Skeletal system

Arms:-

Humerus UlnaRadius

Humerus

he humerus is a long bone in the arm or forelimb that runs from theshoulder to the elbow.T

Anatomically, it connects the scapula and the lower arm (consisting of the radius and ulna), and consists of three sections. The upper extremity consists of a rounded head, a narrow neck, and two short processes (tubercles, sometimes called tuberosities.) Its body is cylindrical in its upper portion, and more prismatic below. Thelower extremity consists of 2 epicondyles, 2 processes (trochlea & capitulum), and 3 fossae (radial fossa, coronoid fossa, and olecranon fossa). As well as its true anatomical neck, the constriction below the greater and lesser tubercles of the humerus is referred to as its surgical neck due to its tendency to commonly get fractured, thus often becoming the focus of surgeons.

Muscles attached to the humerus

The deltoid originates on the lateral third of the clavicle, acromion and the crest of the spine of the scapula. It is inserted on the deltoid tuberosity of the humerus and has several actions including abduction, extension, and circumduction of the shoulder. The supraspinatus also originates on the spine of the scapula. It inserts on the greater tubercle of the humerus, and assists in abduction of the shoulder.

The pectoralis major, teres major, and latissimus dorsi insert at the intertubercular groove of the humerus. They work to adduct and medially, or internally, rotate the humerus.

The infraspinatus and teres minor insert on the greater tubercle, and work to laterally, or externally, rotate the humerus. In contrast, the subscapularis muscle inserts onto the lesser tubercle and works to medially, or internally, rotate the humerus.

The biceps brachii, brachialis, coracobrachialis, and brachioradialis (which attaches distally) act to flex the elbow. (The biceps, however, does not attach to the humerus.)

1

The triceps brachii and anconeus extend the elbow, and attach to the posterior side of the humerus.

The four muscles of supraspinatus, infraspinatus, teres minor and subscapularis form a musculo-ligamentous girdle called the rotator cuff. This cuff stabilizes the very mobile but inherently unstable glenohumeral joint. The other muscles are used as counterbalances for the actions of lifting/pulling and pressing/pushing.

2

Ulna

he ulna is one of the two long bones in the forearm, the other being the radius. It is prismatic in form and runs parallel to the

radius, which is shorter and smaller. In anatomical position (i.e. when the arms are down at the sides of the body and the palms of the hands face forward) the ulna is located at the side of the forearm closest to the body (the medial side), the side of the little finger.

T

Articulation

The ulna articulates with:

trochlea of the humerus, at the right side elbow as a hinge joint with semilunar trochlear notch of the ulna.

the radius, near the elbow as a pivot joint, this allows the radius to cross over the ulna in pronation.

the distal radius, where it fits into the ulna notch.

the radius along its length via the interosseous membrane that forms a syndesmoses joint

it is also called the poisidion

Structure

The long, narrow medullary cavity is enclosed in a strong wall of compact tissue which is thickest along the interosseous border and dorsal surface. At the extremities the compact layer thins. The compact layer is continued onto the back of the olecranon as a plate of close spongy bone with lamellæ parallel. From the inner surface of this plate and the compact layer below it trabeculæ arch forward toward the olecranon and coronoid and cross other trabeculæ, passing backward over the medullary cavity from the upper part of the shaft below the coronoid. Below the coronoid process there is a small area of compact bone from which trabeculæ curve upward to end obliquely to the surface of the semilunar notch which is coated with a thin layer of compact bone. The trabeculæ at the lower end have a more longitudinal direction.

3

Radius

he radius is one of the two large bones of the forearm, the other being the ulna. It extends from

the lateral side of the elbow to the thumb side of the wrist and runs parallel to the ulna, which exceeds it in length and size. It is a long bone, prism-shaped and slightly curved longitudinally. The radius articulates with the capitulum of the humerus, the radial notch and the head of the ulna. The corresponding bone in the lower leg is the tibia.

