Circulation

Chapter 32 Pages 617-639

Major Features and Functions of Circulatory Systems

• Circulatory systems evolved to bring the outside world to each cell in a multicellular organism – The earliest cells were nurtured by the primordial sea in

which they evolved– In complex organisms, individual cells are farther away

from the outside world, but require diffusion for adequate nutrients and to ensure they aren’t poisoned by their own waste

– With the evolution of the circulatory system, a sort of “internal sea” was created, which transports food and oxygen close to each cell and carries away wastes

All circulatory systems have three major parts

– A pump, the heart, that circulating

– Blood – liquid that serves as a medium of transport

– A system of tubes, blood vessels, to conduct the blood throughout the body

Two types of circulatory systems

• Open circulatory systems - invertebrates, including arthropods and mollusks

– One or more simple hearts, network of vessels, and series of interconnected spaces within the body called a hemocoel

– Tissues and organs in the hemocoel are directly bathed by hemolymph - acts as both blood and the extracellular fluid that bathes all cells

Insect Example

• Heart is a modified blood vessel with a series of contracting chambers

• When chambers contract, valves in the heart are pressed shut, forcing the hemolymph out through vessels and into hemocoel spaces throughout the body

• When the chambers relax, blood is drawn back into them from the hemocoel

Closed Circulatory Systems

• Invertebrates - earthworm and active mollusks (squid and octopuses) and all vertebrates – Blood is confined to heart and

blood vessels, which branch throughout the organs and tissues of the body • more rapid blood flow• more efficient transport of

dissolved substances• higher blood pressure than in

open systems

Functions of Vertebrate Circulatory System

• Transport O2 from lungs or gills to tissues

• Transport CO2 from tissues to lungs or gills

• Distribution of nutrients from the digestive system to

body cells

• Transport of wastes and toxic substances to the liver, where they are detoxified, and to the kidneys for excretion

• Distribution of hormones from the glands and organs to the tissues

• Regulation of body temperature by adjustments in blood flow

• Wound healing and blood clotting to prevent blood loss

• Protection against disease by circulating white blood cells and antibodies

Vertebrate Heart

• The vertebrate heart consists of muscular chambers capable of strong contractions

– Chambers called atria collect blood – Atrial contractions send blood into ventricles,

chambers whose contractions circulate blood through the lungs and to the rest of the body

Evolution of the Vertebrate Heart

• Increasingly complex and efficient hearts – The heart has become increasingly complex

– Separation of oxygenated and deoxygenated blood

– Fish (first vertebrates to evolve) has two chambers: a single atrium that empties into a single ventricle • Blood from the ventricle passes first through the gills, where

it picks up O2 and gives off CO2 • Blood then travels from the gills through the rest of the body,

picking up CO2 and returning it to the single atrium

(a) Fish

gill capillaries

body capillaries

ventricle

atrium

Fish Heart

Three Chambered Hearts

• Fish gave rise to amphibians and amphibians to reptiles • Three-chambered hearts consist of two atria and one ventricle • Amphibians, snakes, lizards, and turtles• Deoxygenated blood from the body is delivered to the right

atrium, blood from the lungs enters the left atrium • Both atria empty into the single ventricle • Although some mixing occurs, deoxygenated blood remains in

the right portion of the ventricle and is pumped into vessels that lead to the lungs, while most of the oxygenated blood remains in the left portion of the ventricle and is pumped to the rest of the body

(b) Amphibians and somereptiles

lung capillaries

atria

ventricle

body capillaries

Three Chambered Heart

Four Chambered Hearts

• Some reptiles - crocodiles, birds, and mammals have separate right and left ventricles

