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Introduction to Physiological Principles Physiology
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Category: Anatomy
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Chapter 8 - Circulatory System
Transcript
Circulatory Systems
Limits of Diffusion
Unicellular organisms and some small metazoans lack circulatory systems
Rely on diffusion to transport molecules
Diffusion can be rapid over small distances, but is very slow over large distances
Large animals move fluid through their bodies by bulk flow, or convective transport
Transport can occur over greater distance
Requires energy
Overview
Most metazoans larger than a few cells have circulatory systems
Major function
Transport oxygen, carbon dioxide, nutrients, waste products, immune cells, and signaling molecules throughout the body
Circulatory systems move fluids by increasing the pressure of the fluid in one part of the body
Fluid flows through the body, “down” the pressure gradient
Three main components are needed
Pump or propulsive structures
For example, a heart
System of tubes, channels, or spaces
Fluid that circulates through the system
For example, blood
Types of Pumps
Chambered hearts
Contractile chambers
Blood enters atrium
Blood is pumped out by ventricle
Skeletal muscle
Squeeze on vessels to generate pressure
Pulsating blood vessels
Peristalsis
Rhythmic contractions of vessel wall pumps blood
One-way valves help to ensure unidirectional flow
Closed and Open Circulatory Systems
Open
Circulatory fluid comes in direct contact with the tissues in spaces called sinuses
Circulating fluid mixes with interstitial fluid
Closed
Circulatory fluid remains within vessels and does not come in direct contact with the tissues
Circulating fluid is distinct from interstitial fluid
Molecules must diffuse across vessel wall
Types of Fluid
Interstitial fluid
Extracellular fluid that directly bathes the tissues
Formed by ultrafiltration
Blood
Fluid that circulates within the vessels of a closed circulatory system
Lymph
Fluid that circulates in the secondary circulatory system of vertebrates; the lymphatic system
Carries fluid (lymph) that has filtered out of the vessels
Hemolymph
Fluid that circulates in an open circulatory system
Some Animals Lack Circulatory Systems
Sponges, cnidarians, and flatworms
Lack circulatory systems but have mechanisms for propelling fluids around their bodies
Sponges and flatworms
Flagella or ciliated cells move water within body cavity
Cnidarians
Muscular contractions of body wall pump water in and out of body cavity
Bulk flow of fluid is part of a combined respiratory, digestive and circulatory system
Annelids
Three main classes of annelids
Polychaeta (tube worms)
Oligochaeta (earth worms)
Hirudinea (leeches)
Polychaetes and oligochaetes circulate interstitial fluid with cilia or muscular contractions of body wall
Most have vessels that circulate fluid with oxygen carrier protein
Circulatory system can be open (polychaetes) or closed (oligochaetes)
Molluscs
All have hearts and some blood vessels
Most have open systems
Only cephalopods have closed systems
Very active mode of life
? metabolism
Similar to birds and mammal system!
Figure 8.5
Arthropods
All have one or more hearts and some blood vessels
All have open systems
Insects
Relatively simple open circulatory system
But high metabolic rate?
