Valves and vessels

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Valves structure and function

Ensure blood flows in the right direction - prevent back flow of blood.

Cusps of valves are continuous in their attachment but free towards the centre of the opening.

Bicuspid valve has two cusps.

Tricuspid valve has three cusps- dogs have 2 with several commissural cusps at the free edge.

Cusps are joined by Chordae tendinae (fibrous tissue) to papillary muscles.

Chordae tendinae prevent valves from inverting.

Semilunar valves cusps are semilunar shaped and meet tightly in middle due to thickening of contact areas.

All 4 valves are on the same level in the heart and are supported by annulus fibrosis

valve problems

incompetence of any valves leads to regurgitation of blood

can lead to congestion and heart failure

edges get bumpy so don't close smoothly leaving gaps where blood can leak through

if valves become stenotic then heart has increased work load and increased pressure within its chambers

Valve movement

Open and close due to pressure changes

AV valves open when pressure in ventricle falls below that of the atrium. (Isovolumetric relaxation)

AV valves close as pressure in ventricles rises above that of the atria due to ventricles filling with blood during diastole

Semilunar valves open when pressure in ventricles rises above that of the aorta and pulmonary artery (isovolumetric contraction in systole)

Semilunar valves close when pressure in ventricles fall below that of the aorta and the pulmonary artery (systole)

Body fluid

2/3 of body fluid is intracellular

1/3 of body fluid is extracellular

1/3 of the extracellular fluid is plasma and the remaining 2/3 is interstitial fluid (fluid which moves steadily through extracellular space)

Plasma and ISF are held in equilibrium-- cardiovascular system maintains this

Diffusion

Rapid over short distances

All cells must be close to blood vesssels for exchange

Water soluble molecules diffuse through walls via pores between endothelial cells

Lipid soluble molecules diffuse directly through cell walls

Blood vessels for exchange are capillaries

More active cells require more blood vessels

Capillaries

Where exchange takes place

Fed by artery system

Feed into venous system

Thin walled with large diameter

Blood flow through capillaries is slow

Can be shut off in some body areas due to body demand

Can be completely shut off by arteriovenous anastomosis

Can be partially cut off by metarterioles

Intercellular cleft form between endothelial cells called gap junctions

Adjacent to gap junctions are are tight junctions which form a continuous seal

sinusoids

Very wide capillaries

Large gaps between cells lining walls of capillary

Free communication between blood plasma in sinusoid and surrounding tissues

Allows movement off cells

Slow movement of blood

Arteries

Blood is taken away from heart by arteries

Closer too heart more elastic tissue - withstand higher pressure output.

Further from heart vessels have more smooth muscle for vasoconstriction and dilation . Also contain some elastic tissue

Branch into arterioles which have smaller layer of smooth muscle

Arterioles branch into precapillary arterioles

Collateral circulation in some arteries incase of blockage. May have whole networks (Retia)

Artieries without collateral circulation are called end arteries. These can be found in the brain and coronary tissue.

Some vessels so thick that they need their own blood supply within the adventitia (vaso vasorum)

Pressure reservoir

Precapillary arterioles

Thin layer of smooth muscle

Acts as a sphincter zone regulating blood flow to the capillary bed

Contract to propel blood

Give off side branches incase of blockage--collateral circulation

controlled by body needs - tuning from lion example

arteriovenous anastomoses- cuts out capillary bed

different depending on where they are

Sphincter zones can be used to direct blood through largest direct capillary (referred to as thoroughfare channel/metarterioles/arteriovenous capillaries)

Veins

Blood returns to the heart in veins

Blood movement is passive

Contain smooth muscle and some elastic fibres

Valves present to prevent back flow of blood

When muscles move in body blood in veins is pushed towards the heart

Volume reservoir

Expand and relax to deal with changes in volume

Venules

Smooth muscle in walls

Slow diffusion can still occur

Diapedesis can occur (movement of cells out of lumen) -- process by which WBC move out of circulation to tissues

Layers of blood vessels

Following is true for arteries and veins

Capillaries have no tunica media or adventitia

Tunica intima

Internal layer with endothelial lining

In tissues highly involved in transport endothelial lining has fenestrations where cells are reduced to a thin membrane

Size of fenestrations varies

Fenestrations allow transport of water soluble molecules, proteins and new blood cells

In renal glomerulus cells disappear altogether and blood is only separated from filtrate by basal lamina

Tunia media

Middle tunica media contains varying amounts of smooth muscle and elastic tissue.

