ISAP WK 5 - How is Blood Circulation Regulated?

What are the 5 functions of the circulatory system?

1. carries blood from the heart to tissues and back

2. exchange nutrients, waste products and gases with tissues

3. transports substances such as hormones and enzymes

4. helps to regulate blood pressure within a normal range

5. directs blood flow to the tissues

Describe the structure of blood vessels from lumen to outer wall.

Tunica interna, tunica media, tunica externa

Tunica intima

inner-most layer, consists of 4 layers: endothelium (simple squamous), basement membrane, lamina propria (connective tissue layer), internal elastic membrane consisting of fenestrated layer of elastic fibres

What separates the tunica intima from tunica media?

the internal elastic membrane

Tunica media

the middle layer, consists of smooth muscle cells that are arranged circularly around the blood vessel. Only arteries have an external elastic membrane within the tunica media layer.

What is the purpose of the internal and external elastic membrane?

Present in arteries which helps them handle the high pressure via expanding and recoiling. They provide structural support and elasticity, allowing arteries to accommodate changes in blood pressure.

Tunica externa

outermost layer. composed of connective tissue that ranged from dense (near tunica media) to loose connective tissue that merges with the surrounding connective tissue

What are the three types of arteries?

elastic, muscular and arterioles

Elastic arteries

they have the largest diameter and have greater amount of elastic tissue which allows for expand and recoil. Small amount of muscular tissue. Example includes the aorta and major branches.

Muscular arteries

Thick tunica media, 25-40 layers of smooth muscles which allows for vasoconstriction and vasodilation to regulate blood supply. Examples include medium and small sized arteries.

Arterioles

They decrease in size, diameter and wall thickness with gradual loss of wall layers down to terminal arterioles. Transportation of blood to capillaries

Venules

Tubes of endothelial cells resting on a basement membrane with a few smooth muscle cells. As diameter of venules increases, amount of smooth muscles increases and they become veins.

What are the properties of veins?

thin walls and large lumen, thin tunica interna and tunica media. Tunica externa is the predominant layer. Veins have valves that are created due to folds in the tunica interna.

Veins are compliant and capacitance. What does this mean?

Compliant - veins can easily stretch and expand due to a change of pressure. They can accept a large increase in blood volume with only a small increase in pressure.

Capacitance - veins hold a lot of blood at very low pressure

Explain the structure of capillaries

Endothelial cells sitting on a basement membrane and delicate layer of loose connective tissue on the outside

Function of capillaries?

Rapid exchange of nutrients and metabolites between blood and interstitial fluid

How do substances move cross capillaries?

via diffusion. lipid-soluble and small water-soluble molecules diffuse through the plasma membrane, while larger water-soluble molecules pass through fenestrae (gaps between endothelial cells)

What are the three types of capillaries and what are they classified based on?

Classified based on size and permeability. The three types include continuous, fenestrated and sinusoidal.

Continuous capillaries

no gaps in between endothelial cells eg. muscle, nervous tissue and skin

Fenestrated capillaries

highly permeable due to numerous fenestrae eg. intestinal villi, glomeruli in kidney and endocrine glands

Sinusoidal capillaries

larger in diameter and large fenestrae and basement membrane is sparse or often missing eg. bone marrow and liver

Describe steps of blood flow in capillaries

arterioles —> metarteriole —> thoroughfare channel —> arterial capillaries —> venous capillaries —> vanule

What is the opening and closing of capillaries controlled by?

precapillary sphincters

Where are capillaries more numerous and highly extensive?

In highly metabolic tissues like lungs, liver, kidney and skeletal muscles

Arteriovenous anastomoses

are connections between arterioles and venules that allow blood to bypass capillaries, regulating blood flow and temperature. located at palm of hands and sole of feet

Net hydrostatic pressure

blood pressure within capillaries as a result of the heart’s contraction force

Net osmotic pressure

different in osmotic pressure between the blood and interstitial fluid

Describe the net filtration pressure in relation to net hydrostatic pressure and net osmotic pressure

At the arterial end of capillaries, the net filtration pressure is positive because the net hydrostatic pressure (pressure within capillaries) is greater than the osmotic pressure. More fluid moves out of the capillaries

At the venous end of capillaries, the net filtration pressure is negative because the net hydrotatic pressure is less than the net osmotic pressure. More fluid moves into the capillary

What is blood circulation determined by and what are the two different types?

Determined by pressure, flow and resistance. The two types are laminar flow and turbulent flow.

Laminar flow

blood flows in a streamline - through long, smooth-walled, equal diameter tubes. Blood closes to the outmost layer flows the slowest due to greatest resistance, and blood within the centre flows the fastest due to less resistance

Turbulent flow

blood passes a constriction, a sharpe turn or rough surface caused by numerous small crosswise currents. Common in heart as blood flows past valves.

Blood flow is directly proportional to __________ and inversely proportional to _________. What does this mean?

the difference in pressure; resistance. Blood flows from an area of high pressure to low pressure and when resistance decreases, blood flow increases

Resistance is directly proportional to _______ and _________. And inversely proportional to __________

length of the vessel; viscosity of blood; diameter of the vessel

The rate of blood flow through a tube can be expressed as…..

the volume that passes a specific point per unit of time. mL per min

what is the rate of cardiac output at rest?

approximately 5 liters per minute

equation for blood flow

is defined as the difference in pressure divided by resistance.

