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The Circulatory System

Circulatory Systems in Single-celled Organisms

Single-celled organisms use their cell surface as a point of exchange with the outside environment.

Sponges are the simplest animals, yet even they have a transport system. Seawater is the medium of transport and is propelled in and out of the sponge by biliary action.

Simple animals, such as the hydra and planarian lack specialized organs such as hearts and blood vessels, instead using their skin as an exchange point for materials. This, however, limits the size an animals can attain. To become larger, they need specialized organs and organ systems.

Circulatory Systems in Multicellular Organisms

Multicellular animals do not have most of their cells in contact with the external environment and so have developed circulatory systems to transport nutrients, oxygen, carbon dioxide and metabolic wastes. Components of the circulatory system include

i. Blood: a connective tissue of liquid plasma and cells

ii. Heart: a muscular pump to move the blood

iii. Blood vessels: arteries, capillaries and veins that deliver blood to all tissues

Vertebrate Cardiovascular System

The vertebrate 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 upper chamber of the heart, the atrium (pl. atria), is where the blood enters the heart. Passing through a valve, blood enters the lower chamber, the ventricle.

Contraction of the ventricle forces blood from the heart through an artery.

The heart muscles is composed of cardiac muscle cells.

Arteries are blood vessels that carry blood away from heart. Arterial walls are able to expand and contract. Arteries have three layers of thick walls. Smooth muscles fibers contract, another layer of connective tissue is quite elastic, allowing the arteries to carry blood under high pressure.

The aorta is the main artery leaving the heart.

The pulmonary artery is the only artery that carries oxygen-poor blood. The pulmonary artery carries deoxygenated blood to the lungs. In the lungs, gas exchange occurs, carbon dioxide diffuses out, oxygen diffuses in

Arterioles are small arteries that connect larger arteries with capillaries. Small arterioles branch into collections of capillaries known as capillary beds. Capillaries, are thin-walled blood vessels in which gas exchange occurs. In the capillary, the wall is only one cell layer thick. Capillaries are concentrated into capillary beds. Some capillaries have small pores between the cells of the capillary wall, allowing materials to flow in and out of capillaries as well as the passage of white blood cells.

Changes in blood pressure also occur in the various vessels of the circulatory system. Nutrients, wastes, and hormones are exchanged across the thin walls of capillaries. Capillaries are microscopic in size, although blushing is one manifestation of blood flow into capillaries. Control of blood flow into capillary beds is done by nerve-controlled sphincters.

The circulatory system functions in the delivery of oxygen, nutrient molecules, and hormones and the removal of carbon dioxide, ammonia and other metabolic wastes. Capillaries are the points of exchange between the blood and surrounding tissues. Materials cross in and out of the capillaries by passing through or between the cells that line the capillary. The extensive network of capillaries in the human body is estimated at between 50,000 and 60,000 miles long. Thoroughfare channels allow blood to bypass a capillary bed. These channels can open and close by the action of muscles that control blood flow through the channels.

Blood leaving the capillary beds flows into a progressively larger series of venues that in turn join to form veins. Veins carry blood from capillaries to the heart.

With the exception of the pulmonary veins, blood in veins is oxygen-poor. The pulmonary veins carry oxygenated blood from lungs back to the heart. Venues are smaller veins that gather blood from capillary beds into veins. Pressure in veins is low, so veins depend on nearby muscular contractions to move blood along. The veins have valves that prevent back-flow of blood Blood pressure:

Ventricular contraction propels blood into arteries under great pressure. Blood pressure is measured in mm of mercury; healthy young adults should have pressure of ventricular systole of 120mm, and 80 mm at ventricular diastole.

Higher pressures (human 120/80 as compared to a 12/1 in lobsters) mean the volume of blood circulates faster (20 seconds in humans, 8 minutes in lobsters).

As blood gets farther from the heart, the pressure likewise decreases. Each contraction of the ventricles sends pressure through the arteries.

Elasticity of lungs helps keep pulmonary pressures low. Systemic pressure is sensed by receptors in the arteries and atria. Nerve messages from these sensors communicate conditions to the medulla in the brain. Signals from the medulla regulate blood pressure.

