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CHAPTER 23Circulation
Overview:-Circulatory System-Cardiovascular System-Heart-Blood vessels-Circadian Cycle & ECG-Blood pressure-Blood components
Circulatory Systems & Functions
• Most animals have a circulatory system
– It transports O2 and nutrients to cells
– It takes away CO2 and other wastes
• Every organism must exchange materials with its environment
– The purpose of the circulatory system is to facilitate this exchange
• Capillaries are microscopic blood vessels – They form an intricate network among the tissue
cells
The circulatory system associates intimately with all body tissues
• In jelly and flatworms, the gastrovascular cavity functions in both – digestion – internal transport
Several types of internal transport have evolved in animals
MECHANISMS OF INTERNAL TRANSPORT
• All but the simplest animals have circulatory systems with three main components
– A central pump– A vascular system– The circulating fluid
• Most animals have a separate circulatory system, either open or closed
• Open circulatory system
– The heart pumps blood into large open-ended vessels
– Blood circulates freely among cells
– Many invertebrates, such as mollusks, have open circulatory systems
• Closed circulatory system
– Blood is confined to vessels
– It is distinct from the interstitial fluid
– Earthworms, octopuses, and vertebrates have closed circulatory systems
• The closed circulatory system in vertebrates is called a cardiovascular system
– This system includes the heart and blood vessels
Cardiovascular System
• In the human cardiovascular system
THE HUMAN CARDIOVASCULAR SYSTEM
– The central pump is your heart– The vascular system is your blood vessels– The circulating fluid is your blood
• In humans and other vertebrates, the three components of the cardiovascular system are organized into a double circulation system
The Path of Blood
– There are two distinct circuits of blood flow
• The pulmonary circuit carries blood between the heart and the lungs
• The systemic circuit carries blood between the heart and the rest of the body
Heart- Structure & Function
• The human heart is a muscular organ about the size of a fist
How the Heart Works
– It is located under the breastbone– It has four chambers
• The mammalian heart has two thin-walled atria that pump blood into the ventricles– The thick-walled ventricles pump blood to all other
body organs
Blood vessels – Types & Functions
• If the heart is the body’s “pump,” then the “plumbing” is the system of arteries, veins, and capillaries
Blood Vessels
– Arteries carry blood away from the heart– Veins carry blood toward the heart– Capillaries allow for exchange between the
bloodstream and tissue cells
• All vessels are lined by a thin, smooth epithelium
– Structural differences in the walls of the different kinds of blood vessels correlate with their different functions
• Arteries and veins have smooth muscle and connective tissue
– Valves in veins prevent the backflow of blood
• The walls of capillaries are thin and leaky
– As blood enters a capillary at the arterial end, blood pressure pushes fluid rich in oxygen, nutrients, and other substances into the interstitial fluid
– At the venous end of the capillary, CO2 and other wastes diffuse from tissue cells and into the capillary bloodstream
• The transfer of materials between the blood and interstitial fluid can occur by
– leakage through clefts in the capillary walls– diffusion through the wall– blood pressure– osmotic pressure
• After chemicals are exchanged between the blood and body cells, blood returns to the heart via the veins
Blood Return Through Veins
– By the time blood exits the capillaries and enters the veins, the pressure originating from the heart has dropped to near zero
Circadian Cycle & ECG
• The heart relaxes and contracts regularly
The Cardiac Cycle
– Diastole is the relaxation phase of the heart cycle – Systole is the contraction phase
• Heart valves prevent backflow
• Cardiac output – The amount of blood pumped into the aorta by the
left ventricle per minute
• The pacemaker, or SA (sinoatrial) node, sets the tempo of the heartbeat
The Pacemaker and the Control of Heart Rate
• The pacemaker is composed of specialized muscle tissue in the wall of the right atrium
• The impulses sent by the pacemaker produce electrical currents that can be detected by electrodes placed on the skin
– These are recorded in an electrocardiogram (ECG or EKG)
– Control centers in the brain adjust heart rate to body needs
• In certain kinds of heart disease, the heart’s electrical control fails to maintain a normal rhythm– The remedy is an artificial pacemaker
• A heart attack is damage that occurs when a coronary feeding the heart is blocked
Connection: What is a heart attack?
• How can you avoid becoming a heart disease victim?
– Don’t smoke– Exercise– Eat a heart-healthy diet
• Every year, smoking kills about 430,000 Americans– Many smokers die from lung cancer– Smoking can also cause emphysema
Blood pressure
• The force that blood exerts against the walls of your blood vessels is called blood pressure
Blood Flow Through Arteries
– Blood pressure is the main force driving the blood from the heart to the capillary beds
– A pulse is the rhythmic stretching of the arteries caused by the pressure of blood forced into the arteries during systole
• Blood pressure depends on– cardiac output– resistance of vessels
• Normal blood pressure for adults is below 120 systolic and below 80 diastolic
• High blood pressure is persistent systolic blood pressure higher than 140 and/or diastolic blood pressure higher than 90
– It is also called hypertension
• Pressure is highest in the arteries
– It drops to zero by the time the blood reaches the veins
• Three factors keep blood moving back to the heart
– muscle contractions– breathing– one-way valves
• Blood pressure is measured as systolic and diastolic pressures
Connection: Measuring blood pressure can reveal cardiovascular problems
• Muscular constriction of arterioles and precapillary sphincters controls the flow through capillaries
Smooth muscle controls the distribution of blood
Blood
• The circulatory system of an adult human has about 5 L (11 pints) of blood
Blood
– Just over half of this volume is plasma– Suspended within the plasma are several types of
cellular elements
Red blood cells transport oxygen
• Red blood cells contain hemoglobin – Hemoglobin enables the
transport of O2
• Red blood cells are by far the most numerous type of blood cell– They are also called
erythrocytes
• Each red blood cell contains large amounts of the protein hemoglobin
– Hemoglobin contains iron and transports oxygen throughout the body
– Anemia is an abnormally low amount of hemoglobin or a low amount of red blood cells
White blood cells help defend the body
• White blood cells function both inside and outside the circulatory system – They fight infections and cancer– They are also called leukocytes
– There are about 1,000 times fewer white blood cells than red blood cells
Blood clots plug leaks when blood vessels are injured
• When a blood vessel is damaged, platelets respond– They help trigger the
formation of an insoluble fibrin clot that plugs the leak
• Blood contains two components that aid in clotting– Platelets (thrombocytes) are bits of cytoplasm
pinched off from larger cells in the bone marrow– Fibrinogen is a membrane-wrapped protein found in
plasma
Connection: Stem cells offer a potential cure for leukemia and other blood cell diseases
• All blood cells develop from stem cells in bone marrow– Such cells may prove
valuable for treating certain blood disorders
• New blood cells are continually formed from unspecialized stem cells found in red bone marrow
Stem Cells and the Treatment of Leukemia
– Stem cells differentiate into red and white blood cells and the cells that produce platelets
– Bone marrow stem cells can be isolated and used to treat leukemia
• Leukemia is cancer of the leukocytes
– A person with leukemia has an abnormally high number of leukocytes
– Leukemia is usually fatal unless treated– Not all cases respond to treatment