Insects use a tracheal system for most gas transport
Multiple, contractile “hearts” along dorsal vessel
Crustaceans
Circulatory systems become more complex in larger animals
Small sinuses function as vessels
Some control over distribution of blood flow in body
Structurally open
Functionally closed
Evolution of Circulatory Systems
First evolved to transport nutrients to body cells
Very early began to serve respiratory function
Closed systems evolved independently in jawed vertebrates, cephalopods, and annelids
Increased blood pressure and flow
Increased control of blood distribution
Closed systems evolved in combination with specialized oxygen carrier molecules
High metabolic rates
The Circulatory Plan of Vertebrates
All have closed circulatory systems (except Agnathans)
Muscular, chambered heart contracts to increase the pressure of the blood
Blood flows away from the heart in arteries
Arteries branch to form more numerous, but smaller diameter, arteries
Small arteries branch into arterioles within tissues
Blood flows from arterioles into capillaries
Capillaries are the site of diffusion of molecules between blood and interstitial fluid
Capillaries coalesce to form venules
Venules coalesce to form veins
Veins carry blood to the heart
Vertebrate Blood Vessels
A complex wall surrounding a central lumen
Wall composed of up to three layers
Tunica intima (internal lining)
Smooth, epithelial cells (vascular endothelium)
Tunica media (middle layer)
Smooth muscle
Elastic connective tissue
Tunica externa (outermost layer)
Collagen
Thickness of the wall varies among vessels
Vertebrate Blood Vessels
Figure 8.10
Arterioles have the greatest influence on local blood flow and overall blood pressure
Vein – thinner wall and larger lumen than artery of similar size
Capillaries
Lack tunica media and tunica externa
Continuous
Cells held together by tight junctions
Skin and muscle
Fenestrated
Cells contain pores
Specialized for exchange
Kidneys, endocrine organs, and intestine
Sinusoidal
Few tight junctions
Porous for exchange of large proteins
Liver and bone marrow
Figure 8.11
Jawed Vertebrates – Phylum Chordata
Structure varies depending on respiratory strategy
Water-breathing fish
Single circuit
Some fish have accessory hearts in the tail
Air-breathing tetrapods
Two circuits
Pulmonary - right side
Systemic circuit – left side
Birds and Mammals
Four-chambered heart
Two atria and Two ventricles
Systemic and pulmonary circuits are divided
Pressure can be different in the two circuits
High pressure systemic
Low pressure pulmonary
Oxygenated and deoxygenated blood are completely separate
Amphibians and Reptiles
Heart is only partially divided
Three chambered heart
Two atria and a single ventricle
Blood from both atria flow into the ventricle
Oxygenated and deoxygenated blood can mix
But experiments show that are kept fairly separate
Ventricle pumps blood into pulmonary and systemic circuits
Blood can be diverted between pulmonary and systemic circuits
Physics of Blood Flow
Law of bulk flow: Q = ?P/R
Q = flow
?P = pressure drop
R = resistance (due to friction)
R = 8L?/?r4
L = length of the tube
? = viscosity of the fluid
r = radius of the tube (greatest effect)
Think effects of vasoconstriction or vasodilation
Poiseuille’s equation: Q = ?P ? r4 / 8L?
More detailed version of law of bulk flow
Modeling Circulatory Systems
Like electrical resistors, blood vessels can be arranged in series or parallel
Resistors in series
RT = R1 + R2…
RT increases
Resistors in parallel
1/RT = 1/R1 + 1/R2…
RT decreases
Because of the law of conservation of mass, the flow through each segment of the system must be equal
Figure 8.14
Allows fresh blood to tissues
Blood Velocity
Flow (Q)
Volume of fluid transferred per unit time (a rate)
Velocity
Distance transferred per unit time
Blood velocity = Q/A
A = cross-sectional area of the vessels
Therefore, V is inversely related to total X-sectional Area
For example, total cross-sectional area of capillaries is very large ?velocity is slow ? long time for diffusion
Analogy: River ? Swamp ? River
Transmural Pressure
Pressure exerts a force across vessel wall
Law of LaPlace
T(?) = Pr/w
T = tension in vessel wall
P = transmural pressure
difference between
internal and
external pressure
r = vessel radius
w = vessel wall thickness
Figure 8.15
Aorta ? Large r ? large T (large Stress) ? Thick Wall to Reduce Tension
Compare to Vena Cava??
