Blood Circulation and Internal Environment in Human Beings

Posted on Apr 15, 2024 in Biology

1. Internal Environment

Internal environment is those that provides all cells with all the nutrients they need. The internal environment in human beings is formed by:

• Interstitial plasma. It is a liquid substance that fills the existing spaces between cells. Interstitial plasma needs to be renewed continuously, because new nutrients need to be provided and waste products can’t be allowed to accumulate and must be released. Blood renews interstitial plasma given that it circulates around the whole organism.
• Blood.
• Lymph.

Homeostasis is the ability or tendency of a living organism to keep the conditions of the internal environment

2. Blood Composition

Blood has two components:

a. Blood plasma

• Water with minerals (90%)
• Nutrients: glucose, amino acids, vitamins…
• Waste products: urea, uric acid, CO2…
• Proteins.
• Dissolved hormones.

b. Blood cells

• Erythrocytes or red blood cells.
  • They are the most numerous blood cells.
  • They are disk shaped and don’t have a nucleus.
  • They contain haemoglobin, a red pigment that carries oxygen.
• Leukocytes or white blood cells.
  • All leukocytes have a nucleus.
  • There are three different types: Granulocytes (neutrophils, eosinophils and basophils), monocytes and lymphocytes.
  • They participate in the defense of the organism against infections.
• Thrombocytes or platelets.
  • They are fragments of cells.
  • They contain substances that allow blood to coagulate to stop hemorrhages

3. Functions of the Blood

• It transports nutrients and oxygen to all the cells and collects waste products produced by cell metabolism.
• It contributes to the regulation of body temperature.
• It plays a role in the fight against infections.

4. Blood Circulation

Our cardiovascular system shares nutrients and oxygen between cells and collects waste products that the cells produce during metabolism. It is formed by blood, blood vessels, and the heart.

4.1. Blood Vessels

Blood vessels are responsible for carrying blood around the body. There are three different types:

a. Arteries

• They carry blood from the heart to other organs.
• They have wide and elastic muscular walls.
• As arteries get further away from the heart, they branch out and become thinner (arterioles).

b. Veins

• They carry blood from tissues to the heart.
• They have thinner walls than arteries, and they have valves that stop blood from going backwards.
• They are formed where many venules join together.

c. Capillaries

• They are microscopic blood vessels located in all the tissues.
• They are formed by walls that only have one layer of flat cells (endothelia).
• They connect arterioles and venules.

4.2.1. Heartbeat

A heartbeat is a constant movement performed by the heart that pumps blood from the veins into the atriums, and from there it goes into the ventricles and is finally released into the arteries.

• Heart rate is the number of times the heart beats per unit of time.
• Cardiac output is the volume of blood the heart pumps in one minute.

The different phases of heartbeat are:

1. Atrial systole

• The atriums contract, so blood is propelled into the ventricles.
• The atrioventricular valves (mitral and tricuspid) are opened to allow the blood enters into the ventricles.
• The sigmoid valves are closed.

• The ventricles contract, so blood is propelled into the pulmonary artery and the aorta.
• The sigmoid valves are opened to allow the blood enters into the arteries.
• The atrioventricular valves (mitral and tricuspid) are closed to stop blood from coming back into the atriums.

• The heart relaxes and sucks blood coming from the cava and pulmonary veins into the atriums.
• The sigmoid valves are closed to stop blood from coming back into the heart.
• The atrioventricular valves (mitral and tricuspid) are closed.

4.3. Blood Circuits

Blood circulation in human beings has the following characteristics:

• Circulation is closed. The blood never leaves the blood vessels.
• Circulation is complete. Oxygenated blood proceeding from the lungs never mixes with deoxygenated blood coming from the rest of the body.
• Circulation is double. Blood has to travel twice through the heart in order to complete the whole circuit. Our cardiovascular system goes through 2 separate circuits: systemic and pulmonary

The systemic circuit.

Left ventricle → aorta artery → arterioles → capillaries (reaching all the tissues and organs except the lungs) → the blood provides cells with nutrients and oxygen and removes waste products and carbon dioxide → venules → cava veins → right atrium → right ventricle

The pulmonary circuit.

Right ventricle → pulmonary artery → arterioles → capillaries (reaching pulmonary alveoli in both lungs) → gas exchange → venules → pulmonary veins → left atrium → left ventricle

5. Excretory System

6. The Interaction Process

The interaction process includes a series of processes whose objectives are:

• To adapt the body to changing conditions, in the internal as well as external environment.
• To connect and coordinate the different parts of our body so that they work together.

6.1. Homeostasis

Homeostasis is the ability or tendency of a living organism to keep the conditions of the internal environment (the temperature, the concentration of different molecules in the blood or the amount of water in the tissues). Homeostatic processes work like a negative feedback device. This means that when a variation is detected in a particular element, a series of mechanisms in the body start up to make the variation return to its normal state

6.2. Systems that Participate in the Interaction Process

The sensory organs receive information from the environment (internal and external) in the form of stimuli. The information is transformed into nerve impulses and transmitted to the nervous system. The nervous system receives the information, processes it and gives a response, which is transferred to the effector organs. The effector organs carry out the order and are grouped into the locomotor system, responsible for motor response (movement) and the endocrine system, responsible for sensory response (production of regulating hormones).