(P1 - P2) / R. Where P1 and P2 are pressures in the vessel at points 1 and 2

Equation for resistance

128vl/pieD^4 where v is viscosity of blood, l is length and d is diameter of the vessel

Cardiac Output (CO)

amount of blood pumped by the heart per minute

Blood Pressure (BP)

the force blood exerts against blood vessels walls measured in mmHg

What are the two homeostatic mechanisms involved to control blood flow

local control and extrinsic (nervous and hormonal)

Local control - local metabolic factors

vasodilation increases with increased amount of carbon dioxide, temperature, nitric oxide and decreased oxygen. blood flow increases when tissue metabolism increases

Local control - other local vasoactive substances

• vasodilators: histamine, bradykinins, prostacyclins

• vasoconstrictors: endothelin-1, leukotrienes, thromboxane-A2

Local control - myogenic control

• about blood pressure

• passive stretch of blood vessels, due to blood flow, causes vasocontriction

• decreased stretch, due to sudden drop in blood flow, causes vasodilation

Extrinsic control - nervous control

• controlled by the ANS, particularly the sympathetic division

• rapid (1-30sec)

• sympathetic vasomotor fibres - innervate blood vessels and control vasoconstriction

• the vasomotor centre at lower pons and upper medulla oblongata regulates this constriction

Extrinsic control - hormonal control

• noradrenaline and adrenaline causes vasoconstriction

• adrenaline can also act on beta-2 receptors to cause vasodilation

• angiotensin II and ADH are vasoconstrictors

• ANP (atrial natriuretic peptide) is a vasodilator

Blood pressure

a measure of the force blood exerts against blood vessels walls. it fluctuates between systolic and diastolic pressure with an average of 120/80 for an adult

Mean Arterial Pressure (MAP)

important to ensure blood flow to all areas of the body and is calculated as the average blood pressure in a person's arteries during one cardiac cycle. It is commonly estimated using the formula: MAP = DBP + 1/3(PP), where PP = SBP - DBP.

Name the 4 short term regulations and the overall function

function: responds quickly for a short period of time, sec/mins, neural and hormones

1. chemoreceptor reflex

2. adrenal medullary mechanism

3. CNS’s ischemic response

4. baroreceptor reflex

Name the 3 long term regulation and the overall function

function: mechanisms that do not adapt rapidly to altered blood pressures, primarily by mechanisms that influence kidney function, hours to days

1. renin-angiotension-aldosterone mechanism

2. antidiuretic hormone

3. artial natriuretic mechanism

Baroreceptor reflexes

Baroreceptors that are sensitive to stretch and can detect small changes. Receptors are scattered along the walls of most major arteries such as neck, thorax, carotid and walls of aortic arch.

1. baroreceptors in the carotid sinus and aortic arch monitor blood pressure

2. the GLOSSOPHARYNGEAL (from carotid sinus baroreceptor) and VAGUS NERVES (from aortic arch baroreceptor) conduct action potentials to the cardioregulatory and vasomotor centers in the medulla oblongata.

3. increased parasympathetic stimulation of heart decreases heart rate OR increased sympathetic stimulation of the heart increases the heart rate and stroke volume

4. increased sympathetic stimulation of blood vessels increases vasoconstriction therefore increasing blood pressure

Adrenal Medullary Mechanism

Increase in sympathetic stimulation of the heart and blood vessels which is triggered by increase in physical activity, stress or decrease in blood pressure. Sympathetic nerve fibres stimulates the adrenal medulla which secrete epinephrin and noraepinephrin

Chemoreceptor reflex

Chemoreceptors in the carotid, aortic bodies and medulla oblongata are sensitive to low oxygen levels, low pH levels and increase in carbon dioxide levels. When detected, the parasympathetic is inhibited which increased heart rate. Sympathetic stimulation also increases heart rate, stroke volume, and vasoconstriction.

CNS’s Ischemic Response

An increase in blood pressure in response to lack of blood flow to the medulla oblongata of the brain. An emergency function which results in the vasomotor centre stimulating vasoconstriction.

Renin-Angiotensin-Aldosterone (+ADH) mechanism (RAAS)

1. low blood volume, low MAP or poor renal flow causes renin secretion from juxtaglomerular cells in the kidneys

2. renin catalyses the formation of angiotensin 1 in the blood from a precusror plasma protein, angiotensinogen

3. angiotensin 1 is converted to angiotensin 2 as the blood flows through the lungs

4. angiotensin 2 stimulates aldosterone and anti-diuretic hormone secretion which increase Na, Cl and water reabsorption from renal tubules, which leads to increased blood volume and MAP

ADH mechanism

1. ADH is a vasopressin that works in harmony with RAAS

2. decrease blood pressure detected by baroreceptors release ADH

3. ADH acts on the blood vessels to cause vasoconstriction. It also decreases the rate of urine production by kidneys to help maintain blood pressure (water reabsorption)

Atrial Natriuretic mechanism

1. Atrial Natriuretic Hormone (ANH) is a polypeptide released from cells in the atria of the heart

2. it acts on the kidneys to increase rate or urine production and Na loss via urine

3. dilates arteries and veins

4. causes decrease in blood volume, venous return and decrease in peripheral resistance

Poiseuille’s Law

flow = pie(p1-p2)d^4 / 128vl