Hearts
Cardiac cycle – pumping action of the heart
Two phases
Systole
Contraction and emptying
Spread of excitation
Blood is forced out into the circulations
Diastole
Relaxation and filling
Follows repolarization
Blood enters the heart
Arthropod Heart - Neurogenic
Figure 8.16
Arthropod Cardiac Cycle
Cardiomyocytes contract
Volume of heart decreases; pressure increases
Ostia valves close
Blood leaves the heart via arteries
Stretched ligaments pull apart walls of heart
Volume of heart increases; pressure decreases
Ostia valves open
Blood is sucked into heart
Cardiac ganglion neurons spontaneously and rhythmically depolarize
Thus, arthropod hearts function as both pressure and suction pumps
Vertebrate Hearts
Complex walls with four main parts
Pericardium
Sac of connective tissue that surrounds heart
Outer (parietal ) and inner (visceral) layers
Space between layers filled with lubricating fluid
Epicardium
Outer layer of heart, continuous with visceral pericardium
Contain nerves that regulate heart and coronary arteries
Also extend into myocardium
Myocardium
Layer of heart muscle cells (cardiomyocytes)
Endocardium
Innermost layer of connective tissue covered by epithelial cells (called endothelium)
Myocardium
Two types of myocardium
Compact
Tightly packed cells
Regular pattern
Highly vascularized
Spongy
Loosely connected cells
Some not vascularized
Relative proportions vary among species
Mammals
Mostly compact
Fish and amphibians
Mostly spongy
Arranged as trabeculae that extend into chambers
Fish and Amphibian Hearts
Fish
Four chambers arranged in series
2 primary and 2 auxillary
1 atria / 1 ventricle
Contract in sequence
Figure 8.18a
Amphibians
Three-chambered heart
Two atria, one ventricle
How keep deoxygenated and oxygenated blood separate?
Trabeculae in ventricle
Spiral fold in conus arteriosus
Reptile Hearts
Five-chambered heart
Two atria and three interconnected ventricular compartments
Cavum venosum - leads to systemic aortas
Cavum pulmonale - leads to pulmonary artery
Cavum arteriosum – cavity at base of pulmonary vein
Separation of oxygenated and deoxygenated blood in the ventricle is nearly complete
Birds and Mammals
Four chambers
Two atria
Two ventricles
separated by intraventricular septum
Figure 8.20
Valves
Atrioventricular (AV) valves
Between atria and ventricles
Tricuspid (R) and Bicuspid (L)
Semilunar valves
Between ventricles and arteries
Aortic and Pulmonary
Cardiac Cycle – highly coordinated
Fish hearts
Serial contractions of chambers
Valves are passive
Open and close according to pressure differences
Assure unidirectional flow of blood
In teleosts, noncontractile bulbus arteriosus serves as volume and pressure reservoir
Figure 8.18a
Mammalian Cardiac Cycle
Atria and ventricles alternate systole and diastole
The two atria contract simultaneously, Slight pause
Two ventricles contract simultaneously
Atria and ventricles relax while the heart fills with blood
EDV
ESV
Animation
Ventricular Filling
In birds and mammals ventricles fill passively during diastole
Atrial contraction adds little blood to ventricles
In fish and some amphibians ventricle filled by contraction of atrium
Elasmobranchs may use ventricular suction to pull blood in from veins
Ventricular Pressure
Left ventricle contracts more forcefully and develops higher pressure
Less pressure needed to pump blood through pulmonary circuit
Resistance in pulmonary circuit low due to high capillary density in parallel (results in a large cross-sectional area)
Low pressure protects delicate blood vessels of lung
Systemic and pulmonary circuits have same blood flow
Control of Contraction
Vertebrate hearts are myogenic
Cardiomyocytes produce spontaneous rhythmic depolarizations
Do not require nerve signal
Cardiomyocytes are electrically coupled via gap junctions to ensure coordinated contractions
Action potential passes directly from cell to cell
Pacemaker
Characterized by a lack of a resting membrane potential
Cells with fastest intrinsic rhythm
In the sinus venosus in fish
In the right atrium of other vertebrates
Sinoatrial (SA) node
Pacemaker Cells
Derived from cardiomyocytes
Characteristics of pacemaker cells
Small with few myofibrils, mitochondria, or other organelles
Do not contract
Have unstable resting membrane potential
pacemaker potential
Figure 8.23
Contrast to Nerve and Skeletal Muscle
?t-type Ca2+
Control of Pacemaker Potentials
Increasing heart rate
Norepinephrine released from sympathetic neurons and epinephrine released from the adrenal medulla
More Na+ and Ca2+ channels open
Rate of depolarization and frequency APs ?