T

The word radius is Latin for "ray". In the context of the radius bone, a ray can be thought of rotating around an axis line extending diagonally from center of capitulumto the center of distal ulna. While the ulna is the major contributor to the elbow joint, the radius primarily contributes to the wrist joint. 

The radius is named so because the radius (bone) acts like the radius (of a circle). The ulna acts as the center point to the circle because when the arm is rotated the ulna does not move. The radius (bone) acts like the radius (of a circle) because it rotates around the ulna and the far end, known as the styloid process of the radius, is the distance from the ulna (center of the circle) to the edge of the radius (the circle).

Shape

The radius is one of the two large bones of the forearm, the other being the ulna. It extends from the lateral side of the elbow to the thumb side of the wrist and runs parallel to the ulna, which exceeds it in length and size. It is a long bone, prism-shaped and slightly curved longitudinally. The radius articulates with the capitulum of the humerus, the radial notch and the head of the ulna. The corresponding bone in the lower leg is the tibia.

The word radius is Latin for "ray". In the context of the radius bone, a ray can be thought of rotating around an axis line extending diagonally from center of capitulumto the center of distal ulna. While the ulna is the major contributor to the elbow joint, the radius primarily contributes to the wrist joint. 

The radius is named so because the radius (bone) acts like the radius (of a circle). The ulna acts as the center point to the circle because when the arm is rotated the ulna does not move. The radius (bone) acts like the radius (of a circle) because it rotates around the ulna and the far end, known as the styloid process of the radius, is the distance from the ulna (center of the circle) to the edge of the radius (the circle).

Muscle attachments

4

The biceps muscle inserts on the radial tuberosity of the upper extremity of the bone. Sometimes it may break. The upper third of the body of the bone attaches to the supinator, the flexor digitorum superficialis, and the flexor pollicis longus muscles. The middle third of the body attaches to the extensor ossis metacarpi pollicis, extensor primi internodii pollicis, and the pronator teres muscles. The lower quarter of the body attaches to the pronator quadratus muscle and the tendon of the supinator longus.

Structure

The long narrow medullary cavity is enclosed in a strong wall of compact bone. It is thickest along the interosseous border and thinnest at the extremities, save over the cup-shaped articular surface (fovea) of the head.

The trabeculae of the spongy tissue are somewhat arched at the upper end and pass upward from the compact layer of the shaft to the fovea capituli (the humerus's cup-shaped articulatory notch); they are crossed by others parallel to the surface of the fovea. The arrangement at the lower end is somewhat similar. It is missing in radial aplasia.

5

Chest:-

RibSternum

Rib

umans have 24 ribs (12 pairs). The first seven sets of ribs, known as "true ribs", are directly attached to the sternum through the costal cartilage. Rib 1 is unique and harder to distinguish from other ribs. It is a short, flat, C-shaped bone. The

vertebral attachment can be found just below theneck and the majority of this bone can be found above the level of the clavicle. Ribs 2 through 7 have a more traditional appearance.[1] The following five sets are known as "false ribs", three of these sharing a common cartilaginous connection to the sternum, while the last two (eleventh and twelfth ribs) are termed floating ribs (costae fluitantes) or vertebral ribs. They are attached to the vertebrae only, and not to the sternum or cartilage coming off of the sternum. Some people are missing one of the two pairs of floating ribs, while others have a third pair. Rib removal is the surgical excision of ribs for therapeutic or cosmetic reasons.

H

In general, human ribs increase in length from ribs 1 through 7 and decrease in length again through rib 12. Along with this change in size, the ribs become progressively oblique (slanted) from ribs 1 through 9, then less slanted through rib 12.

The ribcage is separated from the lower abdomen by the thoracic diaphragm which controls breathing. When the diaphragm contracts, the thoracic cavity is expanded, reducing intra-thoracic pressure and drawing air into the lungs. This happens through one of two actions (or a mix of the two): when the lower ribs the diaphragm connects to are stabilized by muscles and the central tendon is mobile, when the muscle contracts the central tendon is drawn down, compressing the cavity underneath and expanding the thoracic cavity downward. When the central tendon is stabilized and the lower ribs are mobile, a contraction of the diaphragm elevates the ribs, which works in conjunction with other muscles to expand the thoracic cavity upward.