• Completely isolate oxygenated and deoxygenated blood

(c) Mammals, crocodiles,and birds

lung capillaries

body capillaries

atria

ventricles

Four Chambered Heart

Four Chambers – Two Pumps• An atrium collects the blood before passing it to a ventricle

which propels it into the body

• One pump, the right atrium and ventricle, deals with deoxygenated blood – Oxygen-depleted blood enters the right atrium through

two large veins - the superior and inferior vena cava – After filling with blood, the right atrium contracts, forcing

blood into the right ventricle – Contraction of the right ventricle sends the oxygen-

depleted blood to the lungs through the pulmonary arteries

Two Pumps, part II

• The second pump, the left atrium and ventricle, deals with oxygenated blood – Oxygen-rich blood from the lungs enters the left atrium

through the pulmonary veins and is squeezed into the left ventricle

– Contraction of the left ventricle sends the oxygenated blood through the aorta to the rest of the body

Heart Valves• Maintain the direction of blood flow

– When the ventricles contract, blood must be prevented from flowing back into the atria • Blood entering the arteries must also be prevented from

flowing back into the ventricles as the heart relaxes – Pressure in one direction opens valves easily, but reverse

pressure forces valves closed • Atrioventricular valves blood flows from atria into the

ventricles • Semilunar valves blood enters the pulmonary artery and

aorta when ventricles contract, but prevent blood from returning as the ventricles relax

Cardiac Muscle Cells

• Cardiac muscle cells are small, branched, and striated – Linked to one another via intercalated discs, appear as

bands between the cells

– Adjacent cell membranes are attached to one another by desmosomes, prevent heart contractions from pulling muscle cells apart

– Intercalated discs also contain gap junctions to allow the electrical signals that trigger contractions to spread from one muscle cell to another, producing synchronous heart muscle contractions

Cardiac Cycle

• The coordinated contractions of atria and ventricles produce the cardiac cycle

– The heart beats in a coordinated fashion • Both atria contract and pump blood into the ventricles • Both ventricles contract and pump blood into the

arteries that exit the heart • All chambers relax briefly before the cycle repeats

– This cardiac cycle lasts less than 1 second Cardiac Cycle

The Cardiac Cycle

Atria contract, forcingblood into the ventricles

Then the ventriclescontract, forcing bloodthrough the arteries tothe lungs and the restof the body

The cycle ends asthe heart relaxes

Deoxygenated blood ispumped to the lungs

Blood fills theatria and beginsto flow passivelyinto the ventricles

Deoxygenatedblood from thebody enters theright ventricle

Oxygenated blood from thelungs enters the left ventricle

Oxygenated bloodis pumped to thebody

321

Blood Pressure

• The cardiac cycle generates the forces that are measured when blood pressure is taken

• Systolic pressure, the higher of the two readings, is measured during ventricular contraction

• Diastolic pressure is the minimum pressure in the arteries as the heart rests between contractions

– A BP reading of less than 120/80 is healthy; higher than 140/90 is defined as high

– High blood pressure, or hypertension, is caused by the constriction of small arteries, which causes resistance to blood flow and strain on the heart

Electrical Impulses Coordinate the Contractions

• The contraction of the heart is initiated and coordinated by a pacemaker, a cluster of specialized muscle cells that produce spontaneous electrical signals at a regular rate

• The heart’s pacemaker is the sinoatrial (SA) node, located in the upper wall of the right atrium

• Electrical signals from the SA node pass freely into the connecting cardiac muscle cells and then throughout the atria

• The electrical signal then passes from the right atrium to a specialized group of muscle cells between the right atrium and right ventricle called the atrioventricular (AV) node

• The signal to contract spreads along specialized tracts of rapidly conducting muscle fibers called the atrioventricular bundle, which sends branches to the lower portion of both ventricles

• Here, the bundles branch further, forming Purkinje fibers that transmit the electrical signal throughout the ventricle

Inexcitable tissueseparates the atria and ventriclesAV node

SA node

AV bundle

AV bundlebranches

An electrical signalfrom the sinoatrial (SA)node starts atrialcontraction

1

The signal entersthe atrioventricular(AV) node, whichtransmits it to theAV bundle with aslight delay