Ephedrine and ephedra
Control of Pacemaker Potentials
Decreasing heart rate
Acetylcholine released from parasympathetic neurons
More K+ channels open
Pacemaker cells hyperpolarize
Time for depolarization ?, frequency APs ?
Extended Action Potentials
Action potentials in cardiomyocytes differ from those in skeletal muscle
Plateau phase
Extended depolarization that corresponds to refractory period and lasts as long as the contraction
Due to voltage-gated Na+ channel inactivation
Caused by Ca2+ entry via
L-type channel
Prevents tetanus
Figure 8.26
Conducting Pathways (Mammalian Heart)
Electrical Signals and Blood Flow
Modified cardiomyocytes
Elongated, pale appearance
Do not contract
Spread depolarization rapidly
Can undergo rhythmic depolarizations
Electrocardiogram (ECG or EKG)
Composite recording of action potentials in cardiac muscle
P wave
Atrial depolarization
QRS complex
Ventricular depolarization
T wave
Ventricular repolarization
Used for clinical diagnosis of problems with conducting system
Figure 8.28
Electrical and Mechanical Events in the Cardiac Cycle
Heart functions as an integrated organ
Electrical and mechanical events are correlated
Changes in pressure and volume of chambers
Blood flow through chambers
Heart sounds
Closing of valves
Blood Flow Through Your Heart and Lungs
Cardiac Output
Cardiac output (CO)
Volume of blood pumped per unit time
Can be modified by regulating HR and/or SV
CO = HR ? SV
Heart rate
Rate of contraction (beats per minute)
Modulated by autonomic nerves and adrenal medulla
Decreased HR (bradycardia)
Increased HR (tachycardia)
Stroke volume
Volume of blood pumped with each beat
Modulated by nervous, hormonal, and physical factors
Control of Stroke Volume
The nervous and endocrine system can also cause the heart to contract more forcefully and consequently pump more blood with each beat (?SV)
Also HR – fast depolarization (T-type Ca2+-channels and “funny” channels)
Figure 8.27
These effects are as a result of stimulation from the sympathetic nervous system
Extrinsic control
Control of Stroke Volume, Cont.
Frank-Starling effect – an increase in end-diastolic volume results in a more forceful contraction of the ventricle and an increase in SV
Due to length-tension relationship for muscle
Heart automatically compensates for increases in the amount of blood returning to the heart (autoregulation)
Intrinsic regulation
Level of sympathetic activity shifts the position of the cardiac muscle length-tension relationship
Figure 8.28
Regulation of Blood Flow
Arterioles control blood distribution
Because arterioles are arranged in parallel, they can alter blood flow to various organs
Vasoconstriction and vasodilation
Changes in resistance alter flow
Control of vasoconstriction and vasodilation
Myogenic Autoregulation
Direct response of the arteriolar smooth muscle
Some smooth muscle cells sensitive to stretch and contract when blood pressure increases
Acts as negative feedback loop
Prevents excessive flow of blood into tissue
Intrinsic (local) factors
Metabolic state of the tissue
Extrinsic factors
Nervous and endocrine systems
Metabolic Activity of Tissues - Intrinsic
Smooth muscle cells in arterioles are sensitive to conditions of extracellular fluid
Levels of metabolites alter vasoconstriction/vasodilation
Blood flow matched to metabolic requirements
Paracrine
e.g. NO
GC – cGMP - Vasodilation
PDE (inactivates)
Tissue damage ? release
Sildenafil
Figure 8.32
Neural and Endocrine Control of Flow
Sympathetic neurons cause vasoconstriction of arterioles
Decreased sympathetic tone causes vasodilation
Other hormones affect vascular smooth muscle
Vasopressin (ADH) from the posterior pituitary causes generalized vasoconstriction
Angiotensin II produced in response to decreased blood pressure causes generalized vasoconstriction
Atrial natriuretic peptide (ANP) produced in response to increased blood pressure promotes generalized vasodilation
Extrinsic (nervous and endocrine) and Intrinsic (paracrine signals related to metabolic activity) work together to influence arteriolar diameter and blood flow.