6

Sternum

he sternum or breastbone, in vertebrate anatomy, is a flat bone. It probably first evolved in early tetrapods as an extension of the pectoral girdle; it is not found in fish. In amphibians and reptiles it is typically a shield-shaped structure, often

composed entirely of cartilage. It is absent in both turtles and snakes. In birds it is a relatively large bone and typically bears an enormous projecting keel to which the flight muscles are attached. Only in mammals does the sternum take on the elongated, segmented form seen in humans. In some mammals, such as opossums, the individual segments never fuse and remain separated by cartilagenous plates throughout life.

T

7

Shoulder:-

Clavicle Scapula

Clavicle

n human anatomy, the clavicle or collar bone is a long bone of short length that serves as a strut between the scapula and the sternum. It is the only long bone in body that lies horizontally. It makes up part of the shoulder and the pectoral girdle and is palpable in

all people, and, in people who have less fat in this region, the location of the bone is clearly visible as it creates a bulge in the skin. It receives its name from the Latin: clavicula ("little key") because the bone rotates along its axis like a key when the shoulder is abducted, and it roughly the same shape as Roman door lock keys.

I

The clavicle is a doubly curved short bone that connects the arm (upper limb) to the body (trunk), located directly above the first rib. It acts as a strut to keep the scapula in position so the arm can hang freely. Medially, it articulates with the manubrium of the sternum (breast-bone) at the sternoclavicular joint. At its lateral end it articulates with the acromion of the scapula (shoulder blade) at the acromioclavicular joint. It has a rounded medial end and a flattened lateral end.

From the roughly pyramidal sternal end, each clavicle curves laterally and anteriorly for roughly half its length. It then forms a smooth posterior curve to articulate with a process of the scapula (acromion). The flat, acromial end of the clavicle is broader than the sternal end. The acromial end has a rough inferior surface that bears prominent line, Trapezoid line and a small rounded projection, Conoid tubercle. These surface features are attachment sites for muscles and ligaments of the shoulder.

It can be divided into three parts. Medial end, lateral end and shaft.

8

Scapula

n anatomy, the scapula (Medical Latin), omo, or shoulder blade, is the bone that connects the humerus (upper

arm bone) with the clavicle (collar bone).IThe scapula forms the posterior (back) located part of the shoulder girdle. In humans, it is a flat bone, roughly triangular in shape, placed on a posterolateral aspect of the thoracic cage.

9

Back:-

IliumSacrumVertobral column

Ilium

he ilium is the uppermost and largest bone of the pelvis, and appears in most vertebrates including mammals and birds, but not bony fish. All reptiles have an ilium except snakes, although some snake species have a tiny bone which is

considered to be an ilium. TThe name comes from the Latin (ile, ilis), meaning "groin" or "flank.

The ilium of the human is divisible into two parts, the body and the ala; the separation is indicated on the top surface by a curved line, the arcuate line, and on the external surface by the margin of the acetabulum.

10

Sacrum

n vertebrate anatomy the sacrum (plural: sacrums or sacra) is a large, triangular bone at the base of the spine and at the upper and back part of the pelvic cavity, where it is inserted like a wedge between the two hip bones. Its upper part connects with the last lumbar vertebra, and bottom part with the coccyx (tailbone). It consists of usually five initially unfused vertebrae which begin to fuse between ages 16–18 and are usually completely fused into a single bone by age 26.

It is curved upon itself and placed obliquely (that is, tilted forward). It is kyphotic — that is, concave facing forwards. The base projects forward as the sacral promontory internally, and articulates with the last lumbar vertebra to form the prominent sacrovertebral angle. The central part is curved outward towards the posterior, allowing greater room for the pelvic cavity.

