6.2 The Blood System Flashcards
What did William Harvey find?
- The heart acted as a pump for the circulation of blood
- Demonstrated that blood flow through the larger vessels is unidirectional, with valves to prevent backflow
- Predicted the presence of numerous fine vessels too small to be seen with contemporary equipment that linked arteries to veins in the tissues of the body
After some simple experiments and observations, what did Harvey propose? (3)
- Arteries and veins were part of a single connected blood network (he did not predict the existence of capillaries however)
- Arteries pumped blood from the heart (to the lungs and body tissues)
- Veins returned blood to the heart (from the lungs and body tissues)
How many chambers do the heart have? What are they?
The human heart is a four chambered organ, consisting of two atria and two ventricles
What do atria and ventricles act as? What do they do?
Atria: acts as reservoirs, by which blood retuning to the heart is collected via veins (and passed on to ventricles)
Ventricles: acts as pumps, expelling the blood from the heart at high pressure via arteries
Why are there two sets of atria and ventricles?
Because there are two distinct locations for blood transport
What does the left and right side of the heart do?
The left side of the heart pumps oxygenated blood around the body (systemic circulation)
The right side of the heart pumps deoxygenated blood to the lungs (pulmonary circulation)
Why does the left side of the heart have a much thicker muscular wall?
It pumps the blood much further - to the body compared to the just the lungs for the right side
What is the function of arteries?
To convey blood at high pressure from the heart ventricles to the tissue of the body and lungs
What are the characteristics of artery structure?
- Narrow lumen (relative to wall thickness) to maintain a high blood pressure
- Have think wall containing an outer layer of collagen to prevent the artery from rupturing under the high pressure
- The arterial wall contains a inner layer of muscle and elastic fibres to help maintain pulse flow (it can contract and stretch)
How does blood flow through the arteries?
In repeated surges called pulses
This blood flows at a high pressure and the muscle and elastic fibres assist in maintaining this pressure between pumps
How does muscle fibres in arteries help withstand high blood pressure and also increase pressure?
- help form a rigid arterial wall that is capable of withstanding the high blood pressure without rupturing
- Muscle fibres also contract to narrow the lumen, which increases the pressure between pumps and helps to maintain blood pressure throughout the cardiac cycle
What benefit comes from the elastic and muscular tissues contributing to the toughness of the walls?
Strong enough to withstand the constantly changing and intermittently high blood pressure without bulging outwards (aneurysm) or bursting
What does the pulse reflect?
each heartbeat and can easily be felt in arteries that pass near the body surface, including those in the wrist and the neck
What do elastic fibres do upon the flow of a pulse through the lumen?
They allow the arterial wall to stretch and expand
What is elastic recoil and how they help?
When the pressure exerted on the arterial wall is returned to the wall and the artery returns to its normal size
The elastic recoil helps to push the blood forward through the artery as well as maintain arterial pressure between pump cycles
What does the contraction of smooth muscle in the artery wall determine?
The diameter of the lumen and to some extent the rigidity of the arteries, thus controlling the overall flow through them
What artery supplies the powerful, continuously active muscles of the heart with blood?
Coronary arteries
What is systolic pressure?
The peak pressure reached in an artery
- It pushed the wall of the artery outwards, widening the lumen and stretching elastic fibres in the wall, this storying potential energy
What happens at the end of each heartbeat?
Pressure in the arteries falls sufficiently for the stretched elastic fibres to squeeze the blood in the lumen
- This mechanism save energy and prevents the minimum pressure inside the artery, called the diastolic pressure, from becoming too low.
What is diastolic pressure?
The lowest arterial blood pressure of a cardiac cycle occurring during diastole of the heart.
When does pressure change during the cardiac cycle?
Left atrium, left ventricle and aorta
What are the 3 layers of the artery wall?
- Tunica Externa - a tough outer layer of connective tissue
- Tunica Media - A thick layer containing smooth muscle and elastic fibres made of the protein elastin
- Tunica Intima - a smooth endothelium forming the lining of the artery.
What do the circular muscles in the wall of the artery form?
A ring
What is vasoconstriction?
When the circular muscles in the wall of the artery contract and the circumference is reduced, and the lumen is narrowed
What does vasoconstriction increase in the arteries?
Blood pressure
What are capillaries?
the narrowest blood vessels with a diameter of about 10um
How are capillary network formed?
Capillaries branch and rejoin repeatedly
What is the function of capillaries?
to exchange materials between the cells in tissues and blood travelling at low pressure
- Transport blood through almost all tissues in the body
What is split in order to turn into capillaries?
Arteries split into arterioles which split into capillaries, decreasing arterial pressure as total vessel volume is increased
What dos the branching of arteries into capillaries ensure?
