The Circulatory System

Jenny McInerney

Circulation video

The Need for a Circulatory System

Organisms need to exchange materials with the external environmentThis must be done on a cellular levelIn most multicelllular organisms, direct exchange with the environment is not possibleDiffusion is inefficient in large organisms

The Need for a Circulatory System cont’d

The circulatory system solves this problem by providing a link between the tissues of a body and the organs that exchange gases, absorb nutrients, & dispose of wastesThe circulatory system solves the diffusion problem by taking substances where they need to go so that the distance they have to diffuse into or out of a cell is never very great

Invertebrate Circulation: Gastrovascular Cavities

Hydras & other cnidarians don’t need a true circulatory system due to the simplicity of their body plan

Instead, they have a gastrovascular cavity: a body cavity filled with fluid that is continuous with the external environment that both digests and distributes nutrients

Figure 42.2

Circularcanal

Radial canal5 cm

Mouth

Invertebrate Circulation: Definition of a Circulatory System

More complex organisms require at least some kind of circulatory systemA ciculatory system has 3 main parts: a circulatory fluid (blood), a set of tubes (blood vessels), and a muscular pump (a heart)The heart circulates blood by increasing the hydrostatic pressure of the blood on the blood vessels, causing it to move down a pressure gradient through a circuit and back to the heart

Invertebrate Circulation: Open Circulatory Systems

Found in insects, arthropods, and most molluscsOne or more hearts pump a fluid called hemolymph into a network of sinuses - body cavities where the hemolymph directly bathes the organs - and then draw the blood back through pores called ostiaNo distinction between blood and interstitial fluidEvolutionary advantages: lower hydrostatic pressure requires less energy to run; lack of extensive blood vessels requires less energy to build and maintain; can serve as hydrostatic skeleton in molluscs

Heart

Hemolymph in sinusessurrounding ograns

Anterior vessel

Tubular heart

Lateral vessels

Ostia

Invertebrate Circulation/Vertebrate Circulation: CLosed Circulatory Systems

One or more hearts pump blood through a network of blood vessels, and nutrients diffuse through capillary beds into the interstitial fluidBlood is distinct from interstitial fluid and is confined to blood vesselsFound in earthworms, squids and octopuses, and all vertebratesEvolutionary advantages: more effective at meeting the metabolic requirements of larger and more complex animals

Interstitialfluid

Heart

Small branch vessels in each organ

Dorsal vessel(main heart)

Ventral vesselsAuxiliary hearts

An Overview of Vertebrate Circulation

Heart comprised of one or two atria (singular, atrium) connected to one or two ventriclesThree kinds of blood vessels: arteries (carry blood away from heart), veins (carry blood to heart), and capillaries (sites of diffusion between blood and interstitial fluid)Blood vessels classified based on blood direction, not content

An Overview of Vertebrate Circulation cont’d

Ateries branch out to become aterioles

Venules converge to become veins

The circulatory systems of the different taxa are variations on this theme

Fish

2-chambered heart

Single curcuit of blood flow

Amphibians

3-chambered heart Double circulation (blood gets pumped again after getting oxygen form the lungs or skin)Mixing of oxygen-rich and oxygen-poor blood in the single central ventricle2 circuits of blood flow

Pulmocutaneous (to the lungs/skin)Systemic (to the rest of the body)

Reptiles (excluding birds)

3-chambered heartDouble circulationPresence of s septum in the middle of the ventricle decreases mixing of oxygen-rich and oxygen-poor blood2 circuits of blood flow

Pulmonary (to lungs)Systemic

Mammals

4-chambered heartDouble circulationNo mixing of blood - two separate ventricles2 circuits of blood flow

PulmonarySystemic

Pathway of Mammalian Circulation

Right ventriclePulmonary arteryCapillary beds in lungsLeft atriumLeft ventricleAortaArterioles and capillaries of head and forelimbs(at same time as) arterioles and capillaries of trunk and hind limbsAnterior (superior) vena cava(at same time as) posterior (inferior) vena cavaRight atriumRight ventricleAnd so on . . .

