending blood rushing through the body in a mere 11 seconds. And though the circulatory system has many purposes, none is as important as sending oxygen throughout the body. And this is accomplished through the most numerous cell is the cat's body -- the red blood cell.
Red blood cells are so numerous in the cat that I'm not even going to bother typing out the numbers. I don't like seeing that many 0s in a row. But there are a lot and they only live for two to six weeks before they must be replaced. Thankfully, the cat's body is constantly making new red blood cells. This starts when tissue, anticipating a shortage of oxygen, sends out a hormone (erthropoietin) through the bloodstream and to the bone marrow in red blood cell construction sites. The hormone signals primitive cells to begin to grow.
The primitive cells develop into rubriblasts, which are sort of like adolescent red blood cells. Each of these rubriblasts divides into two separate cells, both of which continue dividing. Eventually, these two red blood cells (abbreviated to RBCs) become 16. The RBCs are composed mainly of water and hemoglobin. Each hemoglobin molecule contains four iron atoms. This allows for the transportation of over 1 billion molecules of oxygen throughout the cat's body.
The cells continue to grow until hemoglobin accounts for 95% of the dry weight of the red blood cell. This takes about three days (out of a six-day RBC production process). At this point, the cell ejects its nucleus in an act of self-sterilization. This leaves the cell free of all distractions. The cell can now work, and work it does. In fact, RBCs work themselves to death as they carry oxygen to the various organs of the body.
Even the shape of the red blood cell is important. It looks a little like an unperofrated doughnut and it is very flexible. So flexible that an RBC can squeeze through the smallest capillaries to deliver fresh oxygen and take away toxic carbon dioxide.
Without the all-important red blood cell, the cat would not be able to function. It is not an exaggeration to say that the red blood cell is the single most important cell in the circulatory system of the cat.
The circulatory system of the cat is comprised of the heart, a network of arteries and veins, and the spleen (which serves as a blood reservoir). A cat's heart rate, beating from 110 to 140 times a minute, is almost twice the normal heart rate of a human. The cat's body contains half a pint of blood, and this blood is circulated through the body once every 11 seconds. The blood, which is initially blue, goes through the heart to the lungs where it changes color, becoming crimson. The iron atoms in the blood discharge their carbon dioxide and grab some oxygen, which has been freshly inhaled through the lungs.
With the pulmonary part of its journey complete, the blood launches itself on a path through the body. It moves into the aorta and then into the smaller arteries that filter throughout the entire body. The blood slows slightly as it enters even the even smaller arterioles which regulate blood flow to the tissues of the body.
By the time the blood reaches the metarterioles (the conduits between arterioles and capillaries), the blood is moving at its slowest pace. The blood is now a purplish red after being stripped of most of its oxygen. The blood exits the capillaries through venules (little tiny veins) and begins its journey back to the heart. All this takes only 11 seconds.
The bloodstream is important, and not only because it allows for the exchange of oxygen and carbon dioxide throughout the body. The bloodstream is also responsible for the creation of the clots that patch up tears, control osmotic pressure, and adjust the cat's thermostat when the cat is either too warm or too cold by moving heat to other areas of the body. Excess heat is expelled from the body also thanks to the bloodstream. Blood is even responsible for shivering and the warming of the body. The bloodstream defends the cat against disease, helps with digestion after meals, and even helps the cat catch his breath after exertion. In other words, the circulatory system of the cat is perhaps one of the most important systems of all, and not just because it circulates blood.
The complex feline skeleton is composed of 244 separate bones. These bones together with cartilage (the connective tissues that bear weight) form the skeletal system of the cat.
The bones can be classified into three broad categories based upon their shape: long, flat, or irregular. If you think about it, you can probably classify most of the bones yourself simply by thinking abou their shape. The bones in the legs, for example, are long bones. These include the radius, ulna, tibia, and fibula. Flat bones can be found if you look at the scapula, skull, or face. Irregular bones are those that doesn't fit in the other categories and include bones such as the metatarsals and metacarpals.
Things get a little more complicated than that, however, because in additional to have three distinct shapes, the bones of the cat also can come in three varieties of ossification: intramembranous, endochondral, and heterotopic. Heterotopic bones occur after birth, usually under disease conditions, and are formed where you wouldn't normally find bone. These formations can be harmless little extra bits of bone or they can be dangerously close to vital organs. In the last case, they would have to be removed.
Endochondral ossification is the process of bone groth in an area previously occupied by cartilage. The cartilage is slowly replaced by bone. Intramembranous ossification is the development of bone under or with in a connective-tissue membrane.
So why is this important? Because most of the bones in the body are formed using the last two processes. The flat bones of the skull are created through intramembranous ossification while the bones at the base of the skull and in the face and trunk are a combination of intramembrous and endochondral ossification. Bone mass, by which I mean the compact bone of the shafts of the long and flat bones, is formed intramembranously. However, bones grow in length through the endochondral process.
What this all means is that the conformation of the cat is solely determined by the relative prominence of each type of bone growth. Which type of bone growth is prominant is usually dependent on the breed. For example, cats which are lean and lithe, such as the Siamese, have more endochondral ossification. Stockier cats such as the British Shorthair have more intramembranous ossification.