It ensures blood is moving slowly and all cells are located near a blood supply
What happens to capillaries after material exchange has occured?
The capillaries will pool into venules which will in turn collate into larger veins
What are characteristic of the capillaries structure? (4)
- very small diameter which allows passage of only a single red blood cell at a time (optimal exchange)
- The capillary wall is made of a single layer of thin endothelium cells to minimise the diffusion distance for permeable materials
- coated by a filter-like protein gel, with pores between the cells, thus very permeable and allows part of the plasma to leak out and form tissue fluid
- May contain pores to further aid in the transport of materials between tissues fluid and blood
What is plasma?
The fluid in which the blood cells are suspended
What do tissue fluid contain?
Oxygen, glucose, and all other substances in blood plasma apart from large protein molecules, which cannot pass through the capillary wall.
What does tissue fluid do?
Flows between the cells in a tissue, allowing the cells to absorb useful substances and excrete waste products. The tissue fluid then re-enters the capillary network.
What is the effect of the differing permeabilities of capillary walls?
Enabling particular proteins and other large particles to reach certain tissues but not others.
How may capillaries structure vary depending on its location in the body and specific role?
- The capillary wall may be continuous with endothelial cells held together by tight junctions to limit permeability of large molecules
- In tissues specialised for absorption (e.g. intestine, Kidney), the capillary wall may be fenestrated (containing pores)
- Some capillaries are sinusoidal and have open spaces between cells and be permeable to large molecules and cells
What is the flow of blood like in capillaries?
- Very slow
- Low pressure
→ allow for maximal material exchange
How is the high blood pressure in arteries dissipated?
By extensive branching of the vessels and the narrowing of the lumen
What kind of materials do the blood capillaries exchange?
Oxygen and nutrients to cells for respiration
What is the arteriole?
- A small branch of an artery leading into capillaries
- Has high density of muscle cells that respond to various hormone and neural signals to control blood flow to downstream tissues.
What is vasoconstriction and vasodilation of arterioles?
Vasoconstriction of arterioles restricts blood flow to the part of the body that they supply and the opposite process, called vasodilation, increases it
What are the two ends of a capillary called?
Venule and Arteriole
Venule links capillary to vein
Arteriole links capillary to artery
How is material from the bloodstream forced into the tissue fluid and materials from the tissues to enter the bloodstream?
The higher hydrostatic pressure at the arteriole end of the capillary forces material from the bloodstream into the tissue fluid
- Material that exits the capillaries at body tissues include oxygen and nutrients (needed by the cells for respiration)
The lower hydrostatic pressure at the venule end of the capillary allows materials from the tissues to enter the bloodstream
- Materials that enters the capillaries at body tissues include carbon dioxide and urea (wastes produced by the cells)
What is the difference in pressure between the arterial end and the venous end of a capillary?
There is higher hydrostatic pressure at the arteriole end to force the material from the bloodstream into the tissue fluid
Then the lower hydrostatic pressure at the venule end allows materials from the tissue to enter the bloodstream
What is the function of veins?
to collect the blood from the tissues and capillary networks and convey it at low pressure to the atria of the heart
What are the characteristics of veins structure?
- Can dilate = very wide lumen (relative to wall thickness) to maximise blood flow for more effective return
- Can hold more blood than arteries
- Thin wall containing less muscle and elastic fibres as blood is flowing at a very low pressure
- Because the pressure is low, veins possess valves to prevent backflow and stop the blood from pooling at the lowest extremities
Why do veins have valves?
Because the pressure is low, veins possess valves to prevent backflow and stop the blood from pooling at the lowest extremities and maintain circulation
What makes blood hard to flow against the downward force of gravity?
The fact that it is very low pressure
How do veins maintain the circulation of blood and prevent backflow?
Contain numerous one-way valves
What happens to the flow of blood in the veins when skeletal muscles contract? how about the pulse of an artery?
When the skeletal muscles contract, they squeeze the vein and cause the blood to flow from the site of compression
Veins typically run parallel to arteries, and a similar effect can be caused by the rhythmic arterial bulge created by a pulse
What are the differences in the structural characteristics of arteries, capillaries and veins in respective of their functions?
- Arteries have thick walls and narrow lumens because they transport blood at high pressure
- Capillaries have walls that are only a single cell thick because they exchange materials between blood and tissue
- Veins have thin walls with wide lumens and valves because they transport blood at low pressure
Comparison of Blood Vessel Structure
How can you identify arteries, veins and capillaries?