Tunica adventitia

External layer of connective tissue

Great vessels of the heart

Aorta

Receives full output of left ventricle

1st vessels to branch off do so immediately after aortic valve at the aortic bulb

At aortic bulb sinuses of valsalva form - cranial sinus forms right coronary artery and caudosinistral sinus gives rise to left coronary artery

After the aortic bulb the aorta gives rise to the brachiocephalic trunk and subclavian arteries. NB in some species these are combined in the brachiocephalic trunk which moves cranially before dividing

Brachiocephalic trunk gives off common carotid arteries which supply most structures in head

Subclavian arteries (supply the forelimb, neck, cervicothoracic junction and portion of the brain) give off -vertebral artery

-costocervical trunk

-internal thoracic artery

-superficial cervical artery

The vessel left is called the axillary artery which supplies the forelimb and the chest wall.

Internal thoracic artery courses ventrally and caudally in mediastinum supplying the pleura pericardium thymus pectoral muscles and cranial mammary glands. Then tunnels below the diaphragm to continue in abdomen as cranial epigastric artery.

Thoracic aorta continues cranially supplying the vertebrae and ribcage via dorsal intercostal arteries, the caudal extent of the ribcage by dorsal costoabdominal artery and the lungs and oesophagus by bronchoesophageal arteries.

Aorta enters abdomen via aortic hiatus of diaphragm

1st branch of the abdominal aorta is the paired phrenicoabdominal arteries which supply the diaphragm and cranial abdomen.

Then the paired lumbar arteries branch off to supply lumbar tissues

The next to branch is the coeliac artery which gives rise to the gastric, hepatic and splenic arteries

Cranial mesenteric artery arises next and supplies the small intestine and some of the large intestine

The renal arteries branch off and supply the kidneys

Testicular or ovarian arteries branch off supplying the gonads

Caudal mesenteric artery branches off to supply the colon and rectum

Paired deep circumflex iliac arteries branch off to supply the flank

Paired external iliac arteries supply the adductors of the thigh, the groin and caudal mammary glands.

Next is the internal iliac artery which supplies the pelvic viscera and walls, the gluteal muscles and the proximocaudal thigh.

The vessel left of the aorta is called the median sacral artery which becomes the median causal artery to the tail.

Pulmonary trunk

From right ventricle on cranio sinistral aspect of the heart

Slightly dilated after origin at pulmonic valve

Directs caudally and penetrates the pericardium at which point it divides into left and right pulmonary arteries

Each artery goes to the hills of its corresponding lung

Runs along with pulmonary veins and principal bronchus

Cranial and caudal venae cavae

Blood is returned from systemic circulation by cranial vena cava, caudal vena cava and coronary sinus into the right atrium

Cranial vena cava receives blood from the body cranial to the heart as well as from the thoracolumbar area and ribcage via the azygous vein and internal thoracic vein

The azygous vein passes through the aortic hiatus of the diaphragm

Head is drained by internal and external (more prominent) jugular vein

Cephalic veins drain cranial chest and forelimb

Axillary vein joins subclavian vein draining the forelimbs and some of the intercostal area

Vertebral venous plexus drains into the vertebral vein which joins the vena cava cranial to the azygous vein

The above veins combine to form the cranial vena cava

Caudal van cava scies majority of venous return from caudal to the heart

Internal iliac veins drain the rectum and anal region before joining to the external iliac veins which drain the hind limb to form the left and right common iliac veins

Right and left common iliac veins join and are joined by the deep circumflex iliac veins, the testicular/ovarian veins and renal veins to form the caudal vena cava

The portal vein drains most of the gastrointestinal tract and the hepatic veins drain into the caudal vena cava

The caudal vena cava moves cranially through diaphragm via the canal foramen then runs in the plica vena cava to the right atrium

Pulmonary veins

Drain into left atrium in 2 or 3 clumps depending on species

Coronary arteries and veins

Coronary artery supplies heart itself

Coronary sinus receives venous blood from heart wall vial great cardiac vein and the left azygous vein in ruminants and pigs

Left coronary artery is the larger vessel

Left coronary artery passes between left auricle and pulmonary trunk to reach coronary groove

Left coronary artery divides into left inter ventricular artery and the circumflex artery

Left inter ventricular artery runs in the coronary groove to heart apex

Circumflex artery runs in coronary grove to caudal side and terminates near or in right ventricular groove

Venous blood drained by great cardiac vein which opens Ito right atrium via coronary sinus.

Lack of blood supply

If blood supply is cut off it is known as ischaemia

Ischaemia can lead to infarction (death of tissues) and necrosis (death of cells)