Articulation

The sacrum articulates with four bones:

the last lumbar vertebra above

the coccyx (tailbone) below

the illium portion of the hip bone on either side

Rotation of the sacrum forward a few degrees vis-à-vis the ilia is sometimes called "nutation" (L. "nodding"), and the reverse motion "counter-nutation."[3]

It is called the sacrum when referred to all of the parts combined, but sacral vertebrae when referred individually.

11

Vertobral Column

The vertebral column is a bony structure found in Vertebrates. It is formed from the vertebrae.

12

Legs:-

FemurPatellaTibiaFibula

Femur

he femur (pl. femurs or femora), or thigh bone, is the most proximal (closest to the center of

the body) bone of the leg in tetrapod vertebrates capable of walking orjumping, such as most land mammals, birds, many reptiles such as lizards, and amphibians such as frogs. In vertebrates with four legs such as dogs and horses, the femur is found only in the rear legs. The femur is the largest bone in the body. The head of the femur articulates with the acetabulum.

T

In human anatomy, the femur is the longest and largest bone in the body. The average adult male femur is 48 centimeters (18.9 in) in length and 2.8 cm (1.1 in) in diameter at the mid-shaft, and has the ability to support up to 30 times the weight of an adult. The femur forms part of the hip joint (at the acetabulum) and part of the knee joint, above which it is located. There are four eminences, or protuberances, in the human femur: the head, the greater trochanter, the lesser trochanter, and the lower extremity. They appear at various times from just before birth to about age 14. Initially, they are joined to the main body of the femur with cartilage, which gradually becomes ossified until the protuberances become an integral part of the femur bone, usually in early adulthood.

The shaft of the femur is cylindrical and has a rough line, the linea aspera, on its posterior surface.

The intercondylar fossa is present between the condyles at the distal end of the femur. In addition to the intercondylar eminence on the tibial plateau, there is both an anterior and posterior intercondylar fossa (area), the sites of anterior cruciate and posterior cruciate ligament attachment, respectively.

13

Patella

he patella, also known as the knee cap or kneepan, is a thick, circular-triangular bone which articulates with the femur and covers and protects the anterior articular surface of the knee joint. It is the largest sesamoid bone in the

human body.TStructure

The patella is roughly triangular in shape with its base facing proximally (towards the torso) and its tip (apex patellae) facing distally (towards the feet). Its anterior and posterior surfaces are joined laterally (left/right) by a thinner margin and medially (towards centre) by a thicker margin.

Muscles attached

It is attached to the tendon of the quadriceps femoris muscle, which contracts to extend/straighten the knee. The vastus intermedialis muscle is attached to the base of patella. The vastus lateralis and vastus medialis are attached to lateral and medial borders of patella respectively.

The patella is stabilized by the insertion of vastus medialis and the prominence of the anterior femoral condyles, which prevent lateral dislocation during flexion. The retinacular fibres of the patella also stabilize it during exercise.

14

Tibia

he tibia, shinbone, or shankbone is the larger and stronger of the two bones in the leg below the knee in vertebrates

(the other being the fibula), and connects the knee with the ankle bones. The tibia is named for the Greek aulos flute, also known as a tibia. It is commonly recognized as the strongest weight bearing bone in the body.

T

Structure

It is prismoid in form, expanded above, where it enters into the knee-joint, contracted in the lower third, and again enlarged but to a lesser extent towards the ankle joint.

The superior tibiofibular articulation is an arthrodial joint between the lateral condyle of the tibia and the head of the fibula. The inferior tibiofibular articulation (tibiofibular syndesmosis) is formed by the rough, convex surface of the medial side of the lower end of the fibula, and a rough concave surface on the lateral side of the tibia. The tibia is connected to the fibula by an interosseous membrane, forming a type of joint called asyndesmosis. The forward flat part of the tibia is called the fibia, often confused with the fibula.

Strength

The tibia has been modeled as taking an axial force during walking that is up to 4.7 bodyweight. Its bending moment in the sagittal plane in the late stance phase is up to 71.6 bodyweight times millimetre.