3

The signal travelsthrough the AV bundlebranches to the baseof the ventricles

4

Purkinje fibers transmitthe signal to ventricularcardiac muscle cells,causing contraction fromthe base upwards

5

The electricalsignal spreadsthrough the atria,causing them tocontract

2

Purkinjefibers

The Pacemaker and Its Connections

Disorders

– When the pacemaker fails, rapid, uncoordinated, weak contractions called fibrillation may occur

• Treated with a defibrillating machine, which applies a

jolt of electricity, synchronizing the contractions of the ventricular muscle cells, and the pacemaker resumes its normal coordinating function

Heart Rate

• Influenced by nervous system and hormones – On its own, the SA node pacemaker maintains a heart rate

of 100 beats per minute

– Nerve impulses and hormones alter the heart rate • At rest, the parasympathetic nervous system slows the

heart rate to about 70 beats per minute • During exercise and stress, the sympathetic nervous

system increases the heart rate to meet the demand for greater blood flow to the muscles

What Is Blood?

• Blood has two major components

– A liquid or plasma, 55% of total volume

– The cellular portion, 40–45% of total volume • Red blood cells • White blood cells • Platelets

Plasma • Primarily water with proteins, salts, nutrients, and

wastes – 90% water, it contains > 100 different molecules, including

hormones, nutrients, cellular wastes, ions – Proteins are the most abundant dissolved molecules by

weight and include: • Albumin, helps maintain the blood’s osmotic strength • Globulins, antibodies that play an important part in

immune response • Fibrinogen, important in blood clotting

Cellular Components of Blood• Formed in bone marrow

– Of the 3 cell-based components - only the white blood cells are complete, functional cells • Mature RBCs are not cells because they lack a nucleus,

which is lost during development • Platelets are small fragments of cells

– All 3 components originate from blood stem cells which reside in the bone marrow • Stem cells are unspecialized cells that can divide to produce

offspring capable of maturing into one or more types of specialized cells

• Megakaryocyte

Red Blood Cells

• Carry oxygen from the lungs to the tissues – 99% of all blood cells, and 45% of the total volume– Oxygen-carrying red blood cells or erythrocytes – The red color of erythrocytes is caused by the

protein hemoglobin, each hemoglobin binds 4 oxygen molecules, one on each iron-containing heme group• Oxygenated hemoglobin is bright cherry-red color • Hemoglobin becomes bluish as it releases O2 and picks

up CO2 at tissues

Hemoglobin

Red Blood Cells

– Life span of 4 months, replaced by new cells from the bone marrow

– Macrophages (white blood cells) in spleen and liver engulf and break down dead red blood cells

– Iron from erythrocytes is returned to the bone marrow and recycled into new red blood cells

Regulated by Negative Feedback

• Red blood cell count is maintained by a negative feedback system that involves the hormone erythropoietin – Erythropoietin is produced by the kidneys and released

into the blood in response to oxygen deficiency • Stimulates rapid production of new red blood cells by

the bone marrow • When the oxygen level is restored, erythropoietin

production declines and the rate of red blood cell production returns to normal

Oxygen deficiency

Erythropoietinproduction

by the kidneys

Red bloodcell production

in the bone marrow

Restored oxygen level

inhibits

stimulates

causes

stimulates

Red Blood Cell Regulation

White Blood Cells

• Defend the body against disease – Five types of white blood cells or leukocytes• Neutrophils • Eosinophils • Basophils • Lymphocytes • Monocytes

WBC Details

• Cell life spans range from hours to years

• <1% of the cellular portion of blood

• All WBC help to protect the body against disease

• Monocytes, enter tissues and transform into macrophages that engulf bacteria and cellular debris

Platelets

• Cell fragments that aid in blood clotting – Pieces of megakaryocytes, reside in bone marrow • Megakaryocytes pinch off membrane-enclosed pieces

of cytoplasm to form platelets, which enter the blood and play a role in blood clotting • Platelets survive for about 10 days

