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