The Circulatory SystemChapter 49

Blood• Type of connective tissue composed of

– Fluid matrix called plasma– Formed elements

• Functions of circulating blood1.Transportation

2.Regulation

3.Protection

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Blood plasma• 92% water• Contains the following solutes

– Nutrients, wastes, and hormones– Ions

• Na+, Cl–, HCO3, and trace Ca2+, Mg2+, Cu2+, K+, Zn2+

– Proteins• Albumin, alpha () and beta () globulins

• Fibrinogen

– If removed, plasma is called serum

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Formed elements• Red blood cells (erythrocytes)

– About 5 million per microliter of blood

– Hematocrit is the fraction of the total blood volume occupied by red blood cells

– Mature mammalian erythrocytes lack nuclei

– RBCs of vertebrates contain hemoglobin

• Pigment that binds and transports oxygen

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Formed elements• White blood cells (leukocytes)

– Less than 1% of blood cells

– Larger than erythrocytes and have nuclei

– Can migrate out of capillaries into tissue fluid

– Types

• Granular leukocytes– Neutrophils, eosinophils, and

basophils

• Agranular leukocytes– Monocytes and lymphocytes

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Formed elements

• Platelets• Cell fragments that pinch off from larger cells

in the bone marrow• Function in the formation of blood clots

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Prothrombin

Thrombin

Thrombin

Fibrinogen

Fibrin

1. Vessel is damaged, exposing surrounding tissue to blood.

2. Platelets adhere and become sticky, forming a plug.

3. Cascade of enzymatic reactions is triggered by platelets, plasma factors, and damaged tissue.

4. Threads of fibrin trap erythrocytes and form a clot.

5. Once tissue damage is healed, the clot is dissolved.

Formed elements

• All develop from pluripotent stem cells• Hematopoiesis is blood cell production• Occurs in the bone marrow

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Invertebrate Circulatory Systems• Sponges, cnidarians, and nematodes lack a separate

circulatory system• Sponges circulate water using many incurrent pores and

one excurrent pore• Hydra circulate water through a gastrovascular cavity

(also for digestion)• Nematodes are thin enough that the digestive tract can

also be used as a circulatory system

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• Larger animals require a separate circulatory system for nutrient and waste transport

• Open circulatory system– No distinction between circulating and extracellular fluid– Fluid called hemolymph

• Closed circulatory system– Distinct circulatory fluid enclosed in blood vessels and

transported away from and back to the heart

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Invertebrate Circulatory Systems

Vertebrate Circulatory Systems• Fishes

– Evolved a true chamber-pump heart – Have a 2 chambered heart

• Blood is pumped through the gills, and then to the rest of the body

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Vertebrate Circulatory Systems

• Amphibians– Advent of lungs required a

second pumping circuit, or double circulation

– Pulmonary circulation moves blood between the heart and lungs

– Systemic circulation moves blood between the heart and the rest of the body

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Vertebrate Circulatory Systems• Amphibian heart

– 3-chambered heart• 2 atria and 1 ventricle

– Incomplete seperation of two circuits

– Amphibians obtain additional oxygen through their skin

• Reptiles have a septum that partially subdivides the ventricle, further reducing mixing of blood

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Vertebrate Circulatory Systems

• Mammals, birds, and crocodilians– 4-chambered heart– 2 separate atria and 2 separate ventricles– Right atrium receives deoxygenated blood from the body and

delivers it to the right ventricle, which pumps it to the lungs– Left atrium receives oxygenated blood from the lungs and

delivers it to the left ventricle, which pumps it to rest of the body 13

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Evolution of the heart in vertebrates

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Birds

4-chamberheart

2-chamber heart

3-chamberheart

4-chamberheart

CrocodiliansSquamatesTurtlesMammalsAmphibiansFishLancelets

The Cardiac Cycle• Heart has two pairs of valves

– Atrioventricular (AV) valves• Maintain unidirectional blood flow

between atria and ventricles• Tricuspid valve = On the right• Bicuspid, or mitral, valve = On the left