15

Fibula

The fibula or calf bone is a bone placed on the lateral side of the tibia, with which it is connected above and below. It is the smaller of the two bones, and, in proportion to its length, the most slender of all the long bones. Its upper extremity is small, placed toward the back of the head of the tibia, below the level of the knee-joint, and excluded from the formation of this joint. Its lower extremity inclines a little forward, so as to be on a plane anterior to that of the upper end; it projects below the tibia, and forms the lateral part of the ankle-joint.

16

Heart:-

Blood Circulation

he circulatory system is an organ system that passes nutrients (such as amino acids, electrolytes and lymph), gases,

hormones, blood cells, etc. to and from cells in the body to help fight diseases, stabilize body temperature and pH, and to maintain homeostasis.

TThis system may be seen strictly as a blood distribution network, but some consider the circulatory system as composed of the cardiovascular system, which distributes blood, and the lymphatic system, which distributes lymph. While humans, as well as other vertebrates, have a closed cardiovascular system (meaning that the blood never leaves the network of arteries, veins and capillaries), some invertebrate groups have an open cardiovascular system. The most primitive animal phyla lack circulatory systems. The lymphatic system, on the other hand, is an open system.

Two types of fluids move through the circulatory system: blood and lymph. The blood, heart, and blood vessels form the cardiovascular system. The lymph, lymph nodes, and lymph vessels form the lymphatic system. The cardiovascular system and the lymphatic system collectively make up the circulatory system.

Human Cardiovascular System

The main components of the human cardiovascular system are the heart, blood, and blood vesselsIt includes: the pulmonary circulation, a "loop" through the lungswhere blood is oxygenated; and the systemic circulation, a "loop" through the rest of the body to provide oxygenated blood. An average adult contains five to six quarts (roughly 4.7 to 5.7 liters) of blood, which consists of plasma, red blood cells, white blood cells, and platelets. Also, the digestive system works with the circulatory system to provide the nutrients the system needs to keep the heart pumping.

Pulmonary circulation

The pulmonary circulatory system is the portion of the cardiovascular system in which oxygen-depleted blood is pumped away from the heart, via the pulmonary artery, to the lungs and returned, oxygenated, to the heart via the pulmonary vein.

Oxygen deprived blood from the vena cava, enters the right atrium of the heart and flows through the tricuspid valve (right atrioventricular valve) into the right ventricle, from which it is then pumped through the pulmonary semilunar valve into the pulmonary artery to the

17

lungs. Gas exchange occurs in the lungs, whereby CO2 is released from the blood, and oxygen is absorbed. The pulmonary vein returns the now oxygen-rich blood to the heart.

Systemic circulation

Systemic circulation is the portion of the cardiovascular system which transports oxygenated blood away from the heart, to the rest of the body, and returns oxygen-depleted blood back to the heart. Systemic circulation is, distance-wise, much longer than pulmonary circulation, transporting blood to every part of the body.

Coronary circulation

The coronary circulatory system provides a blood supply to the heart. As it provides oxygenated blood to the heart, it is by definition a part of the systemic circulatory system.

Heart

The heart pumps oxygenated blood to the body and deoxygenated blood to the lungs. In the human heart there is one atrium and one ventricle for each circulation, and with both a systemic and a pulmonary circulation there are four chambers in total: left atrium, left ventricle, right atrium and right ventricle. The right atrium is the upper chamber of the right side of the heart. The blood that is returned to the right atrium is deoxygenated (poor in oxygen) and passed into the right ventricle to be pumped through the pulmonary artery to the lungs for re-oxygenation and removal of carbon dioxide. The left atrium receives newly oxygenated blood from the lungs as well as the pulmonary vein which is passed into the strong left ventricle to be pumped through the aorta to the different organs of the body.

Closed cardiovascular system

The cardiovascular systems of humans are closed, meaning that the blood never leaves the network of blood vessels. In contrast, oxygen and nutrients diffuse across the blood vessel layers and enters interstitial fluid, which carries oxygen and nutrients to the target cells, and carbon dioxide and wastes in the opposite direction. The other component of the circulatory system, the lymphatic system, is not closed.

