Cardiovascular III

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The blood vessels

Vein and venules compared to arteries:

Thinner walls

Larger diameter

Closer to the skin

Less muscle and elastic tissue

Functions:

Arteries: have lots of smooth muscle and are very elastic, they arelow resistance tubes important for maintaining an initial high pressure to allow circulation.

Arterioles: Determine relative blood flow in organs at any MAP. Contains smooth muscle for vasodilation and vasoconstriction. The flow of blood depends on resistance which is determined by the radii.

Blood flow = (MAP - venous pressure ~0)/ Resistance of organ

Basal/intrinsic tone is spontaneous contractile activity of arterioles, it can be increased/decreased by signals like neurotransmitters.

Capillaries: have no smooth muscle and function as exchange vessels for gases and solutes. Most cells are no more than 0.1mm from a capillary. They have a large surface area for exchange. Metarteries connect arterioles and venules

Veins: have some smooth muscle which allows veins to alter their capacity and therefore the filling pressure of the heart contributing to the stroke volume. They hold 65-70% of the total blood volume. Valves ensure one way flow, skeletal muscle contraction and respiratory pumps contribute to venous return.

The Endothelium

Endothelial cells are important in regulating local vasodilation e.g nitric oxide production. They are a permeability barrier and secrete cytokines in immune responses

Blood Pressure: Generated by Ventricular Contraction

Ventricular contraction: ventricle contraction during systole, semi lunar valve opens, aorta and arteries expand storing pressure in elastic walls ejecting blood into the arteries. ~1/3 of the Stroke volume leaves the capillaries.

Ventricular Relaxation: Isovolumetric ventricular relaxation during diastole, semi lunar valve shuts, elastic recoil of arteries sends blood into rest of the circulatory system. Volume and pressure decrease slowly but never reach 0.

Max pressure during ejection is systolic pressure. Min pressure just before ejection is Diastolic pressure. The difference between the two is the pulse pressure which is determined by stoke volume, speed of ejection and arterial compliance.

Mean arterial blood pressure = Diastolic pressure + 1/3 (pulse pressure) is the average pressure during cycle, it is the driving force for blood flow, elastic rebound evens and maintains pressure (systolic/diastolic ~ 120/80mmHG). Blood pressure is highest in arteries then falls throughout the circulation.

The mean arterial is determined(directly proportional) by Cardiac output and peripheral resistance . Arterioles are the main site of variable resistance in the systemic circulation.

Blood pressure is affected by and can be conpensated by:

Changes in blood volume, varying cardiac output, varying peripheral resistance(hypertension) and redistributing blood within the system.

Local Mechanisms to maintain BP

This occurs by autoregulation which is independent of nerves/hormones allowing local control of resistance to allow changes in flow or keep flow/pressure constant.

Metabolic: A decrease in oxygen, increase in CO2, low pH and raised [K] causes vasodilation.

Paracrine: increases in flow, release of NO from endothelium causes vasodilation. Other paracrine vasodilation hormones are histamine and bradykinin.

Myogenic: increase in pressure can cause local arteriolar constriction to ensure capillary pressure is not raised. Caused by changes in Ca2+ movement in smooth muscle.

CNS pathways to control BP: The baroreceptor reflex

The main role of the reflex is to keep arterial blood pressure within narrow limits and to be able to initiate changes withing seconds

If peripheral vasodilation occurs, arterial blood pressure will tend to fall, this is detected by baroreceptors in the carotis sinus and aortic arch. The carotid sinus is thinner than other parts of the carotid artery so it streches more in response to pulse pressure.

A rise in blood pressure causes an increase in firing of nerves signal to the brain and vice versa. The cardiovascular centre in the medulla responds by reducing parasympathetic tone to the heart and increasing sympathetic tone to the heart, arterioles and veins.

The Renin Angiotensin Aldosterone System

Renin is a hormone enzyme that is released from the kidney, it results in the production of Angiotensin II. This stimulates thirst, constricts blood vessels and increases soduim and water reabsorbtion by the kidneys. This leads to increased blood volume and peripheral resistance.

The mechanisms that modulate blood pressure ensure:

The circulation works effectively: blood flows and capillary exchange is efficient

Blood supply to the tissue is related to metabolic need: allows for large variations

Lymphatic System:

Structures:

Capillaries with valves

Lymph vessels

Lymph nodes and organs

Roles:

Collect excess ECF and return it to plasma

Immune roles

Small amounts of interstitial fluid enters the lymphatic capillaries by bulk flow, the network ends in two large ducts which drains into the veins.

Measuring BP: Sphygomomanometer

Measures systolic and diastolic BP by:

Inflating a cuff on arm until air pressure > BP which compresses the artery beneath preventing blood flow.

Air is slowly released decreasing the pressure on the artery

Once pressure is just below systolic pressure artery opens slighlty producing a high velocity flow tha causes Korotkoff's sounds. These are detected through a stethoscope. (THESE ARE NOT THE SAME AS SOUNDS HEARD BY CLOSING OF VALVES.

When the cuff pressure reaches diastolic pressure the sounds stop.