- Arteries have thick walls composed of three distinct layers (tunica)
- Veins have thin walls but typically have wider lumen (lumen size may vary depending on specific artery or vein)
- Capillaries are very small and will not be easily detected under the same magnification as arteries and veins
Key components of a human heart
Chambers
- Two atria (singular = atrium) – smaller chambers near top of heart that collect blood from body and lungs
- Two ventricles – larger chambers near bottom of heart that pump blood to body and lungs
Heart Valves
- Atrioventricular valves (between atria and ventricles) – bicuspid valve on left side ; tricuspid valve on right side
- Semilunar valves (between ventricles and arteries) – aortic valve on left side ; pulmonary valve on right side
Blood Vessels
- Vena cava (inferior and superior) feeds into the right atrium and returns deoxygenated blood from the body
- Pulmonary artery connects to the right ventricle and sends deoxygenated blood to the lungs
- Pulmonary vein feeds into the left atrium and returns oxygenated blood from the lungs
- Aorta extends from the left ventricle and sends oxygenated blood around the body
What does the coronary artery do?
Supply the wall of the heart with oxygen and nutrients
What does the septum do?
Contains conducting fibres, which help to stimulate the ventricles to contract
Why do blood in the capillaries of the lungs need to be return to the heart to be pumped again before it goes to other organs?
Blood capillaries in lungs cannot withstand high pressure so blood is pumped to them at relatively low pressure. After passing through the capillaries of the lungs the pressure of the blood is low, so it must return to the heart to be pumped again before it goes to other organs.
What are the two separate circulations human have?
- The pulmonary circulation, to and from the lungs
- The systematic circulation, to and from all other organs, including the heart muscles
What blood does the pulmonary circulation receive that has been returned from where.
What blood does the systemic circulation receive and from where?
The pulmonary circulation receives deoxygenated blood that has returned from the systemic circulation, and the systemic circulation receives blood that has been oxygenated by the pulmonary circulation.
How is the heart a double pump?
It delivers blood under different pressures separately to the two circulations
What does it mean that the contraction of the heart is myogenic?
Meaning that the signal for cardiac compression arises within the heart tissue itself
- In other words, the signal for a heart beat is initiated by the heart muscle cells (cardiomyocytes) rather than from brain signals
- mygenic meaning that the contraction is generated in the muscle itself
What directs the contraction of heart muscle
A specialised cluster of cardiomyocytes within the wall of the right atrium
What is the sinoatrial node?
A group of specialized muscle cells in the right atrium tha initiates the heartbeat
What is the role of the sinoatrial node?
Acts as the primary pacemaker - controlling the rate at which the heart beats (i.e. pace ‘making’)
What happens if the SA node fails?
It is replaced by an electronic device, placed under the skin with electrode impanted in the wall of the heart that initiate each heartbeat in place of the SA node
How does the SA node initiate a heartbeat?
By contracting and simutaneously sends out an electrical signal that spreads throughout the walls of the atria. This can happen because there are interconnections beteen adjacent fibres across which the electrical signal can be propagated. Also the fibres are branched so each fibre passes the signal on to several other. It takes less than a tenth of a second for all cells in the atria to reeive the signal. The propagation of the electrical signal causes the whole of both left and right atria to contract.
What does the time delay of the heart beating allow for?
After the time delay of 0.1 seconds, the electrical signal is conveyed to the ventricles. The time delay allows time for the atria to pump the blood that they are holding into the ventricles. The signal is then propagated throughout the walls of the ventricle, stimulating them to contract and pump blood out into the artieries
How does the electrical conduction of a heart beat occur?
- The sinoatrial node sends out an electrical impulse that stimulates contraction of the myocardium (heart muscle tissue)
- This impulse directly causes the atria to contract and stimulates another node at the junction between the atrium and ventricle
- This second node – the atrioventricular node (AV node) – sends signals down the septum via a nerve bundle (Bundle of His)
- The Bundle of His innervates nerve fibres (Purkinje fibres) in the ventricular wall, causing ventricular contraction
What does the sequence of events of the electrical conduction of a heart beat ensure? Why?
That there is a delay between atrial and ventricular contractions, resulting in two heart sounds
This delay allows time for the ventricles to fill with blood following atrial contractions so as to maximise blood flow
What external signals can regulate the heart rate?
- Nerve signals from the brain can trigger rapid changes, while endocrine signals can trigger more sustained changes
- Changes to blood pressure levels or CO2 concentrations (and thereby blood pH) will trigger changes in heart rate
How can the heart rate be increased or decreased?
By impulses brought to the heart through two nerves from the medulla of the brain
The SA node that sets the rhythm for the beating of the heart responds to signals from outside the heart. These include signals from branches of two nerves originating in. aregion. inthe medulla of the brain called the cardiovascular centre.
Which part of the brain is the pacemaker under autonomic control from?
The pacemaker is under autonomic (involuntary) control from the brain, specifically the medulla oblongata (brain stem)
How do two nerves connected to the medulla regulate heart rate?
