Mammalian Physiology: Placenta, Blood, and Vital Systems

The Role of the Placenta in Mammals

The placenta is the organ from which our group of mammals takes its name: it is the main reason we, the placental mammals, are so much more successful than other mammals. The placenta is composed of several layers of material. It is very richly supplied with blood vessels. In this way, white blood cells and other immune system components are kept within the boundaries of their own systems while nutrients are allowed to pass in, and waste products to pass out, of the embryo's environment. There are at least five different forms the placenta can take in different species of mammals.

Blood Composition and Components

Plasma is the liquid component of the blood. It contains nutrients, wastes, salts, and proteins. Proteins in the blood aid in the transport of large molecules such as cholesterol. Red blood cells, also known as erythrocytes, are flattened, doubly concave cells about 7μm in diameter that carry oxygen associated with the cell's hemoglobin. Red blood cells are continuously manufactured in the red marrow of long bones, ribs, skull, and vertebrae. White blood cells, also known as leukocytes, are larger than erythrocytes, have a nucleus, and lack hemoglobin. They function in the cellular immune response. There are five types of leukocytes, which are important components of the immune system. Platelets are cell fragments that bud off megakaryocytes in bone marrow. They carry chemicals essential to blood clotting.

The Human Respiratory System

The primary function of the respiratory system is to supply the blood with oxygen in order for the delivery of oxygen to all parts of the body. The respiratory system does this through breathing. When we breathe, we inhale oxygen and exhale carbon dioxide. Respiration is achieved through the mouth, nose, trachea, lungs, and diaphragm. Oxygen enters by means of the mouth and the nose, then passes through the larynx and the trachea, which is a tube that enters the chest cavity. The diaphragm is a sheet of muscles that lies across the bottom of the chest cavity.

The Cardiovascular System and Circulation

The cardiovascular system includes a heart, which is a muscular pump that contracts to propel blood out to the body through arteries, and a series of blood vessels. The heart muscle is composed of cardiac muscle cells. Arteries are blood vessels that carry blood away from the heart. Arterial walls are able to expand and contract. The aorta is the main artery leaving the heart. The pulmonary aorta is the only artery that carries oxygen-poor blood. The pulmonary artery carries deoxygenated blood to the lungs. Arterioles are small arteries that connect larger arteries with capillaries. Capillaries are concentrated into capillary beds. The circulatory system functions in the delivery of oxygen, nutrient molecules, and hormones, and the removal of carbon dioxide. Veins carry blood from capillaries to the heart. Venules are smaller veins that gather blood from capillary beds into veins.