Oxygen transportation

About 98.5% of the oxygen in a sample of arterial blood in a healthy human breathing air at sea-level pressure is chemically combined with haemoglobin molecules. About 1.5% is physically dissolved in the other blood liquids and not connected to haemoglobin. The haemoglobin molecule is the primary transporter of oxygen in mammals and many other species.

Development

18

The development of the circulatory system initially occurs by the process of vasculogenesis. The human arterial and venous systems develop from different embryonic areas. While the arterial system develops mainly from the aortic arches, the venous system arises from three bilateral veins during weeks 4 - 8 of human development.

Arterial development

The human arterial system originate from the aortic arches and from the dorsal aortae starting from week 4 of human development. Aortic arch 1 almost completely regresses except to form the maxillary arteries. Aortic arch 2 also completely regresses except to form the stapedial arteries. The definitive formation of the arterial system arise from aortic arches 3, 4 and 6. While aortic arch 5 completely regreses.

The dorsal aortae are initially bilateral and then fuse to form the definitive dorsal aorta. Approximately 30 posterolateral branches arise off the aorta and will form theintercostal arteries, upper and lower extremity arteries, lumbar arteries and the lateral sacral arteries. The lateral branches of the aorta form the definitive renal, suprarrenaland gonadal arteries. Finally, the ventral branches of the aorta consist of the vitelline arteries and umbilical arteries. The vitelline arteries form the celiac, superior andinferior mesenteric arteries of the gastrointestinal tract. After birth, the umbilical arteries will form the internal iliac arteries.

Venous development

The human venous system develops mainly from the vitelline veins, the umbilical veins and the cardinal veins, all of which empty into the sinus venosus.

19

Arteries

rteries are blood vessels that carry blood away from the heart. This blood is normally oxygenated, exceptions made for

the pulmonary and umbilical arteries.AThe circulatory system is extremely important for sustaining life. Its proper functioning is responsible for the delivery of oxygen and nutrients to all cells, as well as the removal of carbon dioxide and waste products, maintenance of optimum pH, and the mobility of the elements, proteins and cells of the immune system. In developed countries, the two leading causes of death, myocardial infarction and stroke, each may directly result from an arterial system that has been slowly and progressively compromised by years of deterioration. (See atherosclerosis).

Description

The arterial system is the higher-pressure portion of the circulatory system. Arterial pressure varies between the peak pressure during heart contraction, called the systolic pressure, and the minimum, or diastolicpressure between contractions, when the heart expands and refills. This pressure variation within the artery produces the pulse which is observable in any artery, and reflects heart activity. Arteries also aid the heart in pumping blood. Arteries carry blood away from the heart. Veins whereas keep blood flowing towards the heart. Except pulmonary arteries, which carry blood to the lungs for oxygenation, all arteries carry oxygenated blood away from the heart to the tissues that require oxygen.

Arterial System

Systemic arteries

Systemic arteries are the arteries of the systemic circulation, which is the part of the cardiovascular system which carries oxygenated blood away from the heart, to the body, and returns deoxygenated blood back to the heart.

Pulmonary arteries

Pulmonary arteries are the arteries of the pulmonary circulation, which is the portion of the cardiovascular system which carries deoxygenated blood away from the heart, to the lungs, and returns oxygenated blood back to the heart.

Type of Arteries

Pulmonary arteries

The pulmonary arteries carry deoxygenated blood that has just returned from the body to the heart towards the lungs, where carbon dioxide is exchanged for oxygen.

Systemic arteries

20

Systemic arteries can be subdivided into two types - muscular and elastic - according to the relative compositions of elastic and muscle tissue in their tunica media as well as their size and the makeup of the internal and external elastic lamina. The larger arteries (>10mm diameter) are generally elastic and the smaller ones (0.1-10mm) tend to be muscular. Systemic arteries deliver blood to the arterioles, and then to the capillaries, where nutrients and gasses are exchanged.