Blood Clotting

• Blood clotting plugs damaged blood vessels • Complex process that plugs damaged blood vessels

and protects excessive blood loss

How it works… – Clotting begins following a break in a blood vessel wall,

exposing collagen fibers that attract platelets, which form a platelet plug

– The platelets and ruptured cells release chemicals that initiate a series of reactions, producing the enzyme thrombin from its inactive form, prothrombin

– Clotting begins following a break in a blood vessel wall, exposing collagen fibers that attract platelets, which form a platelet plug

– The platelets and ruptured cells release chemicals that initiate a series of reactions, producing the enzyme thrombin from its inactive form, prothrombin

Blood Clotting

collagenfibers

prothrombin fibrinogenthrombin

thrombinredbloodcells

bloodvessel

plateletsplateletplug

fibrin

Damaged cells exposecollagen, which activatesplatelets, causing them tostick and form a plug

1 Both damaged cellsand activated plateletsrelease chemicals thatconvert prothrombininto the enzyme thrombin

2 Thrombin catalyzes theconversion of fibrinogeninto protein fibers calledfibrin, which forms ameshwork around theplatelets and traps redblood cells

3

Functions of Blood Vessels

• Arteries to arterioles to capillaries, then into venules, to veins, blood returns to the heart – Except for capillaries, blood vessels have three

cellular layers• Lined with endothelial cells • The second layer is smooth muscle cells • The outermost layer is connective tissue

precapillarysphincter

arteriole

venule

veinartery

capillary

to heartfrom heart

endotheliumvalve

smooth muscleconnective tissue

capillary networkwithin body tissues

Structures of Blood Vessels

jugular vein

aorta

superiorvena cava

carotid artery

lung capillariespulmonary artery

heart

kidney

femoral vein

intestine

inferiorvena cava

liver

femoral artery

The Human Circulatory System

Types and Functions of Blood Vessels

• Arteries and arterioles carry blood away from the heart – The walls are thicker and more elastic than those

of veins – With each heart beat, the arteries expand slightly,

like thick-walled balloons – Arteries branch into smaller diameter vessels

called arterioles, which play a major role in determining how blood is distributed in the body

Arteries and Arterioles

• Arteries and arterioles carry blood away from the heart – The walls are thicker and more elastic than veins – With each heart beat, the arteries expand slightly,

like thick-walled balloons – Arteries branch into smaller diameter vessels

called arterioles, which play a major role in determining how blood is distributed in the body

Capillaries

• Exchange of nutrients and wastes – Arterioles conduct blood into networks of capillaries,

microscopically thin vessels – Capillaries allow individual body cells to exchange

nutrients and wastes with the blood by diffusion • So numerous that most of the body’s cells are no more

than 100 μm from a capillary, close enough for diffusion• Capillaries are so narrow that red blood cells pass

through them single file

Leaky Blood Vessels

– Blood pressure within capillaries causes fluid to leak into the space surrounding the capillaries

– Resulting in extracellular fluid, resembles plasma without the proteins • Primarily water containing dissolved nutrients,

hormones, gases, cellular waste, and WBC• This fluid provides body cells with nutrients and

accepts their wastes

How to Diffuse thru Capillaries

– Gases, water, lipid-soluble hormones and fatty acids diffuse through the endothelial cell membranes

– Small water-soluble nutrients, (salts, glucose, and amino acids) enter the extracellular fluid through narrow spaces between adjacent capillary cells

– Some proteins are carried across the endothelial

cell membrane as vesicles

Osmotic Pressure and Capillaries– Pressure within the capillaries drops as blood travels

toward the venules, and the high osmotic pressure of the blood that remains inside the capillaries draws water back into the vessels by osmosis as blood approaches the venous side of the capillaries

– As water enters the capillaries (diluting the blood), dissolved substances in the extracellular fluid tend to diffuse back into the capillaries