Pulmonary vein

Right atrium

Right ventricle

Posteriorvena cava Capillaries of

abdominal organsand hind limbs

Aorta

Left ventricle

Left atriumPulmonary vein

Pulmonaryartery

Capillariesof left lung

Capillaries ofhead and forelimbs

Anteriorvena cava

Pulmonaryartery

Capillariesof right lung

Aorta

1

10

11

5

4

6

2

9

33

7

8

A Closer Look At the Mammalian Heart

The heart contracts and relaxes in a rhythmic cycle: it pumps blood on the contractions and fills with blood on the relaxations

The contraction phase is called the systole

The relaxation phase is called the diastole

One complete sequence of pumping and filling = the cardiac cycle

A Closer Look at the Mammalian Heart cont’d

Cardiac output: volume of blood the left ventricle pumps into the systemic circuit in a minute

Based on heart rate (in beats per minute)

And on stroke volume - the amount of blood the left ventricle pumps in each contraction

A Closer Look at the Mammalian Heart cont’d

Sets of valves prevent the blood from leaking into the wrong place at the wrong timeAtrioventricular (AV) valves located inbetween each atrium and its connected ventricle are forced shut with each contraction so that the blood can’t leak from the ventricle back into the atriumSemilunar valves located at the exits of each ventricle (the pulmonary artery and the aorta) are forced shut with each relaxation so that the blood can’t leak back from the blood vessel into the ventricle

Semilunarvalve

Atrioventricularvalve

Semilunarvalve

Atrioventricularvalve

Maintaing the Heartbeat

Sinoatrial (SA) node/pacemaker: a region of specialized tissue within the heart that controls the rate and timing of all cardial muscle cell contractions

Cardial muscle cells are self-excitable - they contract without direction from the central nervous system, and each have their own intrinsic pulse/rhythmn - the SA node just makes sure they all beat together

Maintaing the Heartbeat cont’d

Because the pacemaker is located within the heart, vertebrate heats can be called myogenic

Invertebrate hearts are called neurogenic because their pacemakers are located in motor nerves outside the heart

Maintaining the Heartbeat cont’d

Signals pass from the SA node to the atrioventricular (AV) node, which delays them for .1 of a second so that the atria can completely drainThen they are passed to bundle bunches and Purkinje fibers at the apex of the heart that force the ventricles to contract

SA node(pacemaker)

AV node Bundlebranches Heart

apexPurkinjefibers

2 Signals are delayedat AV node.

1 Pacemaker generates wave of signals to contract.

3 Signals passto heart apex.

4Signals spreadThroughoutventricles.

ECG

Blood Vessel Structure and FunctionAteries and veins are composed of 3 layers

Outer layer (connecting, elastic muscle), middle layer (smooth, elastic muscle), inner layer (endothelium, smooth, flat cells designed to reduce resistance to blood flow)

Arteries have thick, elastic walls because they handle the most blood pressure; veins have thinner walls to conduct blood back to the heart at low velocity and pressure, and capillaries have only two thin walls because they are the sites of diffusion between the blood and the interstitial fluid

Artery Vein

100 µm

Artery Vein

ArterioleVenule

Connectivetissue

Smoothmuscle

Endothelium

Connectivetissue

Smoothmuscle

EndotheliumValve

Endothelium

Basementmembrane

Capillary

Blood Velocity

Blood velocity is governed by the law of continuity, which is used to describe the movement of fluid through pipes

As pipe diameter decreases, fluid velocity increases

However, velocity in the capillaries is over 1,000 times greater than that in the capillaries - why?Diameter is measured in total cross-sectional area - the total cross sectional area of the capillaries is greater than that of any other single vessel in the circulatory system

Blood Pressure

Fluids exert pressure on the surfaces they contact; this pressure is what drives fluids through pipes

Fluids flow from areas of high pressure to low pressure

Blood Pressure cont’d

Blood pressure is higher in arteries than in veins, and is highest during ventrical systole (systolic pressure)

Blood pressure is lowest during ventrical diastole (diastolic pressure)

Blood Pressure cont’d

Peripheral resistance: the sudden decrease of velocity that occurs when the arteries hit the arteriolesPressure builds up as the heart continues to exert pressure of the blood to move forward, but it cannot proceed through the smaller arterioles quickly enough to relieve the pressure on the arteries

Blood Pressure cont’d

Blood pressure is determined by cardiac output and peripheral resistance (measured in systolic pressure over diastolic pressure)

Blood Pressure cont’d

Cardiac output increases to maintain blood pressure when arterioles dilate to allow for increased blood flow during heavy exercise

Gravity also affects blood pressure - it takes extra force to pump blood above the level of the heart to the head and brain

