Atrium (heart)

# Overview
- In anatomy, the atrium (plural: atria) refers to a chamber or space. As such it may for example be the atrium of the lateral ventricle in the brain or, popularly, the blood collection chamber of a heart.
- It has a thin-walled structure that allows blood to return to the heart.
- There is at least one atrium in an animal with a closed circulatory system.
- In fish, the circulatory system is very simple: a two-chambered heart including one atrium and one ventricle.
- In other vertebrate groups, the circulatory system is much more complicated such that their circulatory systems are divided into two types: a three-chambered heart, with two atria and one ventricle, or a four-chambered heart, with two atria and two ventricles.
The atrium receives blood as it returns to the heart to complete a circulating cycle, whereas the ventricle pumps blood out of the heart to start a new cycle.

# Human heart
- Humans have a four chambered heart which includes the right atrium, left atrium, right ventricle, and left ventricle.
- The right atrium:
It receives de-oxygenated blood from the superior vena cava and inferior vena cava.
It sends blood to the right ventricle which sends it to the lungs for oxygen.
- It receives de-oxygenated blood from the superior vena cava and inferior vena cava.
- It sends blood to the right ventricle which sends it to the lungs for oxygen.
- The left atrium:
It receives oxygenated blood from the left and right pulmonary veins.
It pumps blood to the ventricle where it is sent to the aorta which takes it to the rest of the body.
- It receives oxygenated blood from the left and right pulmonary veins.
- It pumps blood to the ventricle where it is sent to the aorta which takes it to the rest of the body.
- The atria do not have valves at their inlets.
As a result, a venous pulsation is normal and can be detected in the jugular vein (see: jugular venous pressure).
- As a result, a venous pulsation is normal and can be detected in the jugular vein (see: jugular venous pressure).
- Internally, there is the rough musculae pectinati, crista terminalis which acts as a boundary inside the atrium and the smooth walled part derived from the sinus venosus.
- There is also a fossa ovalis in the interatrial septum which was used in the fetal period as a means of bypassing the lung.

# Anatomy of the Atria

## Right Atrium
- The right atrium forms the right border of the heart.
- The superior vena cava and the inferior vena cava drain systemic venous blood into the smooth posterior wall of the right atrium.
- In contrast, the right atriums anterior wall is ridge-like; it is composed internally of pectinate muscles, a rough muscular wall.

### Right auricle:
- The right atrium contains a right auricle, an ear-like conical muscular pouch that overlaps the ascending aorta.
- The right auricle is an embryonic structural remnant that allows the right atrium to increase its capacity.

### Opening of the superior and inferior vena cava:
- The superior vena cava opens in the interior right atrium at its superior part, at the level of the right third costal cartilage.
- The inferior vena cava opens in the interior right atrium at its inferior part, almost in line with the superior vena cava at approximately the level of the fifth costal cartilage.

### Coronary Sinus:
- It is a venous collection, whereby systemic blood is received at the posterior part of the coronary groove from the cardiac veins into the coronary sinus. The latter is a derivative of an embryonic structure called the venous sinus.
- The opening or orifice of the coronary sinus is located between the right atrioventricular orifice and the inferior vena cava orifice.

### Sinus Venarum:
- The adult right atrium also contains posteriorly a smooth-walled pouch called sinus venarum, on which the coronary sinus, and superior and inferior vena cava bring poorly oxygenated blood to the heart.
- Sinus venarum is formed by the incorporation of the venous sinus (L. sinus venosum), an embryonic structure, into the primordial atrium. As a result, sinus venarum provides further capacity for the right atrium.
- The sinus venorum is separated from the primordial atrium by means of 2 structures: the sulcus terminalis (terminal groove) externally which extends from the front of the superior vena cava to the front of the inferior vena cava, and represents the line of union of the sinus venosus of the embryo with the primitive atrium. Internally, the separation is indicated by the crista terminalis (terminal crest).

### Fossa Ovalis:
- Fossa ovalis is a depression in the inter-atrial septum.
- It is a remnant of the foramen ovale, an embryonic physiological shunt, significant for fetal oxygenation and nutrition from the mothers placenta to fetuss left atrium without passing through the lungs.
- The limbus of the fossa ovalis (annulus ovalis) is the prominent oval margin of the fossa ovalis.
- It is located on the medial wall of the right atrium and circumscribes the septum primum the fossa ovalis anteriorly, posteriorly, and superiorly.

## Left Atrium
- The left atrium is the major component of the heart base.
- The interatrial septum is part of the left atrial wall; it runs posteriorly and to the right. Its wall is slightly thicker than that of the right atrium.
- The interior of the left atrium is characterized by two main pouches:
A larger smooth-walled part, believed to be formed by absorption of parts of embryonic pulmonary veins.
A smaller muscular auricle with pectinate muscles, believed to be remnant of left part of primordial atrium.
- A larger smooth-walled part, believed to be formed by absorption of parts of embryonic pulmonary veins.
- A smaller muscular auricle with pectinate muscles, believed to be remnant of left part of primordial atrium.

### Pulmonary Veins:
- Blood enters the left atrium on the posterior wall via 4 valveless pulmonary veins, arranged in two pairs, left and right.

### Left Auricle:
- Similar to the right atrium, the left atrium also contains a left auricle.
- In contrast to the right auricle which overlaps the ascending aorta, the left auricle forms the superior part of the left border of the heart and overlaps the pulmonary trunk.
- The left auricle is a small muscular region containing pectinate muscles inside.

### Left Atrio-Ventricular Orifice:
- At the level of the left atrio-ventricular orifice, the outflow of oxygenated blood received from the pulmonary veins evacuates the left atrium.