LECTURE 23
ANIMAL PHYSIOLOGY: CIRCULATORY SYSTEM reviewed and cont.
Functions of the circulatory system.
Different organisms have different respiratory proteins that work in conjunction with a metal ion. There are two main types. Some use copper (Cu, e.g., hemocyanin) and others use iron (Fe++, e.g., hemoglobin). The iron ion in our hemoglobin is held in a porphyrin group called heme. The protein part of the molecule varies between species and even between the fetus (HbF) and the adult (HbA). Fetal hemoglobin is coded for by a different gene but is very similar in amino acid sequence to adult hemoglobin. It arose by gene duplication and since it bestows an advantage to the fetus, it was selected for during the evolution of the mammals. The adaptive value of HbF is that it binds oxygen more tightly than HbA and is, therefore, able to "suck" the oxygen from the mother's blood stream.
Proteins that combine with O2 are also found inside cells. The muscle cells have the heme containing protein, myoglobin, which, like HbF is able to draw the O2 away from the hemoglobin in the red blood cells. The ultimate destination of O2 is cytochrome C, another heme containing protein, in the electron transport system in the mitochondria.
Blood. Blood is essentially a salt solution similar to the intracellular fluid and osmotically balanced with the intracellular fluid. In addition to the inorganic ions (K+, Na+, HCO3-, HPO4-, Cl-, Mg2+, Ca2+, etc.) there are protein molecules. In vertebrates the blood contains specialized proteins and cells. Some of the proteins carry molecules such as lipids (HDL and LDL) and ions (transferrin, ferritin). Other proteins include antibodies (gamma globulin fraction) and serum albumin. The specialized cells in our blood are the red blood cells (rbcs) containing the respiratory protein, hemoglobin, to carry gases, the white blood cells (e.g., phagocytes, lymphocytes) which fight infection and the blood platelets which upon disruption release enzymes to catalyze the formation of clots when a "leak in the pipes" is threatened.
The structure of circulatory systems. In simple...small or inactive animals such as sponges, Cnidaria and flatworms....the transport of O2 and CO2, nutrients and wastes is solved simply. The two or three cell layers that form the organism are either close to the external fluid media or the internal fluid of the gut. The rudimentary muscles of the body wall assist in the movement of the molecules needed.
However, in more complex animals with organs and tissues well removed from the sources of gases and nutrients, more is required. Pipes are necessary to convey the molecules to every cell in the organism's body. The organs of circulatory systems are the heart (pump) and blood vessels. The blood is the (connective) tissue that carries the gases, ions and organic molecules. The arteries carry blood away from the heart and the veins carry the blood back to the heart.
(Even the plants that are tall require "pipelines" provided by their vascular tissues. The xylem and phloem are vascular tissues found in complex plants. Their function is to transport water, mineral ions and organic nutrients from roots to leaves and leaves to roots. The plants, however, lack a pump or heart to assist in transporting these molecules.)
Hearts. Hearts are pumps. They are highly muscularized portions of the circulatory system, often with a thin walled atrium, thick walled ventricle and valves to prevent back flow. In animals with one heart, a choice has to be made whether to place it before or after the respiratory organ. If the heart is placed after the respiratory organ (gills) as it is in most invertebrates (e.g., clams lobsters), the blood pressure in the respiratory organ is sacrificed to have a higher blood pressure to the body (systemic circulation). Conversely, if the heart is placed before the respiratory organ as it is in the fish, the higher blood pressure in the gills provides for more efficient gas exchange at the sacrifice of the systemic blood pressure. In the other vertebrates, the oxygenated blood flows back to the heart to be pumped to the entire body (systemic circulation). Since gas exchange is a critical function of a circulatory system, the vertebrates (except the fish) have the heart located before the respiratory organs for maximum flow rate. In cephalopod mollusks (e.g., squid and octopus) there are two gill hearts that pump blood to the gills. The oxygenated blood from the gills flows into another single (systemic) heart which pumps the oxygenated blood to the animal's body.
In mammals and birds (and maybe some reptiles) the heart is actually two hearts side-by-side. The right side of our heart pumps blood to the lungs and the left side collects the blood from the heart and pumps the oxygenated blood to the entire body (systemic circulation). In this way the blood pressure is high for both the pulmonary and systemic circulation.