By either speeding it up or slowing it down:
- The sympathetic nerve releases the neurotransmitter noradrenaline (a.k.a. norepinephrine) to increase heart rate
- The parasympathetic nerve (vagus nerve) releases the neurotransmitter acetylcholine to decrease heart rate
What does the cardiovascular centre receive inputs from?
Receptors that monitor blood pressure and its pH and oxygen concentration
What does the pH of the blood reflect?
The carbon dioxide concentration
What 3 indicators telling the heart rate to speed up or slow down?
- Low blood pressure, low oxygen concentration and low pH all suggest that the heart rate needs to speed up, to increase the flow rate of blood to the tissues, deliver more oxygen and remove more carbon dioxide
- High blood pressure, high oxygen concentration and high pH are all indicators that the heart rate may need to slow down.
What are hormones?
chemical messengers released into the bloodstream that act specifically on distant target sites (like the heart)
How does the heart rate prepare for vigorous physical activity?
Heart rate increase in response to hormonal signalling
- The hormone adrenaline (a.k.a. epinephrine) is released from the adrenal glands (located above the kidneys)
- Adrenaline increases heart rate by activating the same chemical pathways as the neurotransmitter noradrenaline
What is the secretion of epinephrine controlled by?
The brain and it rises when vigorous physical activity may be necessary because of a threat or opportunity. So epinephrine has the nickname “fight or flight hormone”
What does the cardiac cycle describe?
What is it comprised of?
Series of events that take place in the heart over the duration of a single heart beat
It is comprised of a period of contraction (systole) and relaxation (diastole)
Why is there less pressure in the atria and ventricle when blood is returning to the heart?
Due to low volume of blood
When does atrial systole occur?
When ventricles are ~70% full, atria will contract (atrial systole), increasing pressure in the atria and forcing blood into ventricles through the atrioventricular (AV) valves
What makes the first heart sound?
As the ventricles contract, ventricular pressure exceeds atrial pressure and AV valves that linked the atria to the ventricle closes to prevent backflow and makes the first heart sound.
It is the sound of the AV valve closing
What happens when the ventricular pressure exceeds blood pressure in the aorta?
Since both sets of heart valves are closed (AV valve and Semilunar valve), pressure rapidly builds in the contracting ventricle (isovolumetric contraction)
When ventricular pressure exceeds blood pressure in the aorta, the semilunar valve opens and blood is released into either the aorta or pulmonary artery.
What happens to the ventricular pressure when the blood exit the ventricle via the semilunar valves?
Pressure falls
What happens when the ventricular pressure drop below aortic/pulmonary artery pressure?
The semilunar valve closes to prevent back flow (second heart sound)
The AV valves open and blood starts to flow from the atria to ventricle again
How does aortic pressure remain quite high throughout the cardiac cycle?
The muscle and elastic fibres in the artery all maintain blood pressure
Overview of cardiac cycle events
Pressure changes in Left side of the heart during the cardiac cycle
What are the coronary arteries?
Coronary arteries are the blood vessels that surround the heart and nourish the cardiac tissue to keep the heart working
Why are coronary arteries important?
Because blood is pumped through the heart at a high pressure and cannot be used to supply the heart muscle with oxygen and nutrients
What happens if coronary arteries become occluded?
The region of heart tissue nourished by the blocked artery will die and cease to function
What are causes of coronary occlusion?
Atherosclerosis
What is atherosclerosis?
the development of fat tissue called atheroma in the artery wall adjacent to the endothelium.
It is the hardening and narrowing of the arteries due to the deposition of cholesterol
What are atheromas? Where do they develop? What effect do they have?
Atheromas are fatty deposits that develop in the arteries and significantly reduce the diameter of the lumen = restricted blood flow
Low density lipoproteins (LDL) containing fats and cholesterol accumulate and phagocytes are then attracted by signals from endothelium cells and smooth muscle. The phagocytes enguld the fats and cholesterol by endocytosis and grow very large. Sniitg muscle cells migrate to form a tough cap over the atheroma. The artery wall bulges into the llumen narrowing it and thus impeding blood flow.
How does restricted blood flow effect the pressure in the artery as a result of an atheroma?
The restricted blood flow increases the pressure in the artery, leading to damage to the arterial wall (from shear stress)
How is the damaged region causes by restricted blood flow from atheromas effected?
The damage region is repaired with fibrous tissue which significantly reduces the elasticity of the vessel wall
How are atherosclerotic plaques formed?
When the smooth lining of the artery is progressively degraded, lesions form called atherosclerotic plaques.
What happens if the plaque from from the smooth lining of the artery being progressively degraded, ruptures?
Blood clotting is triggered, forming a thrombus that restricts blood flow
What happens if the thrombus thats formed when the plaque ruptures is dislodged?
It becomes an embolus and can cause a blockage in a smaller arteriole
