Vessels They include arteries, arteries capillaries, veins and venules. The walls of arteries and veins have three layers.
- i) THE INTERNA (TUNICA INTIMA)
The inner most layer surrounding the women and consists of an endolheluim of flat pavement cell with polygonal outline. (Simple squamous optithelium).
- ii) TUNICA MEDIA
The middle layer consisting of smooth muscle running mostly round the vessel (i.e. circular).
iii) THE TUNICA EXPERNA (TUNICA ADVENTATA)
is the outermost layer composed of collagen and elastin fibres running mainly along the vessels (i.e. are longitudinal).
FUNCTIONS OF THE 3 LAYERS
- Tunica externa: Protects the blood vessel from wearing out as it moves against other organs surrounding it.
- Tunica intima: In arteries, it has endothelium whose smoothness reduieces the friction used by blood rubbing against it as blood flows along the vessed. In veins, it forms values.
- Tunica media: In arteries, it’s responsible for varying the diameter of the lumen.
- Arteries carry blood away from the heart; veins carry blood to the heart capillaries link arteries to veins. The diameter of arteries and veins gradually reduces as they get further away from the heart.
The smaller arteries are called arteries and smaller veins called venules
BLOOD CAPILLARIES AND THEIR IMPORTANCE
- The are very tiny vessels which offer great resistance to blood flow and form an intimate relationship between the circulatory system and cells.
- The capillaries and the heart form the most important component of the circulatory system because it is in the capillaries where exchange of materials transported in blood and these from cells occur.
- This is achieved because of the following.
- The total cross sectional area. Available for blood flow in the capillaries is very great ( ~ 37,500km)
- They have thin walls; i.e. one cell thick which allows the exchange of materials between blood and tissue fluid via the capillary by diffusion.
- The total surface area available for exchange in them is great ~ (800-1000m2).
- The large volume of blood flawing in them. Total capacity ~ 5 litres. Not all can be filled with blood at all times and thus there is a constant re-directing of blood from organ to organ according to the changing needs of body as shown in the following table.
Capillaries provide a means by which transport of materials can be regulated rings of muscle surround the capillaries at points where they rise from arteries. Under the influence of nerves, hormones or local conditions these sphincter muscles contact or relax thereby decreasing or increasing the flow of blood through them commensurate with needs of the organism.
The table shows that during exercise, there is:-
- Increased blood supply to the skin to increase the rate of dissipation of the heart provided by the increased muscle action and metabolism.
- Increased blood supply to working organs like the heart and skeletal muscles.
- Blood supply to the brain is kept constant. Why??.
BLOOD PRESSURE AND BLOOD FLOW THROUGH THE BLOOD VESSELS
Changes in blood pressure as the blood flows from the heart through arteries, arteries, capillaries, venules, veins.
Venules gradual decrease.
Medulla othongata controls heart best.
Outline + explanation, veins lumen increases in site 30 bip decrease. Bp in arteries decreases, capillaries bp drop in pressure, TSA, resistance to blood flow.
- The blood pressure is highest in arteries and goes on decreasing up to the veins.
- Its lowest in the atria
- This is because the arterial blood pressure on which the rest of the bloods pressure depends is determined by.
- Cardiac output i.e. volume of blood pumped per min.
- Heart rate: – No of beats per minute.
- Stroke volume: – volume pumped per beat.
- Resistance of the blood vessels.
Blood pressure is inversely proportional to vascular resistance and therefore total cross sectional area.
In the large lumen of the contra resistance is very law about 0-2mm Hg/litre/min and thus the mean aortic pressure is high ~ 100mm Hg and increases rapidly in small arteries causing pressure to drop to about 70mm Hg at the beginning of the arterioles.
- A large drop in pressure also results from the ‘leakage’ of plasma from the capillaries during the formation of tissue fluid.
- Consequently the blood volume is very unevenly distributed through the various segments of the circulation even though the volume flowing through each segment is relatively constant.
RELATIONSHIP BETWEEN PRESSURE, TOTAL CROSS SECTIONAL AREA AND VESCULAR RESISTANCE.
Changes in blood velocity and blood distribution.
- In the veins, the large drop in pressure across capillary beds leads to a very low blood pressure in veins.
- Continued friction between blood and the walls of the reduces their pressure further.
- The pressure in the large vein is consequently s lo that its not enough to return the blood to the heart.
Blood flow in veins is assisted by:-
- Exercising skeletal muscles pushing against them which increases the blood pressure in them.