Blood Pressure cont’d

By the time blood gets to the veins, most of the blood pressure has dissipated in the arterioles and capillaries

Blood in the veins is moved partly by contractions of the smooth muscle in the vein walls and mostly by skeletal muscle contractions during physical activity which serve to squeeze the blood through the veins

Capillary FunctionBlood can be diverted to wherever in the body it is needed mostTwo mechanisms regulate the distribution of blood to the capillaries

Contraction or relaxation of the smooth muscles of the arterioles to constrict or increase blood flow to the capillariesPrecapillary sphincters: rings of smooth muscle located at the entrance to the capillaries that can cut off or redirect blood flow

Precapillary sphincters Thoroughfarechannel

ArterioleCapillaries

Venule

VenuleArteriole

20 m

(a) Sphincters relaxed

(b) Sphincters contracted

(c) Capillaries and larger vessels (SEM)

Capillary Function cont’d

Transfer of substances from blood to interstitial fluid can occur through

Simple diffusion through the endomethial cells or through the clefts between adjoining cells

Or through the movement of vesicle made by endocytosis on one side of the cells and exocytosis on the other side

Capillary Function cont’dBlood pressure forces fluid through the capillary clefts and out of the capillaries at the arterial end of the capillary bed, resulting in a net loss of fluid on that end of the capillary bedLarge solutes and plasma proteins remain in the capillaries to create a relatively constant osmotic pressure, while the blood pressure decreases substantially at the venule end of the capillary bedThis causes ~ 85% of the lost fluid to flow back into the capillaries from the interstitial fluid at the end of the capillary bed

At the arterial end of acapillary, blood pressure is

greater than osmotic pressure,and fluid flows out of the

capillary into the interstitial fluid.

Capillary Redbloodcell

15 m

Tissue cell INTERSTITIAL FLUID

CapillaryNet fluidmovement out

Net fluidmovement in

Direction of blood flow

Blood pressureOsmotic pressure

Inward flow

Outward flowPre

ssur

e

Arterial end of capillary Venule end

At the venule end of a capillary, blood pressure is less than osmotic pressure, and fluid flows from the interstitial fluid into the capillary.

Figure 42.14

Fluid Return by the Lymphatic System

Fluid enters the lymphatic system through tiny lymphatic capillaries interspersed throughout the interstitial fluid alongside the capillary bedsOnce it enters the lymphatic system, it becomes known as lymph, though there is little difference in composition between it and the interstitial fluidLymph is conducted through the lymphatic system the same way blood is conducted through the veins - muscle contractionsThe lymphatic system drains into the circulatory system near the junction of the venae cavae with the right atrium, rthius returning the remaining 15% of fluid to the circulatory system

Blood Composition and FunctionBlood consists of several kinds of cells - erythrocytes, leukocytes, and platelets - suspended in a plasma matrixThe cells can be separated from the plasma by using a centrifuge to spin out a sample of bloodWhen the sample is done spinning, the cells will settle to the bottom in a dense red pellet while the plasma will float above it - transparent and straw-coloured

Plasma

Composed of 90% water and various other solutesDissolved ions - electrolytes - collectively act as buffers and help maintain the osmotic balance of blood; moreover, the functioning of muscles relies on stable corresponding concentrations of these ions in the interstitial fluid

Plasma cont’dPlasma also contains plasma proteins: collectively act as buffers against pH changes (the average pH of human blood is 7.4), they help to maintain osmotic balance between the blood and the interstitial fluid, and they contribute to the blod’s visccosity (thickness)Specific classes of plasma proteins also act as escorts for lipidsOther classes of plasma proteins include the imunoglobulins, or antibodies, and fibrinogens, clotting factors which form clots when blood vessels have been injured

Plasma from which these clotting factors have been removed is called serum

Plasma

Plasma also includes other nutrients, metabolic and respiratoy wastes, and hormones

Plasma is about the same composition as interstitial fluid, but with a higher concentration of proteins

Erythrocytes

Also known as red blood cells, erythrocytes are by far the most numerous of the blood cellsTheir function is to provide for the storing and rapid diffusion of oxygenReflected in their structure - a small, round, biconcave (thinner in the middle than at the edges) disk - which maximizes surface area

Erythrocytes cont’d

Lack nuclei - the space is instead taken up by hemoglobin, an iron-containing protein that transports oxygen

Generate ATP through anaerobic respiration so as to preserve the stores of oxygen they carry