Pressure in Vertebrate Circulatory Systems
BP in left ventricle changes with systole and diastole
Pressure decreases as blood moves through system
Pressure and pulse decrease in arterioles due to high resistance
Relatively narrow (vs arteries)
Relatively few (vs capillaries)
Velocity of blood highest in arteries, lowest in capillaries, and intermediate in veins
Figure 8.33
?P
Arteries Act as Pressure Dampeners
Pressure fluctuations in arteries are smaller than those in left ventricle
Aorta acts as a pressure reservoir
Elasticity of vessel wall
Expands during systole
Elastic recoil during diastole
Low compliance
Dampens pressure fluctuations
Also have thick walls
Reduce stress
Figure 8.34
Moving Blood Back to the Heart
Blood in veins is under low pressure
Two pumps assist in moving blood back to the heart
Skeletal muscle
Contraction squeezes vein
Respiratory pumps
Pressure changes in thoracic cavity during ventilation
Valves in veins assure unidirectional flow
Figure 8.35
Veins Act as a Volume Reservoir
Thin, compliant walls
Small ? BP lead to large changes in volume
In mammals, veins hold more than 60% of blood
Vein volume (and venous return) controlled by sympathetic nerves
Venomotor tone
Figure 8.36
Regulation of Blood Pressure
Pressure is the primary driving force for blood flow through organs - Law of Bulk Flow
Rearrange the equation: CO = MAP / TPR
MAP = CO x TPR
Body varies cardiac output (CO) and total peripheral resistance (TPR) to maintain a near constant mean arterial pressure (MAP)
Maintaining MAP is the fundamental requirement
TPR – state of vasoconstriction/vasodilation – metabolic state of tissue
CO varies in response to changes in TPR to maintain MAP in narrow range
Thus, metabolic demand of tissues is ultimate regulator
Homeostatic regulation of Blood Pressure
Figure 8.37
EPO
Baroreceptor Reflex
Baroreceptors
Stretch-sensitive mechano-
receptors in walls of many major blood vessels
Especially carotid arteries and aorta
Send nerve signals to medulla
Baroreceptor reflex regulates MAP
? Parasympathetic
? Ag II and Vasopressin
Figure 8.38
Kidneys Help Maintain Blood Volume
? in blood volume leads to ? in bp, and vice versa
Kidneys excrete or retain water to adjust blood volume (and pressure) to counter changes in MAP
Veins are compliant and can act as volume reservoirs
but not infinitely so!
Figure 8.39
Question?
Suppose the flow of blood into the heart is obstructed. How will the cardiac output and systemic blood pressure change from normal?
CO will fall; systemic arterial bp will fall
CO will rise; systemic bp will rise
CO will fall; systemic bp will rise
CO will fall; systemic arterial bp will fall
CO will remain unchanged; systemic bp will fall
Venous return is impeded, atrial pressure decreases, reduced EDV, reduces CO.
However, the fall in CO is compensated for by an increase in TPR, a fall in urine production and a rise in HR.
Net Filtration Pressure (NFP)
Normal
20 liters out
17 liters in
Xs of 3??