- Presence of values which prevent back flow of blood in them.
- Aspiratory movement, i.e. when breathing in the pressure in the thorax where the heart is located is reduced and this helps to draw blood towards the heart.
- Blood returning via the veins enters the right atrium. The resulting pressure in this chamber forces open the atrio ventricular values (Jricuspid values) and results in blood flawing into the atrio-ventricular opening into the right ventricle.
- When the atrium and ventricle are full of blood, the atrium suddenly contracts propelling all its blood into the ventricle. The antrium contraction spreads from the right atrium over rest of the heart.
- Atrial systole (contraction) is comparatively weak but the ventricles because they are well supplied with muscles contract more powerfully. As a result, blood is forced from the right ventricle into the pulmonary artery. It is prevented from flawing back by artrical ventricular aperture.
- The are prevented from turning inside out by tough strands of connective tissue called tedious chords (heart springs) which run from their underside to the wall of ventricle.
- Once in the pulmonary artery, blood is prevented from flowing back into the ventricle by pocket (semilunan) values guarding the opening of the artery. From the lungs, oxygenated blood returns to the ventricle and aortic (systemic) arch. This movement of blood takes place in the same way as the right side of the heart.
- Deoxygenated blood is pumped from the right ventricle in the pulmonary artery at the same time oxygenated blood is pumped into the aorta.
- Systole is followed by diastole in which relaxation of the ventricle of the heart takes place. This volume increases and pressure decreases enabling the ventricles to be filled with blood again.
- The whole sequence of events i.e. systole (contraction) and diastole (relaxation) is known as the cardiac cycle, and accompanied by electrical activity in the wall of the heart and by sounds corresponding to the classing of various values.
- One cardiac cycle lasts about 0.6-0.8 seconds.
- The heartbeat is the term used to describe one complete cardiac cycle i.e. systole to diastole of atria followed by that of ventricles respectively.
SUMMARY OF EVENTS THAT OCCUR DURING CARIAC CYCLE.
Blood enters atria and ventricles from pulmonary veins and venacasa.
Atria are released and fill with blood ventricles are also relaxed (Thus you can’t talk about atria diastole and ventricular diastole) they are separated like for systole).
- Atria relax
- Ventrice contracts pushing blood away from heart through pulmonary artery and the aorta.
- Pocket values are open
- Bicuspid and tricuspid values are closed.
- The beating produces sounds known at heartbeats which can be heard by putting an car on the chest wall. But an instrument called stethoscope makes the heart beat of the heart muscle but by blood hitting the heart values. The two sounds of the heart beat are described as ‘lub-dub’.
- The first sound is made when blood is forced back against the atrio-ventricular vales as the ventricles contract and the second when a tack flow of the blood hits the semi lunar values in the aorta and pulmonary artery as the ventricles relax i.e. ventricle sytole “lub” sound and ventricular diastole “Dub” sound.
- Blood pressure is measured in millimeters of mercury mm/Hg. Because blood pressure changes from systole to diastole, it is usually recorded as two numbers.
- This first one is the pressure during (systolic pressure).
- The second one is the pressure during diastole (diastolic pressure).
- The blood pressure of a normal healthy adult at rest is
120 – Systolic
180 – Diastolic.
- Blood pressure is measured using a device called sphygmomanometer.
PRESSURE CHANGES IN ATRIA VENTRICLES AND AORTA AND VOLUME CHANGES OF THE VENTRICELS DURING ONE MAMMALIAN CARDIAC (HEART BEAT)
A- Shoes atrium contracting (atria systole)
Atrial pressure exceeds ventricular pressure
Pocket values closed.
B- Ventricle starts to contract (ventricular systole)
Ventricular pressure exceeds atrial pressure so atrial ventricular
values (Bicuspid and Tricuspid value) close producing first sound, lub.
C- Ventricular pressure axceeds aortic pressure.
Thus aortic value open (ie pocket values open)
Blood flows from ventricle into the aorta and the Volume of ventricles fall.
Ventricular pressure falls below aortic pressure.
Closure of aortic value hence second sound, Dub
Volume of ventricle starts to rise.
E- Ventricular pressure fall below atrial pressure.
Blood flows from atrium to ventricle and
Volume of ventricle rises rapidly
F- Atrial pressure exceeds ventricular pressure
Atrium fills with blood from pulmonary vein
Blood flows from atrium to ventricle