– Semilunar valves

• Ensure one-way flow out of the ventricles to the vessels

• Pulmonary valve located at the exit of the right ventricle

• Aortic valve located at the exit of the left ventricle

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The Cardiac Cycle

• Valves open and close as the heart goes through the cardiac cycle

• Ventricles relaxed and filling (diastole)

• Ventricles contracted and pumping (systole)

• “Lub-dub” sounds heard with stethoscope– Lub – AV valves closing– Dub – closing of semilunar valves

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The Cardiac Cycle

• Heart contains “self-excitable” autorhythmic fibers

• Most important is the sinoatrial (SA) node– Located in wall of right atrium– Acts as pacemaker– Autonomic nervous system can

modulate rate

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The Cardiac Cycle

• Each SA depolarization transmitted– To left atrium– To right atrium and atrioventricular (AV) node

• AV node is only pathway for conduction to ventricles– Spreads through atrioventricular bundle– Purkinje fibers– Directly stimulate the myocardial cells of both

ventricles to contract18

The Cardiac Cycle

• Electrical activity can be recorded on an electrocardiogram (ECG or EKG)

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Characteristics of Blood Vessels

• Blood leaves heart through the arteries

• Arterioles are the finest, microscopic branches of the arterial tree

• Blood from arterioles enters capillaries

• Blood is collected into venules, which lead to larger vessels, veins

• Veins carry blood back to heart

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Characteristics of Blood Vessels• Arteries and veins are composed of four tissue layers

– Endothelium, elastic fibers, smooth muscle, and connective tissue– Walls too thick for exchange of materials across the wall

• Capillaries are composed of only a single layer of endothelial cells– Allow rapid exchange of gases and metabolites between blood and body

cells

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Characteristics of Blood Vessels

• Arteries and arterioles– Larger arteries contain more elastic fibers in their

walls than other blood vessels• Recoil each time they receive blood from the heart

– Contraction of the smooth muscle layer of the arterioles results in vasoconstriction

• Greatly increases resistance and decreases flow• Chronic vasoconstriction can result in hypertension

– Relaxation of the smooth muscle layer results in vasodilation

• Decreasing resistance and increasing blood flow to an organ

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Characteristics of Blood Vessels

• Veins and venules– Thinner layer of

smooth muscles than arteries

– Venous pump helps return blood to heart

• Skeletal muscle contractions and one-way venous valves

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Blood flowstoward heart

Vein

Valveclosed

Contractingskeletalmuscles

Openvalve

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The Lymphatic System

• Significant amount of water and solutes in the blood plasma filter through the walls of the capillaries to form the interstitial (tissue) fluid

• Most fluid leaves at the arteriole end of the capillary and returns at the venule end

• Fluid that does not return to capillaries is returned to circulation by the lymphatic system

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The Lymphatic System

• Consists of lymphatic capillaries, lymphatic vessels, lymph nodes, and lymphatic organs (spleen and thymus)

• Excess fluid in the tissues drains into blind-ended lymph capillaries

• Lymph passes into progressively larger vessels with one-way valves

• Eventually drains into subclavian veins

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Cardiovascular Diseases

• Leading cause of death in the United States• Atherosclerosis

– Accumulation of fatty material within arteries– Impedes blood flow

• Arteriosclerosis – Arterial hardening due to calcium deposition

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a. b. c.2000 mm 2500 mm 1000 mm

a-b: © Ed Reschke; c: © Dr. Gladden Willis/Visuals Unlimited.

Blood Pressure

• Measured as systole/diastole• Systole – ventricle contraction• Diastole – ventricle relaxation

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Blood Flow and Blood Pressure• Blood pressure increases with blood volume

• Blood volume is regulated by four hormones– Antidiuretic hormone (ADH)– Aldosterone – encourages kidney to excrete

postassium and retain sodium– Atrial natriuretic hormone – increases sodium

excretion and decreases blood pressure– Nitric oxide (NO) - vasodialator

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