Our hearts have a thin walled right atrium which collects deoxygenated blood from the body and sends it to the lungs via the pulmonary arteries (with deoxygenated blood). The blood then flows through the capillaries surrounding the alveoli in the lung where CO2 is released and O2 is picked up. The capillaries reform to give rise to the pulmonary vein (with oxygenated blood) which leads back to the left atrium. The blood then flows to the thicker walled left ventricle which pumps the blood under pressure to the body. The freshly oxygenated blood is sent first to the coronary arteries that supply the heart and to the aorta which branches into many arteries which in turn supply blood to all the organs and tissues. The carotid artery to the brain also branches off the aorta near its origin.
When a person has a "coronary" we mean s/he has a block (usually an atherosclerotic plaque) in the coronary arteries that supply the heart with fresh oxygen. When this happens, some of the heart tissue is deprived of oxygen and dies (an MI or myocardial infarction).
In the fetus, the blood comes from the umbilical vein (oxygenated blood from the placenta) to the right atrium. Since the lungs are not functioning yet, they are collapsed and only a small portion of the blood goes to nourish them. In the fetus, most of the blood passes directly from the right atrium to the left atrium through a hole called the foramen ovale, which closes up in time. The blood from the left atrium flows to the left ventricle to be pumped to the fetal systemic circulation. If the foramen ovale remains open after birth, a "blue baby" can result because blood from the left atrium mixes with the blood from the right atrium and overloads the right heart.
In simpler animals there is no heart. Either they use the general body wall muscles (sponges, Cnidaria, flatworms) or muscles of the blood vessels (annelids) to move fluids around the body. These peristalsis like movements cause the blood to flow mostly in one direction...the direction of the wave...but there is back flow unless there are valves.
Blood Vessels. Arteries lead away from the heart and, except in the case of the pulmonary circulation, carry oxygenated blood. They are thick walled with elastic fibers and layers of muscle. Their structure explains how they maintain a high blood pressure. The innermost lining of cells is a one layer thick epithelial tissue called the endothelium. The endothelium continues into the arterioles and finally forms the capillaries. Sphincter muscles at the arteriole-capillary junctions regulate the amount of blood to any organ or tissue. These muscles are controlled by hormones and the nervous system.
The capillaries which are fed by arteries and drained by veins, penetrate to within one or a few cells of every cell in the body. Due to the high blood pressure from the heart and maintained by the arteries, the fluid of the blood is forced out of the capillaries. Everything in the blood except the larger protein molecules and the rbcs can get through the capillary wall, even the amoeboid white blood cells. It is in the capillary beds that the most important events occur. Nutrients and hormones are delivered, wastes picked up, and gases exchanged. As the blood pressure drops, the osmotic pressure increases and the fluid reenters the capillaries.
There is, however, a net loss of fluid and white blood cells in the capillary beds. The lymphatic system, an open circulatory system, supplements our closed circulatory system. The lymph vessels begin in capillary beds and collect into larger vessels that eventually drain into the heart. This system collects excess fluid and proteins (e.g., lymph which is blood minus the rbcs) leaked from the blood into the tissues. It also collects fats from the intestine. Another important role is to bring foreign cells, viruses and other material to the lymph nodes. Lymph nodes are scattered throughout the body. They contain large numbers of lymphocytes, produced in the bone marrow, which fight infections.
Venules are the blood vessels that exit the capillary beds and go on to form veins. The veins are thin walled with a larger bore and with less muscle and elastic fibers than arteries. The flow is slow through them and to prevent back flow, there are valves. The body muscles help keep the blood flowing through the veins. The veins are the final vessels in a closed circulatory system.
In a closed system, as found in the vertebrates and annelids, the blood flows through a continuum of blood vessels. However, in many invertebrates such as the mollusks and arthropods, one finds open circulatory systems. Blood flows through vessels for only part of its path and in part of its path it flows into or out of large sinuses which may bathe the specific organ it reaches. These sinuses serve the same purpose as capillaries and blood may recollect into veins or just return in a haphazard way to the heart. The insects can get by with an open circulatory system because they have the highly efficient tracheal respiratory system that delivers oxygen to their cells.