Aorta

The aorta is the root systemic artery. It receives blood directly from the left ventricle of the heart via the aortic valve. As the aorta branches, and these arteries branch in turn, they become successively smaller in diameter, down to the arteriole. The arterioles supply capillaries which in turn empty into venules. The very first branches off of the aorta are the coronary arteries, which supply blood to the heart muscle itself. These are followed by the branches off the aortic arch, namely the brachiocephalic artery, the left common carotid and the left subclavian arteries.

21

Veins

n the circulatory system, veins (from the Latin vena) are blood vessels that carry blood towards the heart. Most veins carry deoxygenated blood from the tissues back to the heart; exceptions are the pulmonary and umbilical veins, both of which

carry oxygenated blood to the heart. Veins differ from arteries in structure and function; for example, arteries are more muscular than veins, veins are often closer to the skin and contain valves to help keep blood flowing toward the heart, whilearteries carry blood away from the heart.

I

Vein Systems

The pulmonary veins carry relatively oxygenated blood from the lungs to the heart. The superior and inferior venae cavae carry relatively deoxygenated blood from the upper and lower systemic circulations, respectively.

A portal venous system is a series of veins or venules that directly connect two capillary beds. Examples of such systems include the hepatic portal vein and hypophyseal portal system

The Thebesian veins within the myocardium of the heart are valveless veins that drain directly into the chambers of the heart. The coronary veins all empty into the coronary sinus which empties into the right atrium.

The Dural sinuses within the dura mater surrounding the brain receive blood from the brain and also are a point of entry of cerebrospinal fluid from arachnoid villi absorption. Blood eventually enters the internal jugular vein.

Function

Veins serve to return blood from organs to the heart. Veins are also called "capacitance vessels" because most of the blood volume (60%) is contained within veins. In systemic circulation oxygenated blood is pumped by the left ventricle through the arteries to the muscles and organs of the body, where its nutrients and gases are exchanged at capillaries, the blood then enter veinules, then veins filled with cellular waste and carbon dioxide. The de-oxygenated blood is taken by veins to the right atrium of the heart, which transfers the blood to the right ventricle, where it is then pumped through the pulmonary arteries to the lungs. Inpulmonary circulation the pulmonary veins return oxygenated blood from the lungs to the left atrium, which empties into the left ventricle, completing the cycle of blood circulation.

The return of blood to the heart is assisted by the action of the skeletal-muscle pump, and by the thoracic pump action of breathing during respiration. Standing or sitting for a

22

prolonged period of time can cause low venous return from venous pooling vascular shock. Fainting can occur but usually baroreceptors within the aortic sinues initiate a baroreflex such angiotensin II and norepinephrine stimulate vasoconstriction and heart rate increases to return blood flow. Neurogenic and hypovolaemic shock can also cause fainting. In these cases, the smooth muscles surrounding the veins become slack and the veins fill with the majority of the blood in the body, keeping blood away from the brain and causing unconsciousness. Jet pilots wear pressurized suits to help maintain their venous return and blood pressure.

The arteries are perceived as carrying oxygenated blood to the tissues, while veins carry deoxygenated blood back to the heart. This is true of the systemic circulation, by far the larger of the two circuits of blood in the body, which transports oxygen from the heart to the tissues of the body. However, in pulmonary circulation, the arteries carry deoxygenated blood from the heart to the lungs, and veins return blood from the lungs to the heart. The difference between veins and arteries is their direction of flow (out of the heart by arteries, returning to the heart for veins), not their oxygen content. In addition, deoxygenated blood that is carried from the tissues back to the heart for reoxygenation in systemic circulation still carries some oxygen, though it is considerably less than that carried by the systemic arteries or pulmonary veins.

Although most veins take blood back to the heart, there is an exception. Portal veins carry blood between capillary beds. For example, the hepatic portal vein takes blood from the capillary beds in the digestive tract and transports it to the capillary beds in the liver. The blood is then drained in the gastrointestinal tract and spleen, where it is taken up by the hepatic veins, and blood is taken back into the heart. Since this is an important function in mammals, damage to the hepatic portal vein can be dangerous. Blood clotting in the hepatic portal vein can cause portal hypertension, which results in a decrease of blood fluid to the liver.