Arteriolar blood pressure forces fluid out of capillaries
Starling principle: NFP = (Pcap – Pif) – (?cap – ?if)
Terms: hydrostatic pressure in the capillary (Pcap) and interstitial fluid (Pif); osmotic pressure in the capillary (?cap) and interstitial fluid (?if)
May result in net filtration or net reabsorption
The Lymphatic System
Collects excess filtered fluid and returns it to circulatory system
Lymph nodes filter lymph to remove pathogens
Lymphocytes
Lymphatic veins and ducts contain valves to prevent backflow
Edema – accumulation of interstitial fluid
Figure 8.41
Affect of Gravity on Blood Pressure
Hydrostatic pressure
Pressure of a column of fluid due to gravity
DP = r ? g ? Dh
DP = pressure difference between two points, r = density of the fluid, g = acceleration due to gravity, Dh = height of the fluid column
Blood flow may be with or against force of gravity
Body Position
Changes in body position can alter bp and flow
Changes relative to gravity
Standing up causes pooling of blood in lower body
? venous return, ? in SV, ? MAP
Baroreceptor reflex brings MAP back to normal
? firing ? ? sympathetic output
? HR and ? SV ? ? CO
Vasoconstriction ? ? TPR
? MAP
Orthostatic hypotension
Low blood pressure upon standing when reflex is too slow
Can lead to fainting
What about Giraffes?
Bending down
? hydrostatic pressure in head would tend to ? filtration
Highly elastic blood vessels that serve as a pressure reservoir
Jugular vein has one-way valves
Standing up
high bp and hydrostatic pressure would tend to ? filtration in lower extremities
Thick-walled and muscular arterioles
Skin on legs is extremely tight
Increases interstitial fluid pressure
Increases efficiency of skeletal muscle pump
NFP = (Pcap – Pif) – (?cap – ?if)
Composition of Blood
Primarily water, containing
Dissolved ions and organic solutes
Blood cells (hemocytes)
Dissolved proteins
Invertebrates
Primarily respiratory pigments
Vertebrates
Carrier proteins such as albumin and globulins
Proteins involved in blood clotting
Blood Proteins
Blood Cells (Hemocytes)
Functions
Oxygen transport or storage
Nutrient transport or storage
Phagocytosis
Immune defense
Blood clotting
Figure 8.44
Great variety of hemocytes, but recently all have GATA TF involved in their development
Red Blood Cells (Erythrocytes)
Most abundant cells in blood of vertebrates
Contain high concentrations of respiratory pigments (such as hemoglobin)
Major function is storage and transport of oxygen
Erythrocytes have evolved independently several times
Also found in various worms, molluscs, and echinoderms
Size of erythrocytes varies among vertebrates
Smaller in mammals than in fish and amphibians
Generally round or oval in shape
Mammalian erythrocytes are biconcave disks
Increased surface area, facilitating oxygen transfer
Lack nucleus, mitochondria, and ribosomes
Cannot divide and have a limited lifespan
Retrieved from http://www.moonbowmedia.com/ei/ct/facts0313.htm, 27/02/06
Vertebrate Blood
Separates into three main components when centrifuged
Plasma
Erythrocytes
Other blood cells and clotting cells
Hematocrit – Fraction of blood made up of erythrocytes
Figure 8.45
White Blood Cells (Leukocytes)
Function in immune response
All are nucleated
Found both in blood and interstitial fluid
Some able to move across capillary walls
Five major types
Neutrophil
Eosinophil
Basophil
Monocyte / macrophage
Lymphocyte
Figure 8.46
Adaptive Immunity
Thrombocytes
Key role in blood clotting
In mammals, thrombocytes are anucleated cell fragments called platelets
In non-mammals, thrombocytes are spindle-shaped cells and classified as leukocytes
Three steps in blood clotting
Vasoconstriction
Platelet plug formation
Platelets stick to break in vessel
Clot formation through coagulation cascade
Blood Cell Formation (Hematopoiesis)
Location of stem cells
Adult mammals
Only in red bone marrow
Fishes
Kidney
Amphibians, reptiles, birds
Spleen, liver, kidney, bone marrow
Signaling factors regulate hematopoiesis
For example, erythropoietin is a hormone released by the kidney in response to low blood oxygen
Stimulates differentiation of stem cells into erythrocytes
Circulatory system during exercise
MAP = CO x TPR
Patterns of blood flow
Retrieved from http://btc.montana.edu/olympics/physiology/pb01.html, 24/03/08
Control of Vasoconstriction/Vasodilation
Table 8.1
Mean Arterial Pressure (MAP)
MAP = 2/3 diastolic pressure + 1/3 systolic pressure
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