– Thus, most of the extracellular fluid is restored to the blood through the capillary walls on the venous side of the capillary network

Veins and Venules

• Carry blood to the heart

• After picking up CO2 and wastes from cells, capillary blood drains into larger vessels, called venules, which empty into larger veins – Walls of veins are thinner, less muscular, and more

expandable than arteries – When veins are compressed, one-way valves keep blood

flowing toward the heart

valveclosed

valveclosed

valveopen

relaxedmuscle

muscle contractioncompresses vein

Valves Direct Blood Flow in Veins

Moving Blood thru Veins

– Pressure changes in the body caused by breathing, as well as contractions of skeletal muscle during exercise, help return blood to the heart by squeezing the veins and forcing blood through them

– Prolonged sitting or standing can cause swollen ankles, without muscle contractions to compress the veins, venous blood pools in the lower legs

– Varicose veins may result from permanently swollen veins in the lower leg as a result of stretched and weakened vein valves

Controlling Blood Flow

• Arterioles carry blood to capillaries; their muscular walls are influenced by nerves, hormones and chemicals

• Arterioles contract and relax in response to the needs of the tissues and organs they supply

Examples of Arteriole Control

– In cold weather, fingers and toes can become frostbitten because arterioles that supply blood to the extremities constrict • Blood is shunted to vital organs (heart and brain) which

cannot function at low temperatures

– On a hot summer day, arterioles in the skin expand to bring more blood to the skin capillaries, so excess heat is dissipated to the air outside

Precapillary Spincters

• Blood flow to capillaries is further regulated by tiny rings of smooth muscle called precapillary sphincters

• They surround junctions between arterioles and capillaries

• Open and close in response to local chemical changes that signal the needs of nearby tissues

The Lymphatic System

• Includes organs and a system of lymphatic vessels, feeds into the circulatory system – Return excess extracellular fluid to the bloodstream

– Transport fats from the small intestine to the bloodstream

– Filter old blood cells and other debris from the blood – Defend the body by exposing bacteria and viruses to white

blood cells

thymus

superiorvena cava

spleen

bonemarrow

thoracicduct

lymph vessels

lymph nodes

The thoracic ductenters a vein thatleads to the superiorvena cava

The Human Lymphatic System

Lymphatic Vessels

• Lymphatic capillaries resemble blood capillaries that branch throughout the body.

• Their walls are only one cell thick, but they are more permeable than blood capillaries

• Unlike blood capillaries (form continuous interconnected network) lymphatic capillaries “dead-end” in the extracellular fluid surrounding body cells

Lymph Capillary Structure

lymphcapillary

extracellularfluid

Pressure forces fluid from the plasmaat the arteriole end of the capillary network

Extracellular fluid enters lymph vessels and the venous endsof capillaries

Lymph is transported into larger lymph vessels and back to the bloodstream

arteriole

capillary venule

1

2

3

Lymph

• From the lymphatic capillaries, the lymph lymphatic vessels increasingly large lymphatic vessels – Vessels resemble veins -

similar walls and one-way valves that control the direction of flow

– Flow of lymph is regulated by internal pressures from breathing and muscle contraction

Transporting Fat from Small Intestine

• The small intestine is supplied with lymph capillaries called lacteals

• After absorbing digested fats, intestinal cells release fat-transporting particles into the extracellular fluid

• These particles are too large to diffuse into blood capillaries, but can move through the openings in lymphatic capillary walls

• They are eventually released into the venous blood along with the lymph

Defend Body and Filter Blood

• Tonsils, thymus, spleen, and hundreds of lymph nodes located along lymphatic vessels

• Spleen, located between the stomach and diaphragm and supplied by vessels of both the lymphatic and circulatory systems, filters the blood – It has a porous interior that is lined with white blood cells,

which engulf old red blood cells and platelets, fragments of dead cells, and foreign matter