Also transports proteins and other materials through out the body.

23

Capillaries

Capillaries are the smallest of a body's blood vessels and are parts of the microcirculation. They are only 1 cell thick. These microvessels, measuring 5-10 μm in diameter, connect arterioles and venules, and enable the exchange of water, oxygen, carbon dioxide, and many other nutrients and waste chemicalsubstances between blood and surrounding tissues. During embryological development, new capillaries are formed by vasculogenesis, the process of blood vesselformation occurring by a de novo production of endothelial cells and their formation into vascular tubes.The term angiogenesis denotes the formation of new capillaries from pre-existing blood vessels.

Types

There are three main types of capillaries:

Continuous - They are continuous in the sense that the endothelial cells provide an uninterrupted lining, and only allow small molecules, like water and ions to diffuse through tight junctions which leave gaps of unjoined membrane which are called intercellular clefts. Tight junctions can be further divided into two subtypes:

1. Those with numerous transport vesicles that are primarily found in skeletal

muscles, finger, gonads, and skin.

2. Those with few vesicles that are primarily found in the central nervous

system. These capillaries are a constituent of the blood-brain-barrier.

Fenestrated - Fenestrated capillaries (derived from "fenestra," Latin for "window") have pores in the endothelial cells (60-80 nm in diameter) that are spanned by a diaphragm of radially oriented fibrils and allow small molecules and limited amounts of protein to diffuse. In the renal glomerulus there are cells with no diaphragms called podocyte foot processes or "pedicels," which have slit pores with an analogous function to the diaphragm of the capillaries. Both of these types of blood vessels have continuous basal lamina and are primarily located in the endocrine glands, intestines, pancreas, and glomeruli of kidney.

Sinusoidal - Sinusoidal capillaries are a special type of fenestrated capillaries that have larger openings (30-40 μm in diameter) in the endothelium. These types of blood vessels allow red and white blood cells (7.5μm - 25μm diameter) and various serum proteins to pass using a process that is aided by a discontinuous basal lamina. These capillaries lack pinocytotic vesicles, and therefore utilize gaps present in cell junctions to permit transfer between endothelial cells, and hence

24

across the membrane. Sinusoid blood vessels are primarily located in the bone marrow, lymph nodes, and adrenal gland. Some sinusoids are special, in that they do not have the tight junctions between cells. They are called discontinuous sinusoidal capillaries, and are present in the liver and spleen where greater movement of cells and materials is necessary.

Anatomy

Blood flows away from the heart via arteries, which branch and narrow into the arterioles, and then branch further still into the capillaries. After the tissue has been perfused, capillaries join and widen to become venules and then widen more to become veins, which return blood to the heart.

Capillaries do not function on their own. The "capillary bed" is an interweaving network of capillaries supplying an organ. The more metabolically active the cells, the more capillaries they will require to supply nutrients and carry away waste products.

A capillary bed can consist of two types of vessels: true capillaries which branch mainly from metarterioles and provide exchange between cells and the circulation. Secondly, capillary beds also consist of a vascular shunt which is a short vessel that directly connects the arteriole and venule at opposite ends of the bed.

Metarterioles provide direct communication between arterioles and venules and are important in bypassing the bloodflow through the capillaries. The internal diameter of 8 μm forces the red blood cells to partially fold into bullet-like shapes and to go into single file in order for them to pass through.

Precapillary sphincters are rings of smooth muscles at the origin of true capillaries that regulate blood flow into true capillaries and thus control blood flow through a tissue.

25

Reference

www.google.com.my

www.wikipedia.org

http://www.google.com.my/imghp?hl=en&tab=wi

Buku Teks Pendidikan Jasmani dan Kesihatan Tingkatan 3

Form 3 Science Textbook

26