Diseases of the Heart and Cardiovascular System

Heart Attack

Cardiac muscle cells are serviced by a system of coronary arteries. During exercise the flow through these arteries is up to five time’s normal flow. Blocked flow in coronary arteries can result in death of heart muscle, leading to a heart attack. Blockage of coronary arteries. Is usually the result of gradual buildup of lipids and cholesterol in the inner wall of the coronary artery. Occasional chest pain, angina pectorals, can result during periods of stress or physical exertion. Angina indicates oxygen demands are greater than capacity to deliver it and that a heart attack may occur in the future. Heart muscle cells that die are not replaced since heart muscle cells do not divide. Heart disease and coronary artery disease are the leading causes of death toddy.

Hypertension, high blood pressure (the silent killer), occurs when blood pressure is consistently above 140/90. Causes in most cases are unknown, although stress, obesity, high salt intake, and smoking can add to a genetic predisposition. Luckily, when diagnosed, the condition is usually treatable with medicines and diet/exercise.

The Vascular System

Two main routes for circulation are the pulmonary (to and from the lungs) and the systemic (to and from the body). Pulmonary arteries carry blood from the heart to the lungs. In the lungs gas exchange occurs. Pulmonary veins carry blood from lungs to heart. The aorta is the main artery of systemic circuit. The vena cave are the main veins of the systemic circuit. Coronary arteries deliver oxygenated blood, food, etc. to the heart.

Animals often have a portal system, which begins and ends in capillaries, such as between the digestive tract and the liver. Fish pump blood from the heart to their gills, where gas exchange occurs, and then on to the rest of the body. Mammals pump blood to the lungs for gas exchange, then back to the heart for pumping out to the systemic circulation. Blood flows in only one direction.

Blood is a bright red viscous fluid which flows through ail the vessels except the lymph vessels, it constitutes 8% of the total body weight. Blood is composed of two portions: formed elements(ceil and cell like structures) ad plasma (liquid containing dissolved substances).

Plasma:

• Plasma is the liquid component of the blood. Mammalian blood consists of a liquid (plasma) and a number of cellular and cell fragment components.

• Plasma is about 60 % of a volume of blood; cells and fragments are 40%. Plasma has 90% water and 10% dissolved materials including proteins, glucose, ions, hormones, and gases.

• It acts as a buffer, maintaining pH near 7.4. Plasma contains nutrients, wastes, salts, proteins, etc. Proteins in the blood aid in transport of large molecules such as cholesterol.

• Formed elements:

(a) Red blood cells

• Red blood cells, also known as erythrocytes, are flattened, doubly concave cells about 7 in diameter that carry oxygen associated in the cell's hemoglobin.

• Mature erythrocytes lack a nucleus. They are small, 4 to 6 million cells per cubic millimeter of blood, and have 200 million hemoglobin molecules per cell.

• Humans have a total of 25 trillion red blood cells (about 1/3 of all the cells in the body).

• Red blood cells are continuously manufactured in red marrow of long bones, ribs, skull, and vertebrae.

• Life-span of an erythrocyte is only 120 days, after which they are destroyed in liver and spleen.

• Iron from hemoglobin is recovered and reused by red marrow. The liver degrades the hem units and secretes them as pigment in the bile, responsible for the color of feces.

• Each second two million red blood cells are produced to replace the dead red blood cells.

(b) White Blood Cells

White blood cells, also known as leukocytes, are larger than erythrocytes, have a nucleus, and lack hemoglobin. They function in the cellular immune response. White blood cells (leukocytes) are less than 1% of the blood’s volume. They are made from stem cells in bone marrow.

There are five types of leukocytes, which are important components of the immune system.

• Neutrophils enter the tissue fluid by squeezing through capillary walls and phagocytizing foreign substances

• Macrophages release white blood cell growth factors, causing a population increase for white blood cells.

• Lymphocytes fight infection.

• T-cells attack cells containing viruses.

• B-cells produce antibodies. Antigen- antibody complexes are phagocytized by a macrophage.

White blood cells can squeeze through pores in the capillaries and fight infectious diseases in intestinal areas

(c) Platelets

• Platelets result from cell fragmentation and are involved with dotting.

• Platelets are cell fragments that bud off megakaryocytes in bone marrow. They carry chemicals essential to blood clotting.

• Platelets survive for 10 days before being removed by the liver and spleen.

• There are 150,000 to 300,000 platelets in each milliliter of blood.

• Platelets stick and adhere to tears in blood vessels; they also release clotting factors. A hemophiliac’s blood cannot clot. Providing correct proteins (clotting factors) has been a common method of treating hemophiliacs. St has also led to HIV transmission due to the use of transfusions and use of contaminated blood products.