The circulatory system is the continuous system of tubes through which the blood is pumped around the body. It supplies the tissues with their requirements and removes waste products.
In mammals and birds, the blood circulates through two separate systems: the first from the heart to the lungs and back to the heart again (the pulmonary circulation), and the second from the heart to the head and body and back again (the systemic circulation).
Arteries, Capillaries, and Veins in Farm Animals
The tubes through which the blood flows are the arteries, capillaries, and veins. The heart pumps blood into arteries that carry it away from the heart. The arteries divide into very thin vessels called capillaries that form a network between the cells of the body. The capillaries then join up again to make veins that return the blood to the heart.
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Structure and Function of Arteries in Farm Animals
Arteries carry blood away from the heart. They have thick elastic walls that stretch and can withstand the surges of high-pressure blood caused by the heartbeat. The arteries divide into smaller vessels called arterioles.
The hole down the center of the artery is called the lumen. The artery walls have three layers of tissue. It is lined with squamous epithelial cells. The middle layer, which is the thickest, consists of elastic fibers and smooth muscle to allow stretching. The outer fibrous layer protects the artery. The pulse is only felt in arteries.
The Pulse in Farm Animals
The pulse is the spurt of high-pressure blood that passes along the aorta and arteries when the left ventricle contracts. As the pulse of blood passes along an artery, the elastic walls stretch. When the pulse has passed, the walls contract, helping to push the blood along.
The pulse is easily felt at certain places where an artery passes near the surface of the body. It is strongest near the heart and becomes weaker as it travels away from the heart. The pulse disappears altogether in the capillaries.
Capillaries and Their Role in Farm Animals
Arterioles divide repeatedly to form a network of capillaries that penetrate the cells of all tissues in the body. These small vessels are called capillaries. The walls of capillaries are only one cell thick, and some capillaries are so narrow that red blood cells have to fold up to pass through them.
Capillaries form networks in tissues called capillary beds. These capillary networks are so dense that no living cell is far from its supply of oxygen and nutrients.
Formation of Tissue Fluid and Lymph in Farm Animals
The thin walls of capillaries allow water, some white blood cells, and many dissolved substances to diffuse through them. These form a clear fluid called tissue fluid (or extracellular fluid or interstitial fluid) that surrounds the cells of the tissues.
The tissue fluid allows oxygen and nutrients to pass from the blood to the cells and carbon dioxide and other waste products to be removed from the tissues.
Some tissue fluid returns to the capillaries, while some flows into the blind-ended lymphatic vessels that form a network in the tissues. Once the tissue fluid enters the lymphatics, it is called lymph, although its composition remains the same.
The lymph vessels have walls that are even thinner than capillaries, allowing larger molecules and particles, such as cancer cells and bacteria, to enter the lymphatic system. These are filtered out as the lymph passes through lymph nodes.
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Veins and Their Function in Farm Animals
Capillaries unite to form larger vessels called venules, which join to form veins. Veins return blood to the heart, and since blood in veins has already passed through capillaries, it flows slowly with no pulse and at low pressure.
For this reason, veins have thinner walls than arteries, though they have the same three layers as arteries. As there is no pulse in veins, blood is squeezed along them by the contraction of skeletal muscles that lie alongside them.
Veins also have valves that prevent blood from flowing backward.
Regulation of Blood Flow in Farm Animals
The flow of blood along arteries, arterioles, and capillaries is not constant but can be controlled depending on the body’s requirements. For example, more blood is directed to the skeletal muscles, brain, or digestive system when they are active.
Regulation of blood flow to the skin’s arterioles is also important in controlling body temperature. The size of the vessels is adjusted by the contraction or relaxation of smooth muscle fibers in their walls.
Oedema and Fluid Loss in Farm Animals
Oedema is the swelling of tissues due to the accumulation of tissue fluid. This may occur because the tissue fluid is prevented from returning to the bloodstream and accumulates in the tissues.
This can be caused by physical inactivity (e.g., long car or plane trips) or imbalances in blood proteins. This is what causes the “pot-belly” of a malnourished child or a worm-infested puppy.
Fluid loss can also be caused by insufficient fluid intake, diarrhea, vomiting, or sudden blood loss due to hemorrhage. The effect is a reduction in blood volume, which decreases blood pressure. This could be dangerous because maintaining constant blood pressure is essential for adequate blood supply to the brain.
To compensate for fluid loss, several mechanisms are activated. First, the blood vessels contract to maintain pressure. Since fluid loss increases the blood’s concentration and osmotic pressure, fluid is drawn into the bloodstream from tissues through osmosis.
The Spleen’s Function in Farm Animals
The spleen is located near the stomach and has a rich blood supply. It acts as a reservoir for red blood cells. In cases of sudden blood loss, such as during a hemorrhage, the spleen contracts and releases large numbers of red blood cells into circulation.
The spleen also destroys old red cells and produces new lymphocytes, although it is not an essential organ, as its removal in adult life causes few problems.
Important Blood Vessels in the Systemic Circulation of Farm Animals
Blood is pumped into the body through the main artery, the aorta, which supplies the head, limbs, and body organs. After passing through a network of fine capillaries, the blood returns to the heart via the largest vein, the vena cava.
Arteries and veins often run alongside each other and have the same name, such as the renal artery and vein serving the kidney, the femoral artery and vein serving the hind limbs, and the subclavian artery and vein serving the forelimbs. Blood to the head passes along the carotid artery and returns to the cranial vena cava via the jugular vein.
A variation in this system is found in the blood vessels serving the digestive tract. Blood from the intestines is carried by the hepatic portal vein to the liver, where digested food is processed.
Blood Pressure in Farm Animals
Blood pressure refers to the pressure of blood against the walls of the main arteries. The pressure is highest as the pulse, produced by the contraction of the left ventricle, passes along the artery (systolic pressure). Pressure is much lower between pulses (diastolic pressure).
Blood pressure is measured in millimeters of mercury. A blood pressure higher than expected is called hypertension, while lower pressure is called hypotension.
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