6.2 The Blood System
S2: Recognition of the chambers and valves of the heart and the blood vessels connected to it in dissected hearts or in diagrams of heart structure.
Diagram: http://ib.bioninja.com.au/_Media/human-heart_med.jpeg The structure of the human heart includes the following key components: - 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
U4: Blood flows through tissues in capillaries. Capillaries have permeable walls that allow exchange of materials between cells in the tissue and the blood in the capillary.
Flow of Blood Blood flows through the capillaries very slowly and at a very low pressure in order to allow for maximal material exchange - The high blood pressure in arteries is dissapated by extensive branching of the vessels and the narrowing of the lumen 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) Structure and Function The function of capillaries is to exchange materials between the cells in tissues and blood travelling at low pressure (<10mmHg) - Arteries split into arterioles which in turn split into capillaries, decreasing arterial pressure as total vessel volume is increased - The branching of arteries into capillaries therefore ensures blood is moving slowly and all cells are located near a blood supply - After material exchange has occurred, capillaries will pool into venules which will in turn collate into larger veins Capillaries have specialised structures in order to accomplish their task of material exchange: - They have a very small diameter (~ 5 µm wide) 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 cells to minimise the diffusion distance for permeable materials - They are surrounded by a basement membrane which is permeable to necessary materials - They may contain pores to further aid in the transport of materials between tissue fluid and blood Capillaries structure may 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. intestines, kidneys), the capillary wall may be fenestrated (contains pores) - Some capillaries are sinusoidal and have open spaces between cells and be permeable to large molcules and cells (e.g. in liver)
U6: Valves in veins and the heart ensure circulation of blood by preventing backflow.
Flow of Blood Blood is at very low pressure in the veins which can make it difficult for the blood to move against the downward force of gravity - The veins contain numerous one-way valves in order to maintain the circulation of blood by preventing backflow Veins typically pass between skeletal muscle groups, which facilitate venous blood flow via periodic contractions - 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
U3: The muscle and elastic fibres assist in maintaining blood pressure between pump cycles.
Flow of Blood Blood is expelled from the heart upon ventricular contraction and flows 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 The muscle fibres help to form a rigid arterial wall that is capable of withstanding the high blood pressure without rupturing - Muscle fibres can also contract to narrow the lumen, which increases the pressure between pumps and helps to maintain blood pressure throughout the cardiac cycle The elastic fibres allow the arterial wall to stretch and expand upon the flow of a pulse through the lumen - The pressure exerted on the arterial wall is returned to the blood when the artery returns to its normal size (elastic recoil) - The elastic recoil helps to push the blood forward through the artery as well as maintain arterial pressure between pump cycles
U12: Epinephrine increases the heart rate to prepare for vigorous physical activity.
Hormonal Signalling Hormones are chemical messengers released into the bloodstream that act specifically on distant target sites (like the heart) Heart rate can undergo a sustained increase in response to hormonal signalling in order to prepare for vigorous physical activity - 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
A1: William Harvey's discovery of the circulation of the blood with the heart acting as the pump.
Our modern understanding of circulatory system is based upon the discoveries of 17th century English physician, William Harvey - Harvey's findings were published in a book commonly called De Motu Cordis - On the Motion of the Heart and Blood Prior to Harvey's findings, scientists held to the antiquated views of the Greek philosopher Galen, who believed that: - Arteries and veins were separate blood networks (except where they connected via invisible pores) - Veins were thought to pump natural blood (which was believed to be produced by the liver) - Arteries were thought to pump heat (produced by the heart) via the lungs (for cooling - like bellows) Based on some simple experiments and observations, Harvey instead proposed that: - 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)
U5: Veins collect blood at low pressure from the tissues of the body and return it to the atria of the heart.
Structure and Function The function of veins is to collect the blood from the tissues and convey it at low pressure to the atria of the heart To this end, veins have a specialised structure in order to accomplish this task: - They have a very wide lumen (relative to wall thickness) to maximise blood flow for more effective return - They have a thin wall containing less muscle and elastic fibres as blood is flowing at a very low pressure (~ 5 - 10 mmHg) - Because the pressure is low, veins possess valves to prevent backflow and stop the blood from pooling at the lowest extremities
U8: The heart beat is initiated by a group of specialized muscle cells in the right atrium called the sinoatrial node. U9: The sinoatrial node acts as a pacemaker.
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 Within the wall of the right atrium are a specialised cluster of cardiomyocytes which direct the contraction of heart muscle tissue - This cluster of cells are collectively called the sinoatrial node (SA node or SAN) The sinoatrial node acts as primary pacemaker - controlling the rate at which the heart beats (i.e. pace 'making') - The SA node triggers roughly 60 - 100 cardiac contractions per minute (normal sinus rhythm) - If the SA node fails, a secondary pacemaker (AV node) may maintain cardiac contractions at roughly 40 - 60 bpm - If both fail, a final tertiary pacemaker (Bundle of His) may coordinate contractions at a constant rate of roughly 30 - 40 bpm The interference of the pacemakers will lead to the irregular and uncoordinated contraction of the heart muscle (fibrillation) - When fibrillation occurs, normal sinus rhythm may be re-established with a controlled electrical current (defibrillation)
S1: Identification of blood vessels as arteries, capillaries or veins from the structure of their walls.
The difference in the structural characteristics of arteries, capillaries and veins is attributable to their respective 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: http://ib.bioninja.com.au/_Media/vessel-comparison_med.jpeg Identification of Blood Vessels Blood vessels can be identified from histological slides or images according to the thickness of their walls: - 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
U10: The sinoatrial node sends out an electrical signal that stimulates contractions as it is propagated through the walls of the atria and then the walls of the ventricles.
The electrical conduction of a heart beat occurs according to the following events: - 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 This sequence of events ensures 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 maximize blood flow
U1: Arteries convey blood at high pressure from the ventricles to the tissues of the body. U2: Arteries have muscle cells and elastic fibres in their walls.
The function of arteries is to convey blood at high pressure from the heart ventricles to the tissues of the body and lungs To this end, arteries have a specialised structure in order to accomplish this task: - They have a narrow lumen (relative to wall thickness) to maintain a high blood pressure (~ 80 - 120 mmHg) - They have a thick wall containing an outer layer of collagen to prevent the artery from rupturing under the high pressure - The arterial wall also contains an inner layer of muscle and elastic fibres to help maintain pulse flow (it can contract and stretch)
U7: There is a separate circulation for the lungs.
The human heart is a four chambered organ, consisting of two atria and two ventricles - The atria act as reserviors, by which blood returning to the heart is collected via veins (and passed on to ventricles) - The ventricles act as pumps, expelling the blood from the heart at high pressure via arteries The reason why there are two sets of atria and ventricles is because there are two distinct locations for blood transport - 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) There is therefore a separate circulation for the lungs (right side of heart) and for the rest of the body (left side of heart) - The left side of the heart will have a much thicker muscular wall (myocardium) as it must pump blood much further
U11: The heart rate can be increased or decreased by impulses brought to the heart through two nerves from the medulla of the brain.
While the basal heart rate is determined within the heart by the pacemaker, it can be regulated by external signals - 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 Nerve Signalling The pacemaker is under autonomic (involuntary) control from the brain, specifically the medulla oblongata (brain stem) 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
