Blood circulation

In organisms, blood circulation systems differ in complexity:

Single circulation (in fish)

1. In less complex organisms, such as fish, blood passes through the heart only once during a complete circuit of the body.
2. This system is referred to as single circulation.
3. Blood is pumped from the heart to the gills for oxygenation and then distributed to the rest of the body before returning to the heart.

Pulmonary circulation involves the movement of deoxygenated blood from the heart to the lungs and back to the heart after oxygenation. The process occurs as follows:

1. Deoxygenated Blood Entry: Blood low in oxygen enters the heart through the vena cava and empties into the right auricle (atrium).
2. Pumping to Lungs: The right auricle pumps the blood into the right ventricle, which then contracts and pumps the blood to the lungs via the pulmonary artery.
3. Oxygenation in Lungs: In the lungs, carbon dioxide is removed from the blood, and oxygen is absorbed.
4. Return to Heart: The oxygenated blood flows back to the heart through the pulmonary vein.

The continuous movement of blood between the heart and lungs is called the pulmonary cycle.

Systemic circulation involves the movement of oxygenated blood from the heart to the rest of the body and the return of deoxygenated blood to the heart.

Double circulation in human beings

The steps are as follows:

1. Transport to the Left Auricle: Oxygenated blood from the lungs is brought to the left auricle (atrium) through the pulmonary vein.
2. Pumping to the Body: The left auricle pumps blood into the left ventricle, which has strong muscles capable of pumping blood to all parts of the body via the aorta, the largest artery.
3. Oxygen Supply to Tissues: The oxygen and nutrients in the blood are delivered to tissues.
4. Return of Deoxygenated Blood: After oxygen is used by tissues, the deoxygenated blood returns to the heart via the vena cava.

The continuous movement of blood between the heart and the body is called the systemic cycle.

Tissue fluid, also known as interstitial fluid, forms through the following process:

Blood Flow through Capillaries:

1. The aorta branches into smaller arteries, which further branch into arterioles and eventually into capillaries.
2. Capillaries have thin walls with tiny pores that allow the exchange of substances.

High Pressure at the Arterial End: At the arterial end of the capillary, high blood pressure forces some fluid out of the capillaries through the pores.

Composition of Tissue Fluid:

1. The fluid contains water, oxygen, hormones, and nutrients.
2. This fluid surrounds and bathes the cells, enabling the diffusion of nutrients and oxygen into cells.

Waste Exchange:

1. Carbon dioxide, nitrogenous wastes, minerals, and heat produced by cells diffuse into the tissue fluid.
2. This waste-laden fluid eventually returns to the bloodstream for removal through organs such as the lungs and kidneys.

Formation of tissue fluid in a capillary bed

At the venous end of the capillary, several factors contribute to the movement of tissue fluid back into the capillaries:

1. Low Blood Pressure: The blood pressure at the venous end of the capillary is lower compared to the arterial end. This reduces the force pushing the fluid out of the capillary.
2. Low Water Potential: Water potential in the blood is low due to the higher concentration of proteins and other solutes within the capillary. This encourages water to move back into the capillary from the surrounding tissue fluid.
3. Higher Pressure of Tissue Fluid: The tissue fluid has higher pressure than the blood in the capillaries at the venous end, which also helps in pushing some fluid back into the capillary.
4. Diffusion: Diffusion plays a role in the movement of gases, nutrients, and waste products, as well as the return of tissue fluid into the capillaries. Oxygen and nutrients diffuse into the cells, while waste products like carbon dioxide move into the tissue fluid for removal.
5. Remaining Tissue Fluid: Some of the tissue fluid, which contains waste products, cannot be reabsorbed by the capillaries. This fluid moves into the lymphatic system, where it forms lymph.

As blood moves from the capillaries, it enters the venous system:

Capillaries to Venules: Capillaries converge to form venules. Venules are small veins that collect blood from the capillary networks.

Venules to Veins: Venules merge to form veins, which are larger blood vessels that carry blood back towards the heart.

The Role of Veins:

• Veins transport deoxygenated blood (except for pulmonary veins) back to the heart.
• Valves in veins help prevent backflow of blood, especially in the lower extremities.

Formation of the Inferior and Superior Vena Cava:

• The inferior vena cava is formed by veins from the lower parts of the body, such as the legs and abdomen.
• The superior vena cava is formed by veins from the upper parts of the body, such as the arms and head.

Vena Cava to the Right Auricle: Both the inferior and superior vena cava merge to form the vena cava, which carries the blood to the right auricle (atrium) of the heart.

Blood circulation is a vital function in the human body, ensuring the proper functioning of cells, tissues, and organs. Below are the key reasons why blood circulation is essential:

Transportation of cell requirements

1. Blood delivers oxygen and nutrients such as glucose, amino acids, and vitamins to all cells in the body.
2. These substances are crucial for cellular respiration, energy production, and overall cell health.

Removal of waste products

1. Blood carries away harmful waste products, including carbon dioxide and metabolic byproducts, from the cells.
2. This prevents the accumulation of waste, which could otherwise lead to toxicity and damage to tissues and organs.

Regulation of body heat

1. Blood circulation helps regulate body temperature by distributing body heat evenly to maintain homeostasis.
2. For instance, during cold weather, blood flow to the skin is reduced to conserve heat, whereas in warm conditions, blood flow increases to release heat through the skin.

Transportation of hormones

Blood serves as a medium for transporting hormones from endocrine glands to target organs where they are needed.

Example:

• Insulin: A hormone produced by the pancreas is transported via blood to regulate blood sugar levels.
• Adrenaline: Released during stress, it prepares the body for action.

Blood pressure is the force exerted by circulating blood on the walls of blood vessels. It is measured using two values:

Systolic Pressure:

1. Occurs when the ventricles of the heart contract to pump blood into the arteries.
2. This is the higher value in a blood pressure reading.

Diastolic Pressure:

1. Occurs when the auricles (atria) of the heart contract, allowing blood to flow into the ventricles.
2. This is the lower value in a blood pressure reading.

Blood pressure is expressed in millimeters of mercury (mmHg) and is measured using an instrument called a sphygmomanometer.

Example: If the systolic pressure is 120 mmHg and the diastolic pressure is 80 mmHg, the blood pressure is recorded as 120/80 mmHg, which is considered normal for a healthy adult.