7. The Neuron

The neuron is the anatomical and physiological unit of the nervous system. They are the most specialized cells in the body, so their capacity of reproduction is very limited. They are made up of:

Neurons can be categorized by different criteria:

• Depending on their structure, neurons can be unipolar, multipolar, or bipolar.
• Depending on their function, neurons are classified into:
  • Sensory neurons. They transmit information from the receptors to the nerve centers.
  • Motor neurons. They transmit information from the nerve centers to the effector organs.
  • Interneurons. They connect the sensory neurons and the motor neurons in the brain and spinal cord.

8. Electric Signals

Neurons generate and transmit nerve impulses. When a neuron is stimulated, some electrical changes originate in the membrane and these transmit the electrical signal from the dendrites towards the axon. There is no physical contact between neurons, there is a small gap between them called synaptic cleft. But neurons are not isolated; they work with one another to establish connections called synapses, which allow the nerve impulses to cross from one neuron to another. The synapse is found between the end of the axon of one neuron and a dendrite of an adjacent neuron. At the axon terminal there are very small vesicles that contain a substance called neurotransmitters. When the nerve impulse gets to the axon terminal, the vesicles break up and the neurotransmitters are released into the synaptic cleft. There they join the dendrite membrane of the adjacent neuron and then the nerve impulse can be started.

9. The Autonomic Nervous System

It regulates and controls involuntary body functions. The higher parts of the autonomic nervous system are located in the grey matter of the spinal cord, in the spinal bulb and in the hypothalamus, from which nerves emerge and send signals to the organs. The autonomic nervous system is divided into 2 parts:

• Sympathetic nervous system
  • Its activates most of the body’s organs so that they work more intensely.
  • This occurs in alert states or under stress.
• Parasympathetic nervous system
  • Its action provokes relaxation and the organs work more slowly and less intensely.
  • This happens while resting or relaxing.

10. The Somatic Nervous System

It connects the sensory receptors of the sensory organs to the central nervous system and this to the skeletal muscle responsible for the movement. Depending on where nerves enter the central nervous system, they are classified into:

• Cranial nerves
  • They enter and leave the brain.
  • There are 12 pairs of cranial nerves, including sensory, motor, and mixed types.
  • They innervate the head, the upper part of the trunk, and some body’s internal organs.
  • They form ganglia nerves on their journey (groups of neuronal bodies)
• Spinal nerves
  • They come out of the spinal cord.
  • There are 31 pairs of mixed nerves. Each mixed root first emerges from the spinal cord as two distinct roots, which then join together to form a single nerve cord. All the sensory nerves enter the spinal cord through the dorsal root and all the motor nerves come out of the ventral root

11. Nerve Actions

12. The Endocrine System

13. The Eye

The globe is surrounded by 3 layers:

• Sclera. It is the outermost layer. It is white and in its frontal part it becomes transparent, forming the cornea.
• Choroid. This is the second layer and it is black; however, in the iris (behind the cornea) it appears colored. In the center of the iris there is a hole, the pupil, which diameter varies to regulate the amount of light that enters into the eye.
• Retina. This is the innermost layer where 2 types of photoreceptors are located: rods and cones. Rods do not differentiate colors, but cones are capable of perceiving colors. Through the back part of the retina, nerve cell extensions of rods and cones form the optic nerve. In this part there are no photoreceptors or sight and is called the blind spot.

In the inner part of the globe there is a biconvex lens attached to the inner wall through muscles, called lens. The lens separates 2 chambers: the anterior chamber, filled with aqueous humor (similar to water), and the posterior chamber, filled with vitreous humor (a more viscous substance).

14. Accessory Organs.

They protect the ocular globe and facilitate its movement:

• Eyebrows. They divert sweat coming from the forehead to protect the eye.
• Eyelids. They are folds of the skin that protect the eye. In their innermost layer they are covered by the conjunctiva.
• Eyelashes. They diffuse light and protect the eye .
• Ocular muscles. They move he ocular globe.
• Lacrimal glands. They secrete tears to avoid dryness and they also contain a substance that kill bacteria.

15. How the Eye Works

The function of the ocular globe is to allow light to stimulate cones and rods in the retina, which produce a nerve impulse that will be carried to the brain by the optic nerve. The image that forms in the retina is inverted, but the brain corrects this and allows us to see the image the right way up. During the journey of the light through the ocular globe, two processes occur:

1. Regulation of light intensity. In this mechanism the pupil intervenes, opening and closing according to light conditions.
2. Focusing on images. It is achieved by modifying the lens: it flattens in order to focus on objects that are far away and widens in order to see objects that are near.

Sight in humans is stereoscopic, which means it can perceive objects in three dimensions, their size…

16. The Ear

A. The Outer Ear

. It is formed by:
● The pinna (the visible ear).
● The external auditory canal that contains glands to secrete protective wax.
● Eardrum. It is a membrane located between the outer ear and the middle ear.
B. The middle ear. It is a cavity in the temporal that starts in the eardrum and reaches two
membranes called the oval window and the round window. It contains three small bones
(ossicles): the hammer (it rests on the eardrum), anvil and stirrup (it rests on the oval
window).
C. The inner ear. It is the deepest section. It is formed by the membranous labyrinth, a group of
layers located in a series of complex cavities of the temporal bone called the bony labyrinth.
Separating the two labyrinths there is a fluid, the perilymph, and inside the membranous
labyrinth there is another type of fluid, endolymph. The membranous labyrinth has several
sections:
● Cochlea. It detects sounds because it contains the organ of Corti, formed by
mechanoreceptors.
● Vestibular system. It is responsible for spatial awareness and balance. It is composed
of 3 semicircular canals, the utricle and the saccule.
The inner ear communicates with the pharynx through the Eustachian tube, that allows to
regulate the pressure on both sides of the eardrum.