Subject - General Science
Chapter : 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 ciliary action. Simple animals, such as the hydra and planaria 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 animal 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.

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

Types of circulatory systems

The open circulatory system

The open circulatory system, examples molluscs and arthropods. Open circulatory systems (evolved in insects, mollusks and other invertebrates) pump blood into a hemocoel with the blood diffusing back to the circulatory system between cells. Blood is pumped by a heart into the body cavities, where tissues are surrounded by the blood. The resulting blood flow is sluggish.

Closed circulatory system

Vertebrates, and a few invertebrates, have a closed circulatory system. Closed circulatory systems (evolved in echinoderms and vertebrates) have the blood closed at all times within vessels of different size and wall thickness. In this type of system, blood is pumped by a heart through vessels, and does not normally fill body cavities. Blood flow is not sluggish. Hemoglobin causes vertebrate blood to turn red in the presence of oxygen; but more importantly hemoglobin molecules in blood cells transport oxygen. The human closed circulatory system is sometimes called the cardiovascular system. The lymphatic circulation, which is also secondary circulatory system collects fluid and cells and returns them to the cardiovascular system.

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.

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

• The heart muscle 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 muscle fibers contract, another layer of connective tissue is quite elastic, allowing the arteries to carry blood under high pressure

• 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

• 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).

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.

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The Heart

The heart is a muscular structure that contracts in a rhythmic pattern to pump blood. Hearts have a variety of forms:

• Chambered hearts in mollusks and vertebrates

• Tubular hearts of arthropods, and aortic arches of annelids.

• Accessory hearts are used by insects to boost or supplement the main heart’s actions.

• Fish, reptiles, and amphibians have lymph hearts that help pump lymph back into veins.

• The basic vertebrate heart, such as in has two chambers. An auricle is the chamber of the heart where blood is received from the body. A ventricle pumps the blood it gets through a valve from the auricle out to the gills through an artery.

The Human Heart

• The human heart is a two-sided, four-chambered structure with muscular walls. An atrioventricular (AV) valve separates each auricle from ventricle. A semilunar (also known as arterial) valve separates each ventricle from its connecting artery.

• The heart beats or contracts approximately 70 times per minute. The human heart will undergo over 3 billion contraction cycles during a normal lifetime.

The cardiac cycle

The cardiac cycle consists of two parts: systole (contraction of the heart muscle) and diastole (relaxation of the heart muscle). Atria contract while ventricles relax. The pulse is a wave of contraction transmitted along the arteries. Valves in the heart open and close during the cardiac cycle. Heart muscle contraction is due to the presence of nodal tissue in two regions of the heart.

Blood

Blood is a bright red viscous fluid which flows through all the vessels except the lymph vessels. It constitutes 8% of the total body weight. Blood is composed of two portions: formed elements(cell 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.

Red blood cells

• Red blood cells, also known as erythrocytes, are flattened, doubly concave cells about 7 µm 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.

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.
  (a) Neutrophils enter the tissue fluid by squeezing through capillary walls and phagocytozing foreign substances
  (b) Macrophages release white blood cell growth factors, causing a population increase for white blood cells.
  (c) Lymphocytes fight infection.
  (d) T-cells attack cells containing viruses.
  (e) 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

Platelets:

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

• 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.

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