Leukocytes

Also known as white blood cells, there are five types of leukocytes:

Monocytes, neutrophils, basophils, esinophils, and lymphocytes

Their collective function is to fight infection

Spend the most time in the interstitial fluid and lympatic system, where they “wage most of their battles” against pathogens, but their numbers in the blood increase temporarily when fighting infection

Platelets

Fragments of cells that aid in the clotting process

No nuclei; originate as pinched-off fragments of large cells in bone marrow

Stem Cells

All of the cells in the bloodstream wear out and are recycled and replaced throughout the lifetime of the individualReplacement cells all develop from a common population of pluripotent stem cells located in the red marrow of bones (particularly the ribs, vertebrae, breastbone, and pelvis)

Stem Cells cont’d

In a negative-feedback system, the body produces erythropoietin (EPO) which stimulates the production of erythrocytes when tissues are not getting enough oxygen

When the tissues signal that they are getting too much oxygen, EPO production slacks off

EPO is used by physicians to treat people with lower-than-normal hemoglobin levels, as in anemia

Stem Cells cont’d

Because of its ability to deliver extra oxygen to muscles and theoretically improve performance, some athletes inject themselves with EPO in a practice called “blood doping”

This has been deemed illegal by the International Olympic Committee and several other sports federations

Stem Cells cont’d

Recently, researchers have made advances in using stem cells to treat certain diseases, such as leukemia

The healthy stem cells would be removed from the patient, set to grow in a culture, then the cancerous stem cells would be removed and replaced with the healthy stem cells

Blood Clotting

Blood contains a self-sealing material called fibrinogen (in its inactive form)Fibrinogen is converted into fibrin (its active form) when clotting factors are released from plateletsThe fibrinogen aggregates into fibrous mesh networks that for the basic framework of the clotThe series of chemical reactions through which clots form are still not fully understood

Blood Clotting

Hemophilia: a disease caused by a genetic mutation in any step of the clotting process

Characterized by excessive bleeding even from small cuts and bruises

Cardiovascular DiseaseAtherosclerosis: accumulation of fatty deposits (plaque) on the interior walls of arteries causing them to harden and lose elasticity, resulting in high blood pressure and inccreased risk of heart attack or stroke

Heart attack: death of heart tissue due to lack of oxygenStroke: death of brain tissue due to lack of oxygenBoth can be caused by a thrombus, or clot, that clogs an artery and block blood flow downstream from it

A thrombus that originates in one area and then is carried somewhere else by the circulatory system is called an embolus

(a) Normal artery (b) Partly clogged artery

Smooth muscleConnective tissue Endothelium Plaque

Cardiovascular Disease cont’d

Hypertension: high blood pressure; can cause or be caused by atherosclerosis, increases risk of heart attack and stroke

Arrhythmia: irregular heartbeat; often congenital and the result of a heart defect

Cardiovascular Disease cont’d

Hypercholesterolemia: excessive amounts of “bad” cholesterol - low-density lipoproteins (LDLs) - as opposed to “good” cholesterol - high-density lipoproteins; also increases risk of heart attack and stroke, associated with obesity/overweight-ness

Cardiovascular Disease cont’d

Widespread symptoms of cardiovascular disease include chest pain, weakness or numbness in limbs, dizziness, increased or irregular heartbeat, fatigue, and excessive sweatingMost cardiovascular diseases can be avoided through a healthful diet (low in animal fats, saturated fats, and salt, and high in fruits and vegetables, whole grains, omega-3s and antioxidants) and exerciseGenetic heart disease (hemophilia, arrythmia) can be treated with medication

How it Relates to AP Biology

Structure and function: the structure of the heart is directly related to its function, as are the structures of blood cells (ex: erythrocytes are biconcave disks to allow for greatest possible surface area with least possible volume)

Regulation: of the heartbeat by the pacemaker, of blood pressure by constriction of blood vessels, of body temperature by the circulatory system itself

Evolution: different hearts in different species are the result of evolution for different purposes

Works Cited

Campbell, Reece. Biology Seventh Edition. San Francisco: Pearson Education Inc., 2005. Print.

Carter, J. Stein. “Circulatory System.” biology.clc.uc.edu. Clermont College. 13 November 2006. Web. 13 April 2012.

“Diseases of the Circulatory System.” circulatory-system.com. Thorium Hosting Solutions. 13 October 2010. Web. 14 April 2012.