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The Mammalian Circulatory System The mammalian circulatory system is a vast network of blood-filled vessels that delivers blood to every cell of the organism The circulatory system is a mass flow system, moving blood on mass from one part of an organism to another The circulatory system is essential for ensuring that materials are transported between the various exchange surfaces of the organism The heart, together with the blood vessels and the blood that they contain forms THE CIRCULATORY SYSTEM The capillaries of the blood system are the sites for exchange of materials between the blood and the tissues The capillary networks surrounding the alveoli of the lungs are the sites at which gas exchange takes place The capillary networks within the villi of the small intestine receive and transport the digested food materials needed by the cells of the organism The circulatory system is linked with exchange surfaces

The Human Circulatory System is a Double Circulation The human heart, except during fetal life, is completely divided into right and left sides The LEFT SIDE of the heart receives OXYGENATED BLOOD from the lungs and then delivers it to the body tissues The heart has four chambers with an upper and a lower chamber on each side The upper chambers are the right and left atria (RA and LA), and the lower chambers are the ventricles (RV and LV) The RIGHT SIDE of the heart receives DEOXYGENATED BLOOD from the body tissues and then delivers it to the lungs Blood always enters the heart along veins, which deliver blood to the atria Blood is pumped away from the heart along arteries, which originate in the ventricles LORD

LORD Left side of the heart receives Oxygenated blood and delivers it to the body tissues LORD Right side of the heart receives Deoxygenated blood and delivers it to the lungs

The Human Circulatory System is a Double Circulation The heart pumps oxygenated blood TO the body tissues and receives deoxygenated blood FROM the body tissues The heart pumps deoxygenated blood TO the lungs and receives oxygenated blood FROM the lungs This circuit in which blood flows from the heart TO the body systems and back to the heart is called THE SYSTEMIC CIRCULATION This circuit in which blood flows from the heart TO the lungs and back to the heart is called THE PULMONARY CIRCULATION The human circulation is a double circulation as there are two distinct circuits along which blood flows

Naming The Blood Vessels The CAROTID ARTERY delivers oxygenated blood to the neck, head and brain THE PULMONARY VEIN delivers oxygenated blood from the lungs to the LEFT ATRIUM of the heart The oxygenated blood enters the LEFT VENTRICLE, which pumps the blood along the AORTA towards the body organs Deoxygenated blood enters the right ventricle from where it is pumped to the lungs along the PULMONARY ARTERY Veins carry deoxygenated blood away from the body organs. The veins join to form two large veins called the VENAE CAVAE Smaller arteries branch from the AORTA in order to deliver this oxygenated blood into the various organs The CORONARY ARTERIES deliver oxygenated blood to the heart muscle The VENAE CAVAE deliver deoxygenated blood to the RIGHT ATRIUM of the heart HEPATIC VEIN RENAL The HEPATIC ARTERY delivers oxygenated blood to the liver HEPATIC PORTAL VEIN The liver receives blood from BOTH the hepatic artery AND the HEPATIC PORTAL VEIN The RENAL ARTERIES deliver oxygenated blood to the kidneys The hepatic portal vein carries blood, rich in the products of digestion, from the gut to the liver

Capillaries and Metabolic Exchange The walls of the capillaries are only one cell thick and composed of flattened or squamous epithelium called the endothelium The endothelium rests on a basement membrane Capillaries are referred to as the exchange vessels, since all the exchange of materials between the blood and living cells takes place through their walls The lumen of the capillary is just large enough for red cells to move along in single file Capillaries are so extensive that they permeate between and around all body cells The human body contains about 40,000 kilometres of capillaries which is enough to circumscribe the earth at the equator At the capillaries, metabolites such as oxygen and nutrients diffuse from the blood into the respiring tissues. Metabolites such as carbon dioxide and other waste materials diffuse from the tissues into the blood As blood in the capillaries is always moving, then steep concentration gradients for oxygen, carbon dioxide and nutrients will persist across the capillary walls

Capillaries and Metabolic Exchange Most materials enter and leave the blood by diffusion According to Fick’s Law... Rate of diffusion = surface area x difference in concentration thickness of exchange surface The efficient rates of diffusion between the blood and body cells occur as a result of: the large surface area presented by the capillaries the large differences in concentration of metabolites between the blood and the cells the thinness of the capillary walls

Capillaries and Metabolic Exchange This cross-section of a capillary shows two endothelial cells forming the wall around the lumen The nucleus of one of the endothelial cells (coloured red) is visible whilst the nucleus of the other cell is located in a different plane and hence not visible within this thin section Capillary Cross Section Endothelial cells The two epithelial cells are separated from one another by very narrow gaps or capillary pores through which most small molecules cross the wall by diffusion LUMEN Capillary pore Some larger molecules pass through the endothelial cells by pinocytosis The wall of the capillary rests upon a basement membrane

Capillaries and Tissue Fluid Formation The cells of the body are bathed in a fluid called tissue fluid Tissue fluid is essential for the efficient exchange of materials between the blood and the cells Tissue fluid is formed at the arteriole end of the capillaries CAPILLARY TISSUE FLUID Arteriole end of capillary Venule end BODY CELLS

Summary of Tissue Fluid Formation At the arteriole end of the capillary, the outward hydrostatic pressure is GREATER than the inward osmotic pull Water, ions and small molecules are filtered out of the blood into the spaces between the cells - this is tissue fluid The loss of fluid from the blood leads to a fall in hydrostatic pressure as the blood approaches the venule end of the capillary At the venule end of the capillary, the inward osmotic pull now exceeds the outward hydrostatic pressure and some of the water re-enters the capillary by osmosis Tissue fluid is drained away from the cells by the lymphatic system and returned to the circulation near the heart CAPILLARY TISSUE FLUID Arteriole end of capillary Venule end BODY CELLS Hydrostatic pressure (kPa) 1.6 1.8 4.3

lymphatic cells tissue fluid small artery lymphatic small vein cells arteriole venule capillaries tissue fluid Tissue fluid is: constantly being formed at the arteriole end of capillary beds essential for the efficient exchange of materials between the blood and the cells is constantly being drained away from the cells by lymph vessels

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