Path of blood through Pulmonary and systemitic circulation

 

Path of Blood through Pulmonary and Systemic Circulation

When a heart contracts and forces blood into the blood vessels, there is a certain path that the blood follows through the body. The blood moves through pulmonary circulation and then continues on through systemic circulation. 

Pulmonary and systemic are the two circuits in the two-circuit system of higher animals with closed circulatory systems. Humans and other mammals have two-circuit circulatory systems: one circuit is for pulmonary circulation (circulation to the lungs, pulmo = lungs), and the other circuit is for systemic circulation (the rest of the body). As each atrium and ventricle contract, blood is pumped into certain major blood vessels, and from there, continues through the circulatory system.

Exchange of Materia

The most important function of blood circulatory system is to transport oxygen
and nutrients to the cells of the tissue and remove the metabolic waste products
from the tissues and transport them back to the organs responsible for their excretion. In the alveoli of the lungs oxygen binds to the
heam part of the haemoglobin, filled in RBCs. Blood also absorbs nutrients, especially
glucose, from villi found in the walls of the small intestine and comes back to the
heart. This blood is pumped into the aorta with full force of the contracted ventricle.
From aorta it moves into branching arteries and then arterioles. When this blood,
loaded with oxygen and nutrients, reaches capillaries, due to the low concentration of
these materials in tissues, oxygen and nutrients diffuse out of the walls of capillaries and enter tissues. The concentration of waste material is more in tissues so they diffuse into the capillaries.
Due to the pressure of blood some water also oozes out of the capillaries as interstitial fluid. This water increases the osmotic pressure outside the capillaries.
Therefore, at venous end of the capillaries water again diffuses back to the capillaries
along with dissolved waste products.

Control of the Capillary Beds. It has been estimated that an adult human being has some 60,000 miles of capillaries with a total surface area of some 800-1000 m (an area greater than three tennis courts). The total volume of this system is roughly 5 liters, the same as the total volume of blood in human body. However, if the heart and major vessels are to be kept filled, all the capillaries cannot be filled at once. So a continual redirection of blood from organ to organ takes place in response to the changing needs of the body. During vigorous exercise, for example,capillary beds in the skeletal muscles open at the expense of those in the viscera. The reverse process occurs after a heavy meal.The table below shows the distribution of blood in the human body at rest and during vigorous exercise. Note the increase in blood supply to the working organs(skeletal muscles and heart). The increased blood supply to the skin aids in the dissipation of the heat produced by the muscles. Note also that the blood supply to the brain remains constant. The total blood flow during exercise increases because of a more rapid heartbeat and also a greater volume of blood pumped at each beat.

Blood Pressure (B.P)

We know that rhythmic pumping of the heart pours the blood in to the arteries.
This pulsation can easily be felt in those arteries (like radial artery) which are near the surface of the skin and we generally call it pulse. The throb we feel (as pulse) is due to
the pressure of the blood which makes the elastic layer of arteries to expand
rhythmically allowing blood to pass through.
The force per unit area that blood exerts on the inside walls of a blood vessel is called blood pressure. It is measured in millimeter of mercury (mm Hg). It is measured with the help of an instrument called sphygmomanometer. 

Blood pressure is of two types

. *Systolic blood pressure which is felt during the ventricular contraction
.*Diastolic blood pressure which is felt during ventricular relaxation.
Systolic B.P.is greater than the diastolic pressure
. The B.P is generally expressed as a ratio in which numerator shows the systolic and denominator depicts increasing age the normal value of BP also increases due to decrease in the elasticity of

the blood vessels..the diastolic B.P. An average, healthy adult human being has a B.P. of 120/80. With The blood pressure is generated by the contraction of the left ventricle. Therefore,pressure is highest in the aorta. As the arteries branch and travel greater distances from the heart the blood pressure decreases. In capillaries the difference between systolic and diastolic pressure disappears. In capillaries the B.P. is about 40 mmHg. It decreases to less than 20 mm Hg when the blood leaves arteries and further drops in venuoles. The pressure of the blood is almost 0 mm Hg when it enters the right atrium from upper and lower vana cavae. 

Certain sensors (nerve endings) are located in the blood vessels of the human

body called Baroreceptors (or baroceptors). They detect the pressure of blood

flowing through them, and can send messages (signals) to the central nervous system to increase or decrease total peripheral resistance and cardiac output. They work as short term blood pressure regulation mechanism. Baroreceptors detect the amount of stretch of the blood vessel walls, and send the signal to the nervous system in response to this stretch.

Baroreceptors can be divided into two categories

a) High pressure arterial baroreceptors

b) Low pressure baroreceptors (also known as cardiopulmonary or volume receptors).

High pressure arterial baroreceptors are present in the aortic arch and the

carotid sinuses of the left and right internal carotid arteries. The baroreceptors found

within the aortic arch enable the examination of the blood being delivered to all the blood vessels via the systemic circuit, and the baroreceptors within the carotid arteries monitor the blood pressure of the blood being delivered to the brain.

Low pressure baroreceptors are found in large systemic veins and in the walls

of the right atrium of the heart. The low pressure baroreceptors are involved with the regulation of blood volume. The blood volume determines the mean pressure

throughout the system, in particular in the venous side where most of the blood is held

The low pressure baroreceptors have both circulatory and renal effects, they produce changes in hormone secretion which have profound effects on the retention of salt and water and also influence intake of salt and water. The renal effects allow the receptors to change the mean pressure in the system in the long term.